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I am using QGIS version 2.12.3.

I have 3 polygon shapefiles: A, B and the intersect of A and B, which we can call C.

All I want is to add a field to the attributes table of each shapefile giving the area (in square m or square km) of each feature in that layer. A, B and C each contain upwards of 10,000 individual features (polygons of protected areas), distributed over every continent.

I have used a global projection (WGS 84/NSIDC EASE Grid-Global EPSG:3975, have also tried 3410), which is listed under 'Projected Coordinate Systems' so I assume it is projected and not geographical, and set this as the coordinate system for the project and each one of the layers.

When I use the field calculation tool and the $area function, the resulting field values are definitely not in square m or square km and bear no resemblance to area as I understand it.

I have run it several times having fiddled with settings that I'm fairly sure I don't understand, and I've ended up with either really tiny values (e.g. 0.0000002345) or random sets of values including negative numbers.

Can anyone identify where I've gone wrong and suggest how I can go about completing this fairly simple task?

ElephantTwelve
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    Can you post the EPSG code of that particular WGS 84 projection? Because what first comes to mind is: you are using degrees and not meters as units. – sig_renato Feb 10 '16 at 21:14
  • Have you checked http://gis.stackexchange.com/questions/30006/how-to-calculate-polygon-areas-and-perimeters-using-qgis?rq=1 – underdark Feb 10 '16 at 21:22
  • The EPSG is 3975, have also tried 3410. Both say 'units=m' in the description. Have set measurement units in project properties to meters as well. If these CRS are in degrees, what global projection can I use that will allow area calc in meters? Thanks – ElephantTwelve Feb 10 '16 at 21:32

2 Answers2

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WGS 84/NSIDC EASE Grid-Global seems to be a stereographic projection, which does not preserve areas. This is the reason why you can't get meaningful results.

The most simple solution is to reproject your layer to a UTM projection suited to your area of interest.

ArMoraer
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  • Hi, many thanks for this, but my area of interest is the whole world. The analysis does not allow breaking down any further. Any advice on a global projection that will allow area calculation? – ElephantTwelve Feb 10 '16 at 21:34
  • Ok, maybe have a look at this post. It is advised to use a cylindrical equal-area projection. – ArMoraer Feb 10 '16 at 22:19
  • Hello, thanks so much for your advice, but I have had a look and the projections I was using ARE equal area cylindrical projections and are listed as such. In theory, then, I should be able to calculate area in sq meters? Where else could I be going wrong? Thanks again. – ElephantTwelve Feb 10 '16 at 22:38
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All used shp-Files have the EPSG 31468.

I run the model, add a field and calculate the area the sum of all records will be 10275 m²enter image description here

When I do the same with field calculator form the gui the sum of all records will be 10311 m². When I use a other gis I get always this result enter image description here

klausk
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