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I have downloaded ESA global land cover tiff file and opened it with QGIS 3.6. To measure Africa separetly I used polygon file of this continent as a mask. I got the result that total area of Africa is 29882300 km2 using r.report plugin. Africa is 30370000 km2 reported in Wikipedia. Why did I get this distinction and how can I reduce it? Difference is 487700 km2. I think this divergence in values is unacceptable.

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    What projection are you using? Any projection is a trade-off of area, shape, and distance, so it's not surprising that there's a difference. Data in the wikipedia article appear to be pulled from various sources; it's possible that some of these are in various projections. – smiller May 14 at 20:19
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    Which projection did you use? – MrXsquared May 14 at 20:20
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    Area (and length) calculations are very dependent on how many points make up a feature. Did you include Madagascar? – mkennedy May 14 at 20:49
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    you won’t believe it, but the difference in sources depends on the original scale in which the sources were created and the more accurate the original scale, the more complex and precise your calculations ... – Cyril May 15 at 6:25
  • I have downloaded tiff file from here: www.esa-landcover-cci.org – Ferenc Mics May 15 at 15:58
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Geographic vs. Projected Coordinate Systems

The CRS (Coordinate Reference System) of the files you are using is EPSG:4326, which is the identifier of the plain WGS84 in EPGS's nomenclature.

Since WGS84 is a geographic coordinate system (aka latitude-longitude), your files are not projected: a geographic coordinate system in fact does not "speak" km2, but only "understands" angles (and local distance from the Datum center, of course).

To calculate areas and distances in meters/km/miles, you first have to project the raster into a projected coordinate system, for example any WGS 84 / UTM xx. Which actually means "Universal Transverse Mercator (projection based on) WGS 84 (Datum)".

In your specific case, since you are measuring areas over the whole African continent, you need a projection which maintains equal areas. I would suggest Africa_Albers_Equal_Area_Conic which in QGIS is named also EPSG:102022.

Other projections might seem nice, because they resemble more closely the idea of Africa we have, but areas measurements would be way off.

To reproject the raster file:

  • Right-click on the raster -> Export -> Save as
  • select a projected CRS (best if you choose the appropriate UTM),
  • save, and you will have the projected raster added to the layer list
  • make sure your project is also in the same projected CRS
  • now you can measure/calculate areas in km2

Further reading:

https://en.wikipedia.org/wiki/Geographic_coordinate_system

https://en.wikipedia.org/wiki/Spatial_reference_system

http://www.fao.org/3/y4816e/y4816e0f.htm

http://www.georeference.org/doc/guide_to_selecting_map_projections.htm

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