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(I'm working in QuantamGIS)

I intersected two layers creating a new layer. Within the attibute table of this new intersect layer i would like to calculate the new areas.

However, it just doesn't calculate the correct area size. The layers are all in the same CRS and the canvas units are in meters. However I am unable to recalculate the correct area.

To check if the problem was created by me I created a polygonal area, calculated the size and came out to about the right size, however the same polygon area within the intersect layer just gives really inconsistent small numbers.

Is there any good tutorial or procedure that describes the intersect action, thus creating new polygons, with the right CRS and then recalculating the areas?

underdark
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Jan-Willem De Jong
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    If you calculate the areas of the original polygons, do you get expected values? Is it possible to get a sample of your layers? And one more (annoying), questions: are you sure all layers are in the same meter-based CRS? – Micha Apr 25 '15 at 12:12
  • Yes if i calculate the original file i get the expected values because the area sizes are listed in one of the columns (data from government). Here's a link to the 3 shapefiles: https://jumpshare.com/b/kJa890HSHBbGK1V0Z3BZ I want to recalculate the size of the polygons in the new 'intersect' shp – Jan-Willem De Jong Apr 25 '15 at 12:46

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Sorry, but the layers are not in the same CRS. The Catchments is in an Albers Equal area projection (units-meters) and the other two are in Haartebeesthoek94, with degree as units. So your area calculations are in square degrees.

If you reproject the transmissivity and intersect polygons to Albers Eq Area (with the "Save As..." option in QGIS) then recalculate the areas, it should then work out.

Micha
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  • the files i shared aren't projected correctly, i know that. they're the basic files which im working with. – Jan-Willem De Jong Apr 25 '15 at 15:45
  • Reprojecting them and saving them doesn't work. Use the vector tool 'Define current project' doesn't work either. – Jan-Willem De Jong Apr 26 '15 at 08:40
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    What do you by "Reprojecting doesn't work". What do you get if you do "Save As..." and choose the Albers Equal Area projection of the Catchments layer? You should NOT use the "Define current project". That often confuses folks. – Micha Apr 26 '15 at 16:10
  • 'Save as' of the correct projection doesn't work. Even restarting qgis doesn't help. I've been reprojecting all the shapefiles in ArcGIS which works fine. – Jan-Willem De Jong Apr 27 '15 at 06:47
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    And when you reproject with ArcGIS what do you get? Are the new shapefiles then in the Albers Equal Area projection? I you have the original data, and you can share it, we could have a look... – Micha Apr 28 '15 at 12:48