Accuracy Assessment and Comparison of National, European and Global Land Use Land Cover Maps at the National Scale—Case Study: Portugal

Abstract

The free availability of Sentinel-1 and 2 imageries enables the production of high resolution (10 m) global Land Use Land Cover (LULC) maps by a wide range of institutions, which often make them publicly available. This raises several issues: Which map should be used for each type of application? How accurate are these maps? What is the level of agreement between them? This motivated us to assess the thematic accuracy of six LULC maps for continental Portugal with 10 m spatial resolution with reference dates between 2017 and 2020, using the same method and the same reference database, in a bid to make the results comparable. The overall accuracy and the per class user’s and producer’s accuracy are compared with the ones reported by the map producers, at the national, European, or global level, according to their availability. The nomenclatures of the several maps were then analyzed and compared to generate a harmonized nomenclature to which all maps were converted into. The harmonized products were compared directly with a visual analysis and the proportion of regions equally classified was computed, as well as the area assigned per product to each class. The accuracy of these harmonized maps was also assessed considering the previously used reference database. The results show that there are significant differences in the overall accuracy of the original products, varying between 42% and 72%. The differences between the user’s and producer’s accuracy per class are very large for all maps. When comparing the obtained results with the ones reported by the map producers for Portugal, Europe or globally (depending on what is available) the results obtained in this study have lower accuracy metrics values for all maps. The comparison of the harmonized maps shows that they agree in 83% of the study area, but there are differences in terms of detail and area of the classes, mainly for the class “Built up” and “Bare land”.