2013/8/23 Lynkos <[email protected]>: > 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 _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
