Area calculation of classified raster layer

Question

I am working with QGIS 3.22.4 and want to analyse the output of MaxEnt. Therefore, I have a raster layer showing the habitat suitability for my species in the whole oceans as .asc-data type. I have already classified the raster using the raster calculator: ("a@1" < 0.25)*1 + (( "a@1" >= 0.25) AND ("a@1" < 0.5))*2 + (("a@1" >=0.5) AND ("a@1" <0.75))*3 + ("a@1" >=0.75)*4

This generated a raster layer where every pixel has a value of 1, 2, 3 or 4. So far so good.

Now I want to calculate the area of every class in a metric unit. But either I try calculating the area with CRS: 4326 - WGS 84 and the area is not calculated in a unit I can work with (it is a superscript 02) or I try calculating the area with different projected CRS (e.g. 54004 - World Mercator; 3857 - WGS 84 Pseudo Mercator) and get orders of magnitude that can not be correct. Like for the whole ocean I get a total of somewhat around 42.500m²:

Answer 1

You have to use a CRS that

• 1 is a projected (ot geographic) CRS, thus units not in degrees, but in meters (or any other length unit) and

• 2 does not distaort area. So you need an equal area CRS. Mercator projection disorts area heavily, so never use it for area calculations. For local areas, you could use one of the UTM zones, but this does not work for a worldwide dataset.

What to do? Use an equal area projection like e.g. World Cylindrical Equal Area (ESRI:54034). Reproject your layer to this CRS, than calculate area.

Another option is to calculate ellipsoidal areas: in QGIS field calculator, use the function $area(). Like this, you can get "correct" measurements regardless of the layer's CRS (but depending on the ellipsoid set for the CRS).

For background, see https://gis.stackexchange.com/a/408399/88814 and https://gis.stackexchange.com/a/397278/88814