[Qgis-user] Calculating polygon areas

[Lynkos]

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.





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