I seem to remember that you can choose to calculate projected area (or distance) or on the chosen geoid (spherical). I doubt this should make that much of a difference in your case, but maybe still worth checking.
Maybe you should make a test with some geometries of known area to see whether QGIS is actually wrong. E.g. squares of 1, 10 and 100 m width. @Goyo: if it was square degrees Sarah is getting, the difference would be a lot larger than just 8%... Peter -----Ursprüngliche Nachricht----- Von: [email protected] [mailto:[email protected]] Im Auftrag von Goyo Gesendet: Freitag, 23. August 2013 14:20 An: Lynkos Cc: qgis-user Betreff: Re: [Qgis-user] Calculating polygon areas 2013/8/23 Lynkos <[email protected]>: > Everything else I try and do in QGIS works just fine. But whenever I > try to calculate polygon areas, I come unstuck :-(. > I have been given a shape file with a series of polygons representing > the distribution of a particular plant, with the request to calculate > the surface area in square metres. This I have done in two ways, using > the "Add geometric columns" vector tool and with the field calculator. > Both methods give me identical results, but there are two problems: > 1) With the shape file, I was also given a list containing some of the > surface areas calculated previously using a different programme > (Garmin Mapsource). In these cases, the value I get from QGIS is > consistently about 8% higher. I tend to favour the precision of the > QGIS value, but as I will need to explain this difference, I wondered > if anyone else had experienced this sort of discrepancy. > 2) Although the values seem to be calculated correctly, I have a > problem with the way they are expressed. The units for the project are > set to metres, so I imagine that is what I should be getting in the > calculated columns. Instead, whatever variations I experiment with, I > always get the numbers with the format #.####### etc. Below are some > examples, with the value as it appears after calculation in QGIS on > the right and as it should be in square metres on the left: > > 1.14576732 11457 sq m > > 8.15223017 8152 sq m > > 1.51271706 151 sq m > > Any ideas what I'm doing wrong and how to correct it? > Thanks as ever for your help and patience, Sarah. Maybe your shapefile has a geographic projection and you are getting square degrees? Goyo _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
