Abstract
Abstract. Traditionally the accuracy assessment of a hard raster-based land use land cover (LULC) map uses a reference data set that contains one LULC class per pixel, which is the class that has the largest area in each pixel. However, when mixed pixels exist in the reference data, this is a simplification of reality that has implications for both the accuracy assessment and subsequent applications of LULC maps, such as area estimation. This paper demonstrates how the use of class proportions in the reference data set can be used easily within regular accuracy assessment procedures and how the use of class proportions can affect the final accuracy assessment. Using the CORINE land cover map (CLC) and the more detailed Urban Atlas (UA), two accuracy assessments of the raster version of CLC were undertaken using UA as the reference and considering for each pixel: (i) the class proportions retained from the UA; and (ii) the class with the majority area. The results show that for the study area and the classes considered here, all accuracy indices decrease when the class proportions are considered in the reference database, achieving a maximum difference of 16% between the two approaches. This demonstrates that if the UA is considered as representing reality, then the true accuracy of CLC is lower than the value obtained when using the reference data set that assigns only one class to each pixel. Arguments for and against using class proportions in reference data sets are then provided and discussed.
Highlights
1.1 Accuracy assessment of land use land cover (LULC) mapsLand use land cover (LULC) maps are usually produced through the classification of satellite imagery
The aim of this paper is to propose that the accuracy assessment of raster-based LULC maps, in particular those with low and medium spatial resolutions, should be performed with reference databases that include further information about reality, by identifying, for each pixel, the proportion of that pixel that is occupied by each of the classes in the chosen nomenclature
This paper recommends that the labelling protocol used to create reference data sets for assessing the accuracy of crisp LULC maps, in particular when using photo interpretation, should be based on the assignment of one class to each spatial unit but the percentage of the spatial unit occupied by each class
Summary
1.1 Accuracy assessment of LULC mapsLand use land cover (LULC) maps are usually produced through the classification of satellite imagery. For maps with a low or medium spatial resolution, many of the pixels may, in reality, include several classes, i.e., they are mixed pixels In this case, the class that occupies the largest proportion of the pixel is usually assigned to that pixel (Li et al, 2014). The class that occupies the largest proportion of the pixel is usually assigned to that pixel (Li et al, 2014) Because of all these difficulties, the accuracy assessment of the per-pixel based LULC maps is a very important step in the map production, as it will define its usability in different applications, ensuring that the map is of sufficient quality for each application
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