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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