<div dir="ltr"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;color:#000066">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. </div>
<div class="gmail_default" style="font-family:arial,helvetica,sans-serif;color:#000066"><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">2013/8/23 Peter Schürch <span dir="ltr"><<a href="mailto:p.schuerch@meierpartner.ch" target="_blank">p.schuerch@meierpartner.ch</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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.<br>
<br>
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.<br>
<br>
@Goyo: if it was square degrees Sarah is getting, the difference would be a lot larger than just 8%...<br>
<br>
Peter<br>
<br>
-----Ursprüngliche Nachricht-----<br>
Von: <a href="mailto:qgis-user-bounces@lists.osgeo.org">qgis-user-bounces@lists.osgeo.org</a> [mailto:<a href="mailto:qgis-user-bounces@lists.osgeo.org">qgis-user-bounces@lists.osgeo.org</a>] Im Auftrag von Goyo<br>
Gesendet: Freitag, 23. August 2013 14:20<br>
An: Lynkos<br>
Cc: qgis-user<br>
Betreff: Re: [Qgis-user] Calculating polygon areas<br>
<div class="HOEnZb"><div class="h5"><br>
2013/8/23 Lynkos <<a href="mailto:lynkosmail@gmail.com">lynkosmail@gmail.com</a>>:<br>
> Everything else I try and do in QGIS works just fine. But whenever I<br>
> try to calculate polygon areas, I come unstuck :-(.<br>
> I have been given a shape file with a series of polygons representing<br>
> the distribution of a particular plant, with the request to calculate<br>
> the surface area in square metres. This I have done in two ways, using<br>
> the "Add geometric columns" vector tool and with the field calculator.<br>
> Both methods give me identical results, but there are two problems:<br>
> 1) With the shape file, I was also given a list containing some of the<br>
> surface areas calculated previously using a different programme<br>
> (Garmin Mapsource). In these cases, the value I get from QGIS is<br>
> consistently about 8% higher. I tend to favour the precision of the<br>
> QGIS value, but as I will need to explain this difference, I wondered<br>
> if anyone else had experienced this sort of discrepancy.<br>
> 2) Although the values seem to be calculated correctly, I have a<br>
> problem with the way they are expressed. The units for the project are<br>
> set to metres, so I imagine that is what I should be getting in the<br>
> calculated columns. Instead, whatever variations I experiment with, I<br>
> always get the numbers with the format #.####### etc. Below are some<br>
> examples, with the value as it appears after calculation in QGIS on<br>
> the right and as it should be in square metres on the left:<br>
><br>
> 1.14576732 11457 sq m<br>
><br>
> 8.15223017 8152 sq m<br>
><br>
> 1.51271706 151 sq m<br>
><br>
> Any ideas what I'm doing wrong and how to correct it?<br>
> Thanks as ever for your help and patience, Sarah.<br>
<br>
Maybe your shapefile has a geographic projection and you are getting square degrees?<br>
<br>
Goyo<br>
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</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br><div dir="ltr"><font style="color:rgb(0,51,0)"><span style="font-family:'comic sans ms',sans-serif"><font size="4"><b>Sarah Gregg</b></font></span></font><br>
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