[Qgis-user] Calculating polygon areas

[Goyo]

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