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My CRS is WGS 84. I intersected two files containing polygons. Each file has an attribute table stating the area of each of the polygons (in m2). After intersecting them I navigate to: Vector >> Geometry Tools >> Add Geometry Attributes. The area of the new polygons are calculated but they are orders of magnitude different to the original polygons. For instance, when comparing a new (smaller) polygon to the original (larger) polygon (which are only slightly different), the new area is calculated as 28.08 and the original is 243,943,129,649 m2

There is no way that these can be the same units. How can i find out the units of the new polygons?

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Geometry calculations and operations (area, length, buffer) are done based on the CRS of the layer. So, if your layer is in WGS84, your area is measured in square-degree. Not really useful.

Either save your layer using any projected CRS, or you run something like area(transform($geometry,'current EPSG','projected EPSG')).

  • Thanks. That's very useful. I used ArcGIS about 5 years ago and seem to remember that you could set a global coordinate system and a projected coordinate system. In QGIS it seems you can only choose one. So if I select 'World Mollweide' in the 'Coordinate reference systems of the world' menu area, should this be sufficient to calculate an area in the appropriate units? Thanks – ChrisB Apr 30 at 15:37
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    Setting a CRS is not transforming the layer. So not, that is not sufficient. Right click your layer and save it using a projected CRS. – Erik Apr 30 at 15:40
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You are using latitude and longitude as coordinates which is not a projected coordinates system, and thus the units are degrees. Measuring areas or distances using these units will result in completely false values, since you expect them to be in meters.

You should reproject your polygons in a different CRS that uses meters as units, and then measure the areas.

If you are still willing to use WGS84 as CRS you can refer to this answer on GIS SE How can I measure area from geographic coordinates?

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