Thanks for your contribution. The problem (resolved with a little help!) in fact lay with the CRS of the layer. When corrected, it works fine, Sarah.
2013/8/23 Peter Schürch <[email protected]> > 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 > -- *Sarah Gregg* immagini - parole - territorio - natura Web: www.wild-eyes.eu
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