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I have a world map divided into zones and need to compute the area for each polygon. I tried using Vector > Geometry Tools > Export/Add geometry columns and successfully get the area and perimeter but I don't know the units of this measurement.

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I assumed this was in square meters because the Layer CRS is WGS84 and I have 'Units for area measurement' in Project > Project Properties set to square meters but it seemed off by couple orders of magnitude. If I assumed these measurements were in square kilometers the sum of all the areas is still off by about 20% from the earths surface area.

marked as duplicate by mgri, Oto Kaláb, whyzar, Kersten, AndreJ qgis Jul 26 '17 at 18:54

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The output area and perimeter are in CRS unit measurement. If you are working in WGS84 Lat/Long the output will be in degrees. If you are working in a projected CRS, the output could be in meters or feet (as NAD83 state plane). ArcMAP let you decide the area/perimeter unit, QGIS always work in CRS unit. You can use field calculator to add area into a different unit, as $area / 10000 to add a field with the area in hectares.

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