[Qgis-user] Calculating polygon areas

[Peter Schürch]

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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