Review Article | Open Access
Liangyun Liu, Xiao Zhang, Yuan Gao, Xidong Chen, Xie Shuai, Jun Mi, "Finer-Resolution Mapping of Global Land Cover: Recent Developments, Consistency Analysis, and Prospects", Journal of Remote Sensing, vol. 2021, Article ID 5289697, 38 pages, 2021. https://doi.org/10.34133/2021/5289697
Finer-Resolution Mapping of Global Land Cover: Recent Developments, Consistency Analysis, and Prospects
Land-cover mapping is one of the foundations of Earth science. As a result of the combined efforts of many scientists, numerous global land-cover (GLC) products with a resolution of 30 m have so far been generated. However, the increasing number of fine-resolution GLC datasets is imposing additional workloads as it is necessary to confirm the quality of these datasets and check their suitability for user applications. To provide guidelines for users, in this study, the recent developments in currently available 30 m GLC products (including three GLC products and thematic products for four different land-cover types, i.e., impervious surface, forest, cropland, and inland water) were first reviewed. Despite the great efforts toward improving mapping accuracy that there have been in recent decades, the current 30 m GLC products still suffer from having relatively low accuracies of between 46.0% and 88.9% for GlobeLand30-2010, 57.71% and 80.36% for FROM_GLC-2015, and 65.59% and 84.33% for GLC_FCS30-2015. The reported accuracies for the global 30 m thematic maps vary from 67.86% to 95.1% for the eight impervious surface products that were reviewed, 56.72% to 97.36% for the seven forest products, 32.73% to 98.3% for the six cropland products, and 15.67% to 99.7% for the six inland water products. The consistency between the current GLC products was then examined. The GLC maps showed a good overall agreement in terms of spatial patterns but a limited agreement for some vegetation classes (such as shrub, tree, and grassland) in specific areas such as transition zones. Finally, the prospects for fine-resolution GLC mapping were also considered. With the rapid development of cloud computing platforms and big data, the Google Earth Engine (GEE) greatly facilitates the production of global fine-resolution land-cover maps by integrating multisource remote sensing datasets with advanced image processing and classification algorithms and powerful computing capability. The synergy between the spectral, spatial, and temporal features derived from multisource satellite datasets and stored in cloud computing platforms will definitely improve the classification accuracy and spatiotemporal resolution of fine-resolution GLC products. In general, up to now, most land-cover maps have not been able to achieve the maximum (per class or overall) error of 5%–15% required by many applications. Therefore, more efforts are needed toward improving the accuracy of these GLC products, especially for classes for which the accuracy has so far been low (such as shrub, wetland, tundra, and grassland) and in terms of the overall quality of the maps.
Land cover is an essential climate variable and is widely used in numerous studies concerned with, for example, global climate change, Earth system modeling, natural resource management, food security, and conservation biology [1–3]. In recent decades, many global land-cover (GLC) products have been successfully generated using satellite remote sensing data, and GLC mapping is moving toward providing datasets with higher spatial resolution and finer classification [3–5]. The GLC products that have a coarse spatial resolution, such as the global land-cover classification product at 1 km (GLC_2000)  and IGBP (International Geosphere-Biosphere Programme) DISCover land-cover classification product (IGBP_DISCover) , the 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) land-cover product (MOD12Q1) , and the 300 m global land-cover map (GlobCover)  and the European Space Agency (ESA) Climate Change Initiative land-cover product (CCI_LC) , have been used in many studies. However, these coarse-resolution GLC products do not provide enough spatial detail for studies on, for example, urban expansion and resource management [11, 12]. As pointed out by Herold et al. , more efforts based on the use of higher-resolution satellite data should be made to improve the classification accuracy of heterogeneous landscapes, which is the most significant challenge for improving GLC mapping. Furthermore, fine-resolution (∼30 m) GLC products provide rich spatial information at the scale of most human activity, thus offering greater flexibility for application in a range of studies [11, 12, 14].
The free availability of fine-resolution satellite remote sensing data (especially Landsat archive and Sentinel-2 data), together with improved computing and storage capacity, has resulted in the availability of GLC products with a resolution of 10–30 m; these products include GlobeLand30 , FROM_GLC , GLC_FCS30 [15, 16], ESA-S2-LC20 , and FROM_GLC10 . Many global thematic fine-resolution land-cover products have also been generated. For example, there are several global impervious surface datasets, including NUACI , ESA GHSL , GAIA , and MSMT_IS30 ; global inland water body datasets, including GLCFGIW , G3WBM , JRC_GSW , and GLAD_Water ; global cropland datasets, including GFSAD30 , GCAD30 , and FROM_GC ; and global forest-cover datasets, including TreeCover-2010 , GFCC30TC , and GFC30 .
These fine-resolution GLC datasets have been generated as a result of various initiatives, so they differ in terms of classification systems, satellite data used, and classification methodologies [33, 34]. For example, there are huge discrepancies between CGLS-LC100, ESA-S2, and FROM_GLC, which have an overall spatial agreement of 56.87% and a disagreement of 43.13% for Africa . Consequently, it is difficult to assess which GLC dataset fits user application best [36, 37]. Although many campaigns have been conducted with the aim of collecting reference datasets for validating fine-resolution GLC maps, their availability and role in applications outside their area of intended use have been very limited because of the inconsistencies between legends, sampling strategies, and response design protocols used [33, 38]. According to reports relating to global validation campaigns, there are large differences between the overall accuracies of fine-resolution GLC products: 64.9% for FROM_GLC , 80.3% for GlobeLand30 , and 82.5% for GLC_FCS30 . In spite of the validation assessments provided by the developers of these products, some third-party researchers have found considerably lower accuracies for some regions when verifying GLC products [39, 40].
The need to assess the quality of the increasing number of fine-resolution GLC datasets and check their suitability for different user applications is leading to additional workloads. The objectives of this study were (1) to review recent developments related to current fine-resolution GLC products, (2) to compare the available 30 m GLC maps and quantify their spatial consistency, and (3) to summarize the trends in and prospects for fine-resolution GLC mapping.
2. Consistency and Accuracy Assessment Metrics
The area-based and pixel-based consistencies are widely used to assess the agreement between different global products. The area-based analysis mainly focuses on the consistency of the areal proportions of each LC type between the different GLC products [39, 41], whereas the pixel-based analysis is concerned with the spatial (dis)agreement for each LC type between the different GLC products [34, 41, 42]. In this study, both an analysis of the consistency of the current 30 m GLC products and an analysis for different classes (impervious surface, forest, cropland, and water body) of current global 30 m thematic products were performed to give a more comprehensive understanding of the process of global land-cover mapping.
2.1. Analysis of the Consistency of Current 30 m GLC Products
First, the area percentage of each land-cover type in each GLC product was calculated to give an overview of the consistency between the current GLC products. Then, in order to intuitively illustrate the spatial consistency between the different products, the spatial superposition method  was employed to obtain the pixel-by-pixel spatial correspondence for the different products. A maximum value (equal to 3) was assigned for highly consistent pixels when the target land-cover types for the three LC products were exactly the same; a value of 2 was assigned for moderately consistent pixels when only two land-cover types agreed; a minimum value (equal to 1) was assigned for low-consistency pixels when the three LC products all had different target land-cover types . To obtain more detailed information about the spatial consistency between the different products, the spatial superposition method was also applied to each pair of products. Pixels for which the target land-cover type was the same for both products were considered “consistent,” whereas pixels with different target land-cover types were labeled “inconsistent.” The spatial consistency distribution for all three products and for each pair of products is given based on a spatial resolution of 0.05°. In the spatially aggregating step, the consistency metric was firstly calculated for each 30 m pixel; then, the proportion of highly consistent (PHC) pixels, proportion of moderately consistent (PMC) pixels, proportion of low-consistency (PLC) pixels within each 0.05° grid were calculated and stored in three channels. Therefore, the spatial consistency was assessed at a resolution of 30 m, while illustrated at a resolution of 0.05°.
2.2. Analysis of the Consistency of Current Global 30 m Thematic Products
The analysis of the consistency of the current global 30 m thematic products focused on the impervious surface, forest, cropland, and water body classes. First, the fractional areas for each thematic class were calculated at a spatial resolution of 0.05°. The total meridional and zonal fractional areas were then calculated for each class for each 0.05° longitude and latitude bin. To quantitatively evaluate the consistency between each pair of thematic products, the linear regression approach was applied, and scatter plots for each pair of products were made. Based on the linear regression, the root-mean-squared difference (RMSD) and coefficient of determination () were calculated as follows: where is the fractional thematic class area for thematic product in the th grid cell; is the fractional thematic class area for thematic product in the th grid cell; and is the total number of grid cells in the thematic product. The RMSD value was used to reflect the discrepancy between the two thematic products in each pair: where is the average fractional thematic class area for thematic product and is the average fractional thematic class area for thematic product . The higher the value of , the better the fit between a pair of thematic products.
3. Review of the Current Global 30 m Products
3.1. Review of Global 30 m Land-Cover Products
3.1.1. Overview of the Global 30 m Land-Cover Products
There are three 30 m global land-cover products that are currently available free of charge: GlobeLand30 , FROM_GLC , and GLC_FCS30 . Some basic information about these three products is listed in Table 1.
The GlobeLand30 product, which is one of the most widely used land-cover products, was developed by using the pixel-object-knowledge-based (POK) method and multitemporal Landsat and HJ-1 A/B imagery . This product has been validated as having an overall accuracy of 80.3% based on 154,586 validation samples from across the globe . FROM_GLC was the first global 30 m land-cover product. Currently, the 2015 version (which has a 30 m resolution) and the 2017 version (which has a 10 m resolution) are available. FROM_GLC30-2015, which was produced using multitemporal Landsat imagery combined with over 90,000 visually interpreted training samples and random forest classifiers , has been validated as having an overall accuracy of 80.6% . The GLC_FCS30 product, which contains a large variety of land-cover types, was developed on the GEE platform by combining time series of Landsat imagery, a global prior training dataset obtained from GSPECLib , and local adaptive random forest models . GLC_FCS30-2015 has been validated as having an overall accuracy of 82.5% and a kappa coefficient of 0.784 based on 44,043 global validation samples .
There are also some differences between the classification systems employed by the three different GLC products. GlobeLand30 consists of 10 basic land-cover types, whereas FROM_GLC30 uses a two-level hierarchical classification system with 10 land-cover types at level 1, which is the same classification system used by GlobeLand30. GLC_FCS30 employs the Climate Change Initiative land-cover dataset (CCI_LC) classification system, which has 16 level 1 land-cover types. As a uniform classification system is the basis of the comparative analysis of different GLC products, these different classification systems were transformed into a target classification system with 10 basic land-cover types based on the study of Gao et al.  before the consistency analysis was carried out. Details of the classification systems used in the three GLC products and the correspondence between them are shown in Table 2.
3.1.2. Review of the Accuracy Assessments of the Three GLC30 Products
There have been numerous validations of the 30 m GLC products at both the regional and global scales. Table 3 summarizes the reported accuracies. Generally, the accuracies vary according to the validation dataset used. All producers provide accuracy reports for their own products based on different global validation datasets. The reported accuracy of GLC_FCS30 is the highest—an overall accuracy of 82.5% —as against 80.3% for GlobeLand30  and 64.9% for FROM_GLC . Since GlobeLand30 and FROM_GLC were released earlier, numerous validations of these products have been conducted, and the reported accuracies have ranged from 46.0% for Central Asia  to 88.9% for the European Union  and from 57.71% for Indonesia  to 80.36% for Beijing, China , for GlobeLand30 and FROM_GLC, respectively. Recently, two sets of reported results for GLC_FCS30-2015 gave an accuracy of 65.59% for Indonesia  and 84.33% for the European Union .
Only three published studies simultaneously compared the accuracy of the three GLC products using the same validation dataset. Based on the 44,043 global validation points derived from the reference land-cover datasets that are currently available , GLC_FCS30-2015 was reported to achieve the best accuracy (82.5%), compared to 75.9% for GlobeLand30-2010 and 59.1% for FROM_GLC-2015 . Gao et al.  investigated the performance of the three 30 m GLC products for the European Union and found that GlobeLand30-2010 had the highest overall accuracy of ; this was followed by for GLC_FCS30-2015 and for FROM_GLC-2015. Kang et al.  investigated the performance of the three 30 m GLC products for Indonesia and found that the accuracy of the 30 m GLC products requires further improvement in tropical rainforest regions: the highest overall accuracy was 65.59% for GLC_FCS30-2015. Accuracies of 61.65% and 57.71% were found for GlobeLand30-2010 and FROM_GLC-2015, respectively.
Furthermore, the mapping accuracy varies for different land-cover types. For land-cover types that have relatively pure spectral properties or occupy a large proportion of the Earth’s surface, such as forest, cropland, and water, relatively high accuracies have usually been found. In contrast, complex land-cover types such as shrubland, grassland, and wetland are often confused with other types. For example, for the forest class, according to their producers, the mapping accuracies are 94.0% (GLC_FCS30) , 76.5% (FROM_GLC30) , and 83.6% (GlobeLand30) . For shrubland, the accuracies reported by the producers are significantly lower at 56.8% (GLC_FCS30) , 33.9% (FROM_GLC30) , and 72.6% (GlobeLand30) . A similar trend was found by the comparative studies. In , the accuracies found for cropland were 99.0% (GlobeLand30), 83.6% (GLC_FCS30), and 63.5% (FROM_GLC30); these were much higher than accuracies for wetland, which were 35.8% (GlobeLand30), 36.2% (GLC_FCS30), and 0.0% (FROM_GLC30). In the accuracy comparison analysis made by Zhang et al. , the accuracies for cropland were 94.0% (GLC_FCS30), 4.9% (FROM_GLC30), and 92.6% (GlobeLand30), whereas the accuracies for wetland were 61.8% (GLC_FCS30), 3.3% (FROM_GLC30), and 52.6% (GlobeLand30).
3.2. Review of the Global Impervious Surface Maps
3.2.1. Overview of the Global Impervious Surface Maps
Impervious surfaces are a special land-cover type that usually relates to surfaces that are covered by anthropogenic materials such as roads, driveways, sidewalks, parking lots, and rooftops which prevent or retard the entry of water into the soil . Mapping the global distribution and dynamics of global impervious surfaces using satellite remote sensing data is extremely important to improve our understanding of the intensity of human activity and of global change.
In this study, eight global 30 m impervious surface products were acquired and investigated. Some details of these products are presented in Table 4. Most global impervious surface products cover the long time period stretching from 1985 to 2020 and include multiepoch impervious surface maps; the exceptions to this are MSMT_IS30, HBASE, and GUF. In this study, an analysis of the spatial consistency between these eight impervious surface products in 2015 was conducted. Three products were not available—GlobeLand30 and HBASE/GMIS for 2010 and GUF for 2012. The temporal variations between 2010 and 2015 were, therefore, neglected, which may have led to a certain amount of bias, especially in the case of developing countries. Most of the impervious surface products (MSMT_IS30, NUACI, GAIA, FROM_GLC, GlobeLand30, and GHSL) use similar definitions of “covered by anthropogenic materials which prevent water penetrating into the soil” for impervious surfaces. HBASE defines impervious surfaces as the sum of all surfaces constructed by humans and the area surrounding these surfaces, including urban green spaces; GUF defines impervious surfaces from the perspective of spatial structures—“impervious surfaces should show obvious vertical components”—and so does not define roads, parking lots, airport runways, etc. as impervious surfaces. Lastly, almost all impervious surface products are produced using classification-based methods, including independent random forest classification (MSMT_IS30 and FROM_GLC), a combination of segmentation, classification, and prior knowledge (GlobeLand30 and HBASE), or a combination of unsupervised classification, supervised classification, and prior knowledge (GHSL and GUF). The NUACI is different as it uses a novel built-up index to generate a global multiepoch impervious surface product; GAIA uses normalized scores (the exclusion-inclusion method) and knowledge-driven temporal checking methods to produce annual global impervious surface products.
3.2.2. Review of the Accuracy Assessments of the Global Impervious Surface Maps
Table 5 lists the accuracy metrics produced by various researchers at different scales for eight global 30 m impervious surface products. Firstly, from the perspective of the global accuracy of each product, the overall accuracy of the impervious surface class in GAIA is 89%, which exceeds the values of 80% in GlobeLand30, 81%–84% in NUACI, and 84.18% in GHSL using the corresponding validation points. At the regional scale, MSMT_IS30 has been validated as having an overall accuracy of 95.1% and a kappa coefficient of 0.898 for 15 test sites of size  and as having the user’s and producer’s accuracies of 73.61% and 99.07%, respectively, in Indonesia  and and , respectively, in the European Union . As for the GlobeLand30 impervious layer, an overall accuracy of 88.4% was found for 15 test sites , along with accuracies of 91.37% in Beijing , 80.54% in Malaysia , and 67.86% for 450 test sites . GHSL has also been widely used and assessed and has been validated as having an overall accuracy of 90.3% for 15 test sites , as well as accuracies of 89.21% in Malaysia  and 70.64% for 450 test sites . This product has been shown to have an accuracy of 96.28% and a kappa coefficient of 0.3233 in Europe  and a kappa coefficient of 0.562 for 26 counties of the US . Lastly, using 15 test sites, Zhang et al.  found that the overall accuracies of NUACI, FROM_GLC, and HBASE were 85.6%, 89.6%, and 88.0%, respectively. Kang et al.  and Gao et al.  found that the producer’s and user’s accuracies of FROM_GLC were 81.31% and 52.41%, respectively, in Indonesia and and , respectively, in the European Union. Wang et al.  analyzed the performance of HBASE and found that it had an overall accuracy of 97.9% and a kappa coefficient of 0.90.
In summary, although the global 30 m impervious surface products have improved markedly in recent decades, there is still a lack of a comprehensive accuracy assessment for these global products that uses a common validation dataset. Furthermore, the accuracy metrics of these global impervious surface products vary according to the geographical region and the validation datasets used.
3.3. Review of the Global Forest Maps
3.3.1. Overview of the Global Forest Maps
Forest is an important natural land-cover type that usually describes surfaces covered by trees or by a canopy that exceeds a certain tree-cover percentage and height . Monitoring global forest-cover distribution and disturbances using remote sensing techniques is critical for understanding the intensification of human activities, climate change, and the global carbon cycle. In recent decades, as the intensity of human activities has increased and free access to medium- and high-resolution remote sensing imagery has become available, global forest mapping and monitoring has experienced a transition from coarse-resolution to medium- and high-resolution mapping. In this study, seven global 30 m forest products were acquired and investigated. These products are listed in Table 6.
All of these forest products define the forest land-cover type using the tree-cover percentage (TCP) and tree height (TH); however, they do not all use the same thresholds for the TCP and TH. For example, a TCP of 15% is used as the threshold for forest by GLC_FCS30 and FROM_GLC, 10% is used by GlobeLand30 and GFC30, and 49% is used by the TreeCover product. GLC_FCS30 and FROM_GLC have a TH threshold of 3 m, whereas 5 m is used by the GLADForest and GFC30 products. In terms of the methodology used, supervised classification methods are widely used: for example, GLC_FCS30, FROM_GLC, and GFC30 are generated using training samples and supervised classification models and GlobeLand30 is produced by combining supervised classification with an object-based segmentation method. The decision tree regression model is used by TreeCover and GLADForest to generate continuous forest-cover percentage products. The GFCC30TC forest product is derived by combining the 250 m MODIS Vegetation Continuous Fields (VCF) tree-cover layer and Landsat imagery.
3.3.2. Review of the Accuracy Assessments of the Global Forest Maps
To help with understanding the accuracy of each global 30 m forest map, Table 7 lists the accuracy metrics of each forest product based on the results of different studies. It should be noted that the forest land-cover type exists as an independent layer in GLC_FCS30, FROM_GLC, and GlobeLand30 and also as a single thematic element in the GFCC30TC, TreeCover-2010, GLADForest, and GFC products.
For the products where forest is an independent layer (GLC_FCS30, GlobeLand30, and FROM_GLC), the producer’s accuracies are 94.0%, 92.4%, and 76.5%, respectively, and the user’s accuracies are 90.4%, 84.1%, and 80.5%, respectively, at the global scale. Kang et al.  and Gao et al.  comprehensively assessed the forest layers in these three different products at a regional scale, and the results indicated that GlobeLand30 and GLC_FCS30 have the same producer’s and user’s accuracies and that these values were higher than those of FROM_GLC. Furthermore, Moreno-Sanchez et al.  found that the overall accuracy of “forest” in GlobeLand30 was higher in tropical forest areas than in temperate forests, and Tsendbazar et al.  confirmed that the accuracy of the forest layer in GlobeLand30 is poorer in Africa than in other regions. Overall, the accuracy of the forest land-cover type in GlobeLand30 and GLC_FCS30 was found to be better than that in FROM_GLC.
Secondly, the global thematic forest products GFCC30TC and GFC30 have been validated as having accuracies of 90% and 90.94% at the global scale. GFCC30TC performs differently in different geographical regions: for example, it underestimates the high-canopy forest (canopy ) but overestimates the low-canopy forest (canopy ) in the boreal forest in Finland  and has a higher accuracy over North America (82% in Maryland)  than in South America (where the producer’s and user’s accuracies are and , respectively) . TCC-2010 has been shown to have a higher accuracy than other forest products: for example, the producer’s and user’s accuracies were shown to be and , as against and for GFCC30TC, in South America . In Kyrgyzstan, TCC-2010 was found to have an overall accuracy of 94.50% against 88.27% for GlobeLand30 . Finally, GLADForest, which is the most widely used forest change product, has been validated to have an overall accuracy of for forest loss and for forest gain based on only 1500 validation points . However, regional validations have indicated that this product has an overall accuracy of 81% in Maryland , 66%–56% in Indonesia , and in Costa Rica .
Therefore, as shown by the accuracy metrics in Table 7, the global thematic forest products (GFCC30TC, TreeCover-2010, GLADForest, and GFC30) generally have a higher overall accuracy than the global “forest layer” products (where forest is an independent layer), and the regional accuracies are usually lower than the global values.
3.4. Review of the Global Cropland Maps
3.4.1. Overview of the Global Cropland Maps
Cropland is an important land-cover type related to global food and water security. Cropland has a close relationship with human activities and usually consists of land cultivated with plants that are harvested for food, animal feed, and fibers. In recent decades, as the global population has rapidly grown, the growing demand for food has put added pressure on the food supply system, particularly because of the demand for grains used as livestock feed in regions where food security remains elusive . Therefore, accurate global mapping of cropland is of great importance to assessing and monitoring global food security and production potential. In this study, six global 30 m cropland products were acquired. These products are summarized in Table 8.
Most global cropland products (GLC_FCS30, GlobeLand30, GCAD30, GFSAD30, and FROM_GC) contain single-epoch cropland information; the exceptions are the FROM_GLC and GlobeLand30 cropland products. In this study, the nominal year of 2015 was selected for analyzing the consistency between the global products. However, the GCAD30 stopped sharing after GFSAD30 was released and two of these products (FROM_GC and GlobeLand30) were available for 2010. The temporal variations between 2010 and 2015 were, therefore, neglected, and this may have led to a certain amount of bias in the comparison. Furthermore, from the point of view of the definition of cropland, there are obvious differences between these global cropland products. For example, GLC_FCS30 defines cropland using only human management as the criterion and FROM_GC excludes “permanent pastures.” GlobeLand30, FROM_GLC, and GFSAD30 define cropland from a functional perspective as being land cultivated with plants that are harvested for food, animal feed, and fiber; however, FROM_GLC excludes fruit trees and classifies them as belonging to “forest.” Finally, in terms of the methodology used, these products can be divided into three groups: GLC_FCS30 and FROM_GLC are produced using pixel-based supervised classification; GlobeLand30, GCAD30, and GFSAD30 use a combination of pixel-based classification and object-based segmentation; and FROM_GC is composited from four previously produced cropland maps.
3.4.2. Review of the Accuracy Assessments of the Global Cropland Maps
The results of assessments of the accuracy of five global 30 m cropland maps are summarized in Table 9. Similar to the forest land-cover type that was discussed in Section 4.2, cropland exists as a single layer in the GLC_FCS30, FROM_GLC, and GlobeLand30 land-cover products and also as an independent thematic product in FROM_GC and GFSAD30. (Note that GCAD30 has not been available since the release of GFSAD30.)
Looking first at the products in which cropland is an independent layer, in GLC_FCS30, GlobeLand30, and FROM_GLC, cropland has been shown to have the producer’s accuracies of 88.0%, 88.2%, and 47.7% and the user’s accuracies of 83.9%, 88.7%, and 74.7%, respectively . The accuracy of the cropland class in GlobeLand30 and FROM_GLC varies according to the geographical zone and validation dataset used; for example, Zhong et al.  found an overall accuracy of 79.61% and kappa coefficient of 0.58 for FROM_GLC as against 76.23% and 0.52 found by Lu et al. . GlobeLand30, which is the most widely used land-cover product , was demonstrated to have an overall accuracy of 80% in Africa, America, and Asia , an accuracy of 79.61% and a kappa coefficient of 0.58 in China , an accuracy of 80.61% in Shaanxi Province, China , an accuracy of 86.98% and a kappa coefficient of 0.692 in East Africa , an accuracy of in Tanzania , and an accuracy of 79.5% over five West African countries .
FROM_GC and GFSAD30 are two widely used thematic cropland products—this applies to GFSAD30 in particular. GFSAD30, which was derived and optimized from GCAD30 , was assessed for different continents and countries. Globally, an overall accuracy of 91.7%, a producer’s accuracy of 83.4%, and a user’s accuracy of 78.3% were found using 19,171 validation points [27, 72]. Regionally, this product was found to have an overall accuracy of 94% in Australia and China , an overall accuracy of 90.8% and a weighted accuracy of 93.8% in Europe, the Middle East, Russia, and Central Asia , a weighted overall accuracy of 94.5% in Africa [75, 76], an overall accuracy of 88.1% in Southeast and Northeast Asia , and an average user’s accuracy of 73.1% over five West African countries . As for the FROM_GC cropland product, it was assessed to have a significant linear relationship with FAOSTAT data (with an of 0.97) globally , and overall accuracies of 77.67% and were demonstrated for Shaanxi Province, China  and Tanzania , respectively. FROM_GC and GFSAD30 matched for 68.8% of cropland samples from Africa [75, 76].
Comparisons between GlobeLand30 (in which cropland is a single land-cover layer) and GFSAD30 (in which cropland is an independent thematic product) have also been conducted. The results have indicated that the two products have an overall similarity of 88.8% and a kappa coefficient of 0.7 in Europe, the Middle East, Russia, and Central Asia . GlobeLand30 was shown to have a higher overall accuracy than GFSAD30 over five West African countries .
3.5. Review of the Global Water Maps
3.5.1. Overview of the Global Inland Water Maps
Inland water refers to fresh and brackish water bodies such as lakes, reservoirs, and rivers . These water bodies play a vital role in the global and regional hydrological and biogeochemical water cycles [23, 79]. However, due to their sensitivity to changes in the environment and climate, the distribution of water bodies varies over time. The resulting variations in water bodies have significant implications for the water balance and regional biodiversity . In this study, three 30 m global inland water thematic products (G1WBM, JRC_GSW, and GLADWater) and three 30 m GLC products (GLC_FCS30, FROM_GLC, and GlobeLand30) were acquired for carrying out a spatial consistency analysis of inland water bodies for 2015. Details of these products are shown in Table 10.
Apart from JRC_GSW and GLADWater, for which annual water maps are available for periods of a number of years (1984–2015 and 1999–2018, respectively), these products were only produced in certain individual years. Specifically, G1WBM was only produced in 2010, GLC_FCS30 is only available for 2015, FROM_GLC is only available for 2010, 2015, and 2017, and GlobeLand30 was only produced in 2000 and 2010. Because 2015 was chosen as the nominal year for comparison and because there was no G1WBM and GlobeLand30 available for 2015, any discrepancies between 2015 and the actual years used may also have caused some discrepancies in the results. Regarding the definitions of “water body” used by the different products, the three thematic water products were distinguished between the permanent and seasonal water types based on the frequency of the presence of water occurrence, whereas the three GLC products were not distinguished between the permanent and seasonal water bodies. In addition, there were also differences between the definitions of permanent water and seasonal water used by the three thematic products. G1WBM defines permanent water as corresponding to pixels where water is present more than 70% of the year; however, JRC_GSW defines pixels as permanent water only if the water is present throughout the year. GLADWater also labels as permanent water the pixels where the difference between the maximum frequency and the minimum frequency of the presence of water over three consecutive years is less than or equal to 33% and the average frequency over these years is greater than 90%. In order to reduce the discrepancies caused by these different definitions of water bodies, we combined the permanent and seasonal water types in the thematic products into one water class. The three GLC products are all derived using supervised classification methods. However, two of the thematic water products (G1WBM and JRC_GSW) are generated based on classification rules and globally consistent thresholds; only GLADWater is produced using a supervised classification method.
3.5.2. Review of the Accuracy Assessments of the Global Water Maps
The results of the assessment of the accuracy of the six products that include global inland water maps are summarized in Table 11. Among the three GLC products, the results for GlobeLand30 and FROM_GLC include regional and global results, whereas the assessment of GLC_FCS30 was performed for Indonesia only. Among the global thematic inland water products, global results were available for JRC_GSW and GLADWater, whereas, for G1WBM, only the report for the United States could be found.
For GlobeLand30 and FROM_GLC, the water class has a satisfactory global accuracy. The overall accuracies of this class in GlobeLand30-2000 and GlobeLand30-2010 are 96.51% and 96.48%, respectively ; the user’s and producer’s accuracies are 81.97% and 85.66%, respectively, in FROM_GLC-2010  and 81.97% and 86.39%, respectively, in FROM_GLC-2017 . At the regional scale, the accuracy was found to vary with the location. For example, Guo et al.  found that the overall accuracy for water in FROM_GLC-2017 was 95.87% in West Asia–Northeast Africa, 89.75% in Central and Eastern Europe, 87.01% in Central Asia, and 15.67% in South Asia. Also, the user’s and producer’s accuracies for GlobeLand30-2010 were 93.44% and 54.42%, respectively, in Indonesia  and 94.00% and 79.00%, respectively, in China . Additionally, it was found that the producer’s accuracy (which is related to the omission error) was generally much lower than the user’s accuracy at the regional level. For instance, the user’s and producer’s accuracies of the water class in GLC_FCS30, GlobeLand30, and FROM_GLC in Indonesia were 97.74% and 60.19%, 93.44% and 54.42%, and 89.97% and 68.52%, respectively . Also, the user’s and producer’s accuracies of the water class in FROM_GLC-2015 in Africa were 93.50% and 66.90%, respectively . These results indicate that these GLC products are not sufficiently accurate to fully support the analysis of water bodies at the regional scale and that they are more likely to omit water bodies in specific regions.
In the three thematic inland water products, the accuracies of the permanent water body class are quite high. The global commission and omission accuracies of this class in JRC_GSW are greater than 99% and 97%, respectively . Also, for the United States, more than 85% of the permanent water bodies in G1WBM were proved to be accurately mapped . For GLADWater, the global user’s and producer’s accuracies are and , respectively . However, the accuracies for the seasonal water body class are much lower than those for permanent water bodies. Pekel et al.  reported that the omission accuracy of the seasonal water body class in JRC_GSW was only about 75%. The accuracy of seasonal water in JRC_GSW was found to be even lower in the research of Pickens et al. , which gave the user’s and producer’s accuracies of and , respectively. Similarly, the user’s and producer’s accuracies for GLADWater were only and , respectively, based on the time series of stable seasonal water bodies . These results show that it was still difficult to accurately detect seasonal water bodies.
4. Assessment of the Spatial Consistency of the Current Global 30 m Products
4.1. Spatial Consistency of the Three GLC30 Products
All these 30 m GLC datasets were aggregated to a spatial resolution of 0.05° for visual comparison of the spatial pattern, in which the land-cover type in each 0.05° grid was determined by the land-cover type with the largest proportion. Figure 1 shows the spatial distributions of the different land-cover types in the three fine-resolution GLC maps both at the global scale with a resolution of 0.05° (Figure 1(a)) and at four typical inconsistent regions with a size of and a resolution of 30 m (Figure 1(b)).
Overall, the three products are consistent and roughly capture the actual distribution of land-cover types at the global scale. Specifically, bareland is mainly found between latitudes 15°N–50°N and 15°S–35°S, including parts of Central Asia and the western United States. In the northern hemisphere, cropland is mainly found in India and southern China in Asia, mid- to high-latitude parts of Europe, and the southeastern United States; in the southern hemisphere, cropland is mainly found in southern Brazil and northern Argentina. The forest class is concentrated in rainforest areas (including the Amazon rainforest, African rainforest, and Indian–Malay rainforests) and high-latitude areas. However, the three GLC30 products show large inconsistency in the transition zones or heterogeneous regions (as illustrated in Figure 1(b) A–D). For example, the disagreement is noticeable in the transition zones from bareland to grassland or cropland (Figure 1(b) A, B) and in the heterogeneous mountainous areas with complicated land-cover types (Figure 1(b) C, D).
Figure 2 illustrates the percentage areas covered by each class according to the three GLC products. The percentages show roughly the same pattern for the different products: forest has the highest percentage area, accounting for about 30% of the global total, followed by grassland, bareland, and shrubland.
The area percentages of grassland, shrubland, and tundra in the GLC_FCS30-2015 product are very different from those in the other two products. The shrubland percentage in GLC_FCS30-2015 (11.38%) is higher than that in FROM_GLC30-2015 (7.47%) and GlobeLand30-2010 (5.18%); in contrast, the grassland and tundra percentages are significantly smaller than those in the other two products. In particular, the tundra percentage (4.79%) in GLC_FCS30-2015 is about half of the area in the other two products. Furthermore, the area percentages of cropland and wetland in FROM_GLC30-2015 are much lower than those in GLC_FCS30-2015 and GlobeLand30-2010. In FROM_GLC30-2015, wetland accounts for only 0.02% of the global total, whereas in GLC_FCS30-2015 and GlobeLand30-2010, the corresponding percentages are 2.24% and 2.52%, respectively. In FROM_GLC30-2015, cropland covers only 7.44% of the total area—much lower than the figures of 13.96% for GlobeLand30-2010 and 15.64% for GLC_FCS30-2015.
Figure 3 illustrates the spatial consistency between the three GLC products. It is obvious from Figure 3 that the spatial distribution of the three products is relatively consistent in homogeneous regions such as the northern part of the African continent (mainly bareland) and the northern region of South America (forest). However, the spatial consistency is low in heterogeneous or transition zones such as Australia and southern Africa and the transition zone from the vegetation to nonvegetation classes in Africa and also central North America. It can be seen from Figure 1(b) A–C that the disagreement in these areas mainly occurs in areas labeled as grassland, shrubland, or bareland. This is mainly due to the maximum fraction of vegetation coverage that the different classification systems permit in areas defined as not being covered by vegetation. For example, the maximum fraction of vegetation coverage in bareland, as defined by the GlobeLand30 product, is 10%, whereas the maximum fraction defined by GLC_FCS30 is 15%. Therefore, using the former definition, sparse shrubland or grassland will be classified as shrubland, whereas using the latter definition, sparse shrubland and grassland will be wrongly classified as bareland.
To further analyze the spatial consistency between each pair of GLC products, the superposition method was applied to each pair of products. Figure 4 shows the spatial distribution of the proportion of consistent pixels within each grid cell for each pair of the compared GLC products (Figure 4(a)) and the spatial consistency at four typical inconsistent regions with a size of and a resolution of 30 m (Figure 4(b)). Among all of the pairs, GlobeLand30-2010 and FROM_GLC30-2015 have the highest consistency with the consistent area accounting for about 63.35% of the total area; in terms of consistency, this is followed by GLC_FCS30-2015 and GlobeLand30-2010 (59.73%) and then GLC_FCS30-2015 and FROM_GLC30-2015 (57.11%). These results may have been affected by the conversion between the different classification systems. Also, similar to the results for the spatial consistency between the three different products, there is a low degree of consistency in the areas with complex land-cover types and in the transition zones from the vegetation to nonvegetation classes. For example, there is a low degree of consistency in the heterogeneous regions with complex land-cover types and in the transition zones from the vegetation to nonvegetation classes. For example, there is significant disagreement between each pair of products in the transition zones from bareland to grassland or cropland in Africa (Figures 1(b) A and 4(b) A). The same phenomenon can be seen in high-latitude heterogeneous mountainous areas with complicated land-cover types (Figures 1(b) D and 4(b) D). Further analysis showed that the areas of inconsistency between GlobeLand30-2010 and GLC_FCS30-2015 corresponding to transition zones are mostly distributed in areas which are classified as grassland by the GlobeLand30-2010 product but as cropland or shrubland by GLC_FCS30-2015.
4.2. Spatial Consistency of Global 30 m Impervious Surface Maps
Figure 5 illustrates the fraction of impervious surfaces across the globe according to the seven global 30 m impervious surface products using a grid size of 0.05°. Overall, all these products accurately capture the global patterns of impervious surfaces. For example, impervious surfaces can be seen to be mainly found between 20°N and 60°N (including central and eastern parts of North America, the whole of Europe, and eastern parts of China). This agrees with the general patterns of global economic development and population distribution. However, considering the patterns shown in more detail, NUACI-2015, FROM_GLC-2015, and GAIA-2015 clearly produce lower estimates of the amount of impervious surface cover over America and India compared with other products.
To quantitatively assess the spatial consistency between the seven global impervious surface products, the total meridional and zonal areas classified as impervious surfaces were calculated using 0.05° steps (Figure 6). It can be seen from the meridional histogram that all of the products share similar curve shapes that exhibit significant volatility. Four peak intervals in the longitude direction are also captured; these include the region between 60°W and 100°W covering the central and eastern parts of North America, the region between 10°W and 40°E covering the whole of the European Union, the region between 60°E and 90°E, which covers India, and the region between 100°E and 120°E, which covers the east of China and Southeast Asia. Looking at the different products in more detail, using a 0.05° step, HBASE-2010 and GAIA-2015 have the biggest and smallest impervious surface areas among these products, respectively. The values for GAIA-2015 in the 10°W–40°E region (Europe) are clearly low, which is consistent with the results shown in Figure 5.
Similarly, the zonal histogram illustrates that all the global products accurately capture the global distribution patterns of impervious surfaces and shows that this cover type is mainly concentrated between 20°N and 60°N, which is where most of the world’s population and economic activity are also concentrated. HBASE-2010 has the biggest area classified as impervious surfaces in almost all latitude regions except for 45°N–55°N, where GHSL-2015 has a bigger area. In terms of the total area classified as impervious surfaces, HBASE-2010 has the largest area, followed in order by MSMT_IS30-2015, GHSL-2015, GlobeLand30-2010, FROM_GLC-2015, NUACI-2015, and GAIA-2015 (see Figure 6).
Together with the area-based analyses, scatter plots were also used to analyze the spatial consistency between the different global impervious surface products (Figure 7). In this study, MSMT_IS30-2015 was selected as the reference because it has been demonstrated  to have a better performance than most other products. First, it was found that the two products that are most consistent with each other are MSMT_IS30-2015 and GHSL-2015: the relationship between these two products gave an of 0.783, an RMSD of 0.038, and a regression slope of 0.921, with the points in the scatter plot being distributed around the 1 : 1 line. The second best agreement was found between FROM_GLC-2015 and MSMT_IS30-2015, for which was 0.740 and the RMSD was 0.042; in this case, there was a better agreement between the points in the “high-density” regions than in the “low-density” regions, which resulted in a regression slope of less than 1. In GlobeLand30-2010, there is a temporal interval of 5 years between 2010 and 2015 and a minimum mapping unit of pixels, and so it was expected that GlobeLand30-2010 would have a smaller impervious area than MSMT_IS30-2015; in this case, the points in the scatter plot are mainly located below the 1 : 1 line. HBASE-2010 is the only dataset that has a bigger impervious surface area than MSMT_IS30-2015—the slope of the regression is 1.07. This is especially the case in “high-density” regions because, in HBASE, impervious surfaces are defined as comprising all types of man-made built-up surfaces and the areas surrounding them that are functionally linked to those surfaces (e.g., urban green spaces) . Lastly, NUACI-2015 and GAIA-2015 clearly produce underestimates of the impervious surface area (using MSMT_IS30-2015 as a reference) and most of the points in the scatter plots are distributed below the 1 : 1 line, especially in the case of GAIA-2015, for which is 0.491, the RMSD is 0.063, and the slope of the regression is 0.568.
4.3. Spatial Consistency of Global 30 m Forest Maps
Figure 8 illustrates the areal proportions of forest across the globe according to the seven global 30 m forest maps after aggregating the data to a resolution of 0.05°. Overall, all of the global forest maps accurately capture the real patterns in the forest land-cover type. For example, forests can be seen to be mainly concentrated in rainforest areas, including the Amazon rainforest, African rainforests, and Indian–Malay rainforests, and in high-latitude areas where needleleaf forests are found. In high-latitude areas, the forest cover according to GFCC30TC-2015 is obviously sparser than the other forest products; TreeCover30-2010 and GLADForest-2018 give higher densities than the other forest products in the African rainforest areas.
Figure 9 illustrates the distribution of forest in the meridional and zonal directions in steps of 0.05° according to the seven global forest products. Specifically, in terms of the meridional direction, all the forest products capture the global forest spatial patterns, including three peak intervals between 90° and 50°W (the Amazon rainforest), 0° and 40°E (which includes the African rainforests), and 60° and 150°E. There is greater consistency between these products in the western hemisphere than in the eastern hemisphere. It can also be seen that GFCC30TC-2015 generally exhibits the smallest forest areas while TreeCover-2010 exhibits the largest areas, particularly between 0° and 40°E (which includes the African rainforests).
The consistency between the seven products is also illustrated by the forest area curves for the zonal direction. Overall, there is a greater agreement between the products in the southern hemisphere than in the northern hemisphere. The greatest difference occurs between GFCC30TC and the other products around 60°N (where the total forest area in TreeCover-2010 is nearly twice that in GFCC30TC-2015). Looking at the patterns in the curves, the seven forest products can be divided into three groups: GFCC30TC-2015 on its own, GFC30-2015 and GlobeLand30-2010, and the remaining four products as the final group.
Figure 10 shows scatter plots of the seven global 30 m forest products after aggregation to a spatial resolution of 0.05° with the GLC_FCS30-2015 forest layer used as the reference dataset. GLADForest-2018 has the best agreement with GLC_FCS30-2015 and has an of 0.742 and an RMSD of 0.197. This is followed by TreeCover-2010 (, ), FROM_GLC-2015 (, ), GFC30-2018 (, ), GlobeLand30-2010 (, ), and GFCC30TC-2015 (, ). Except for the plot of GLC_FCS30-2015 against GFCC30TC-2015, in which the “high-density” scatter points are mainly distributed below the 1 : 1 line (which also explains why GFCC30TC-2015 has a sparser forest cover than the other products over high-latitude areas in Figures 8 and 9), it can be seen that most of the points in these scatter plots are concentrated around the 1 : 1 line. Finally, from the regression slope coefficients, it can be seen that the spatial consistency between GLC_FCS30, TreeCover-2010, and GLADForest is more significant than that between other pairs of products (the slopes of TreeCover-2010 and GLADForest are closer to 1.0).
4.4. Spatial Consistency of Global 30 m Cropland Maps
Figure 11 illustrates the spatial distributions of the crop class in the five global 30 m cropland products after aggregation of the data to a resolution of 0.05°. Overall, the greatest spatial consistency is between GlobeLand30-2010, FROM_GC-2010, and GFSAD30-2015. In GLC_FCS30-2015, there is a greater extent and density of cropland in Africa than in the other products. The spatial patterns of cropland on the African continent in the CCI_LC land-cover product and GLC_FCS30-2015 are similar. Furthermore, FROM_GLC-2015 obviously underestimates the amount of cropland compared to other cropland products, especially in North America and Eastern Europe. Comparing FROM_GLC-2015 and FROM_GC-2010, it can be seen that, in FROM_GC, which is derived from the FROM_GLC cropland layer, the extent and density of cropland are greater and the omissions in Europe and North America are greatly decreased.
Figure 12 illustrates the total cropland area in the meridional and zonal directions using steps of 0.05°. The meridional curves indicate that the global distribution of cropland varies a lot with longitude. FROM_GLC-2015 has the lowest cropland area at most longitudes, which also explains the omissions in FROM_GLC-2015 at the global scale. Also, compared with GFSAD30-2015 and GlobeLand30-2010, the amount of cropland in GLC_FCS30-2015 is larger in the 0°–40°E region (which includes Africa) and between 100°E and 130°E; GLC_FCS30-2015 basically agrees well with GFSAD30-2015 at other longitudes. The area classed as cropland in FROM_GC-2010 generally falls between the FROM_GLC-2015 value and the values in the three other products, which also indicates that the FROM_GC is a significant improvement on the FROM_GLC cropland product.
The zonal cropland area curves show that, globally, cropland is mainly distributed in the northern hemisphere. The greatest agreement is between GFSAD30-2015 and GlobeLand30-2010. The GLC_FCS30-2015 values are obviously high in the 25°S–15°N region and are consistent with GFSAD30-2015 and GlobeLand30-2010 elsewhere. The FROM_GLC-2015 and FROM_GC-2015 cropland products clearly have low values in the 30°N–60°N region (which includes the croplands in North America, Europe, and eastern Asia).
Figure 13 displays scatter plots for the five global 30 m cropland products after aggregation of the data to 0.05° and using the GLC_FCS30-2015 cropland layer as a baseline. It can be seen that there is a significant agreement between GLC_FCS30-2015, GlobeLand30-2010, and GFSAD30-2015 because most of the points are concentrated around the 1 : 1 line in these plots. However, the points in the plots showing the relationship between GLC_FCS30-2015, FROM_GLC-2015, and FROM_GC-2010 deviate a lot from the 1 : 1 line and are mainly located below it. In addition, combining the spatial patterns and the area statistics from Figures 11 and 12, it can be concluded that FROM_GC-2010 still suffers from commission and omission errors even if, visually, it appears to capture the overall distribution of cropland at the global scale. According to the accuracy metrics shown in Figure 13, there is a high degree of spatial consistency between GLC_FCS30-2015 and GFSAD30-2015, for which , , and the regression slope is 0.842. This is followed by the GLC_FCS30-2015–GlobeLand30-2015 pair, for which , , and the regression slope is 0.800. It should be noted that some of the points in this scatter plot are located below the 1 : 1 line because the density and extent of cropland in GLC_FCS30-2015 in Africa are higher.
4.5. Spatial Consistency of Global 30 m Water Maps
Figure 14 shows the spatial distribution of the water class in the six global 30 m inland water maps after aggregation of the data to a resolution of 0.05°. Overall, the spatial patterns in the inland water maps are reasonably consistent. For instance, there is a relative abundance of water bodies in the eastern part of North America and the southern regions of Asia, whereas water bodies are relatively sparsely distributed in Africa. However, some discrepancies still exist between these products in specific regions. For example, there are obviously more water bodies in the western part of South America in FROM_GLC than in the other five products.
The spatial consistency between the six global inland water maps can also be seen from the meridional and zonal distributions of this class based on 0.05° intervals that are shown in Figure 15. These six products have similar trends in the latitudinal and longitudinal directions. However, there are still some obvious disagreements in specific regions.
The distribution of inland water between 15°W and 30°W, 60°W and 80°W, and 50°E and 120°E is quite different in the different maps. Between 15°W and 30°W and 60°W and 80°W, the size of the water area in FROM_GLC-2015 is much greater than in the other products. Also, in both FROM_GLC-2015 and GLADWater-2015, between 50°E and 120°E, there is a much greater water area than in the other products. These patterns are also noticeable in Figure 15, where it can be seen that, in FROM_GLC-2015, there is a greater number of water bodies in northwestern Africa (15°W to 30°W), Iceland (15°W to 30°W), western South America (60°W to 80°W), and eastern North America (60°W to 80°W). Also, in GLADWater-2015 and FROM_GLC-2015, there are more water bodies in mainland Asia (50°E to 120°E).
There are also differences in the latitudinal distribution of inland water between the products. In FROM_GLC-2015 and GLADWater-2015, there is a greater area of water than in the other products from 25°N to 35°N and above 70°N. This can also be seen from Figure 15. In mid- and high-latitude regions, FROM_GLC and GLADWater exhibit higher proportions of water than the other four products (Figure 15).
In order to quantitatively analyze the differences between the six global inland water maps, scatter plots of the GLC_FCS30-2015 water map against the other products were made using a grid and the results are shown in Figure 16. The results show that, for all five pairs of products, the value of is greater than 0.82 and all the products have a high degree of consistency with the water map in GLC_FCS30—the values of range from 0.823 (G1WBM-2010) to 0.913 (JRC_GSW), and the RMSD values range from 0.106 (G1WBM-2010) to 0.068 (JRC_GSW). The inconsistency between GLC_FCS30-2015 and GlobeLand30-2010 and G1WBM-2010 may be partly because these three products were produced in different years. Moreover, since the algorithm used to develop G1WBM-2010 is mainly intended to accurately detect permanent water bodies , the uncertainty in the detection of seasonal water bodies may also be a cause of some of the differences between G1WBM-2010 and GLC_FCS30-2015.
Firstly, the synergy between the spectral, spatial, and temporal features derived from multisource satellite datasets can improve the classification accuracy of fine-resolution GLC products. Over recent decades, the volume and number of types of satellite data have grown due to various breakthroughs, providing great opportunities for accurate land-cover mapping. Taking the mapping of impervious surfaces as an example, due to the complicated spectral and spatial structures associated with these surfaces, it is very difficult to produce accurate large-area impervious surface products using only optical imagery. For example, Liu et al.  used Landsat imagery to generate a global 30 m impervious surface product, but this map suffered from serious omission errors, especially in rural areas and small cities [22, 106]. Furthermore, as free access to Sentinel-1 RADARSAT and ENVISAT synthetic-aperture radar (SAR) imagery has become available, the combination of optical imagery and SAR imagery has been demonstrated to significantly improve the accuracy of the mapping of impervious surfaces. This is because optical imagery can capture surface reflectance characteristics, whereas SAR data can be used to provide information on the structure and dielectric properties of surface materials [22, 106, 118]. Recently, the rapid development of nighttime light remote sensing has also helped to promote accurate impervious surface mapping. For example, Goldblatt et al.  combined nighttime light data and Landsat imagery to characterize the built-up land cover in three geographically diverse countries: India, Mexico, and the US. Zhang et al.  combined multisource (optical, SAR, nighttime light, and DEM) datasets and spectral and textural features from time series stacks to generate a global 30 m impervious surface product that had an overall accuracy of 95.1% and a kappa coefficient of 0.898.
Secondly, the local adaptive stratified classification scheme is essential for accurate large-scale land-cover mapping. Due to the spectral heterogeneity of land-cover types, it is very difficult to generate an accurately classified product using a single classification model . The stratified classification scheme is more appropriate for regional or global mapping that has a complicated classification system [19, 44]. For example, to improve the mapping accuracy, Chen et al.  presented a split-and-merge strategy for handling the classification of 10 different land-cover types: i.e., first, each class in a prioritized sequence was identified, and then the ten types were merged together. Zhang et al.  directly used an existing global 30 m impervious surface map as an independent land-cover type in the global land-cover product, which was superimposed over the global land-cover classifications. In addition, as the distribution of the land-cover types varies with the region, the use of a local adaptive classification scheme is more appropriate [19, 44]. For instance, Liu et al.  divided the global land surface into geographical tiles and developed a local random forest model for each tile using the corresponding training samples, then generated a global impervious surface map (MSMT_IS30) by mosaicking 948 geographical tiles. This map, developed using local adaptive classifiers, has a much higher accuracy than the single classifier-based products: MSMT_IS30 has an overall accuracy of 0.95, which compares with 0.89 for FROM_GLC-2015, 0.903 for GHSL-2015, 0.884 for GlobeLand30-2010, and 0.880 for HBASE-2010 . Furthermore, the classification accuracy of fine-resolution GLC maps can also be improved using deep learning algorithms. For example, using a deep convolutional neural network- (CNN-) based classifier, an accurate 30 m large-scale land-cover map was generated, achieving a classification accuracy of 84.4%, compared to 79.9% for RF and 80.2% for SVM 80.2% .
Thirdly, over the last four decades (since the launch of the first Landsat satellite in 1972), land-cover updates based on a time series change detection approach have made it possible to obtain information about land-cover types and changes at a high resolution. Land-cover change (LCC) is, in fact, the key information needed for global change research. Although the existing global land-cover products partially meet users’ requirements in terms of performance, none of these global land-cover products meets the requirements in terms of interannual stability due to the limited classification accuracy of results if the GLC map is generated independently for each year. Thus, these temporally independent land-cover products cannot be used to support LCC analysis [4, 8, 122–124]. To deal with this issue, change-based land-cover mapping has been adopted by the remote sensing community to generate temporally consistent GLC products. Using the traditional approach, first, an annual land-cover dataset is generated, and then some time series postclassification processing steps are implemented to ensure temporal consistency and logical transition, which requires a large number of subjective rules [125–127]. Recently, the continuous change detection approach has been widely applied to the detection of land-cover changes using time series of remote sensing images. Using this approach, changed pixels are first identified using dense time series of images; the changed regions are then updated using temporal segments instead of specific epochs [128–130]. In this way, an opportunity for generating accurate and consistent annual fine-resolution GLC products that can be used for the analysis of global land-cover changes is provided.
Finally, cloud computing platforms greatly facilitate global fine-resolution land-cover mapping. The volume of data required for generating a fine-resolution GLC product is huge, and this task was previously very labor- and computationally intensive . Figure 17 outlines the development of some representative fine-resolution land-cover products over the years, and it can be seen that the early land-cover products (e.g., the National Land-Cover Database (NLCD)  and China’s land-use and land-cover (LUCC) product ) were mainly based on manual, visual interpretation, which is very laborious and time-consuming. Later, the Landsat dataset became freely available to the public and more fine-resolution remote sensing datasets (e.g., Landsat-8 and Sentinel-2) appeared. In order to promote the automation of land-cover mapping, sample-based classification strategies were employed to reduce the amount of labor required compared to the manual editing approach. Some fine-resolution GLC products, such as GlobeLand30  and FROM_GLC , were then developed. However, the amount of labor required for the collection of training samples was still a limitation , and the amount of computation required remained a challenge for scientists until cloud-based computing platforms, such as GEE, became available. GEE has proved its potential in many different applications, especially in land-cover mapping, and the Landsat and Sentinel datasets have been extensively utilized by GEE users . Due to GEE’s powerful computing ability , the production of fine-resolution GLC products can now be completed in a few hours or days . Also, because of the massive data reserves held within GEE, the training datasets can be automatically derived from existing land-cover products and multisource remote sensing datasets without any labor input . Gong et al.  automatically developed the 10 m FROM_GLC10 by transferring an existing 30 m resolution sample set to the 10 m Sentinel-2 imagery using the GEE platform. Zhang et al.  used the freely available multisource data and the programming environment within GEE to construct a spatiotemporal spectral library to automatically generate GLC_FCS30. Therefore, we can expect more and more GLC products with higher resolutions, finer classification systems, more accurate classification, more real-time releases, and more frequent updates. However, perhaps too many GLC products are being generated by the remote sensing community too quickly. Does having so many datasets really benefit or mislead data users? It is a particularly urgent task to assess the quality of the increasing GLC datasets and their suitability for different user applications. As also reported in numerous research studies, the mapping accuracy varied noticeably for different validation datasets. Therefore, rigorous and transparent validation is much more important than ever.
Most human impacts on the land cover can be captured at a resolution of 30 m, and the 30 m GLC products have been widely used in many applications. In this study, the accuracy and consistency of, and the recent progress made by, 30 m GLC products were investigated. Despite the great efforts made toward improving mapping accuracy in recent decades, the current 30 m GLC products still suffer from a relatively low accuracy—their accuracies range from 46.0% to 88.9% for GlobeLand30-2010, 57.71% to 80.36% for FROM_GLC-2015, and 65.59% to 84.33% for GLC_FCS30-2015. The reported accuracies for the global 30 m thematic maps vary from 81.0% (NUACI) to 95.1% (MSMT_IS30) for the impervious surface products, 65.5% (GLADForest) to 97.0% (GlobeLand30) for the forest products, 76.23% (FROM_GLC30) to 91.70% (GFSAD30) for the cropland products, and 81.97% (FROM_GLC) to 98.50% (JRC_GSW) for the inland water products. Given that the target maximum error is 5–15% either per class or for the overall accuracy , most of the current land-cover maps still do not meet the accuracy demands of many applications. In addition, the consistency and comparability of different GLC maps show a good overall agreement in terms of overall spatial patterns but a limited agreement for some vegetation classes (such as shrub, tree, or grassland) in specific areas, mostly transition zones. Unfortunately, land-cover change primarily occurs in these transition zones, which means that it is extremely essential to improve mapping accuracy in these zones. So far, all these temporally independent GLC products cannot be used to support LCC analysis due to the limited classification accuracy. Therefore, more efforts are needed toward improving the classification accuracy and generating temporally consistent GLC products, especially for classes for which the accuracy is currently low (such as shrub, wetland, tundra, and grassland) and for regions with low accuracy (heterogeneous or transition zones, tropical rainforest regions).
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.
This research was funded by the National Natural Science Foundation of China (41825002), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA19090200), and the Key Research Program of the Chinese Academy of Sciences (grant number ZDRW-ZS-2019-1).
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