[Qgis-user] Calculating polygon areas
Peter Schürch
p.schuerch at meierpartner.ch
Fri Aug 23 06:54:09 PDT 2013
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: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] 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 <lynkosmail at gmail.com>:
> 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
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