[Qgis-user] Calculating polygon areas

[Lynkos]

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 <p.schuerch at meierpartner.ch>

> 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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-- 
*Sarah Gregg*
immagini - parole - territorio - natura
Web: www.wild-eyes.eu
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