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I'm using QGIS 3.8. I have dowloaded GLOBCOVER 2009 raster land cover map and ecoregions shapefile. Polygon of ecoregions are used for mask to cut raster file. On new layer r.report was performed to measure land cover types.

I have found difference between area of polygon and excised raster layer. For example, Ural montane forest and tundra ecoregion is 175533 km2 and the result of r.report:

enter image description here

Total area is 116576,1 km2. Difference is 58956,88 km2. What causes the difference?

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    In what projection did you do your analysis? – Vince Jul 27 at 19:33
  • I used WGS84 projection. – Ferenc Mics Jul 28 at 16:55
  • Then how did you calculate area? – Vince Jul 28 at 17:10
  • I used r.report for raster layer and field calculator $area for vector layer. – Ferenc Mics Jul 29 at 7:02
  • The difference should be explained by comparing the documentation for the $area function and the r.report algorithm. $area uses "the current project's ellipsoid setting and area unit settings". It's not obvious how r.report does it, but you can probably figure it out if you dig further into the GRASS manual. Here's a link to the r.report documentation: grass.osgeo.org/grass76/manuals/r.report.html – csk Jul 29 at 16:46

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