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I am trying to calculate the area of a polygon following the step by step mentioned in all the questions related to this issues in this website. Nonetheless, the result of the area calculation is about 6.1 and of course it is wrong cause I was expecting something more than 50000 km². I don't know if I am mistaking in some point or there is a bug in the qgis version I have. I have:

  • changed the layer projection to UTM (In this case is UTM 23);
  • changed the project layer projection to UTM;
  • calculated in attributes/field calculator/geometry/$area.

Still the result is something weird. This is the polygon I am working Três Marias Polygon

underdark
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morebru
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  • I just looked at your shapefile, and I found an area of ~ 51000 units. I didn't reproject the layer, and I set the project SRC to EPSG:4326 (same as the layer). – ArMoraer Jun 19 '16 at 12:48
  • @ArMoraer could you please make a step by step what you have done to find that 51000 unit? I am not an expert on that. – morebru Jun 19 '16 at 13:18

2 Answers2

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DO NOT use Set Layer CRS unless you are told to do so. It will corrupt your data.

Instead, use Save As ... to a new filemane and different CRS for vector data, and Raster -> Projections -> Warp for rasters.

AndreJ
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Seems like you calculated the area after you erroneously defined the layer's crs to be a projected UTM crs from a geographic one.

This is wrong.

Export your polygon layer to a new layer with the required CRS, then change the map canvas CRS to the same one. This way your calculation will be correct

Example: Here is your polygon when defined in WGS 84 enter image description here

This is your polygon when defined in UTM 23S for both layer and canvas enter image description here

While the shape is preserved, the units are in meters (the x/y is the centerpoint). This way, the program thinks that your shape is only a few meters in width and height, and thus you get the ~6 square meter area.

As the shape is actually in degrees, you need to convert it to a CRS with units in meters like UTM 23S.

enter image description here

After the conversion, you can set the map CRS to be the same and perform your $area calculation, with the results being in meters.

HDunn
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  • I understood what you said. But still, I am not expert on GIS, so could you set a step by step, since the opening layer and so on? – morebru Jun 19 '16 at 13:20
  • I tried to reproduce your eemple, bur still something is wrong, because it is returning a value abou 4.3 of area calculation Coulld you make a step-by-step for me to calculate the area? – morebru Jul 31 '16 at 18:25