[Qgis-user] Calculating polygon area

[Alexandre Neto]

Hello Sarah,

I believe its a Coordinate System Problem. You data is probably in a
Geographic CRS (Lat Long). That means that using field calculator
the measuring are being made in Degree instead of meters.

Best way to do what you need is to reproject your data in some Projected
CRS (choose a good one for your specific area, generally the official
country CRS, or continent CRS). And then make the measurements in Field
calculator.

If you have just a few polygons, you simply use the same projected CRS in
your QGIS Project, set On the Fly Reprojection, and using the
identification tool on each polygon, see the geometries measurements in
derived (quite boring...).

Hope it helps.

Alexandre Neto



On Thu, May 2, 2013 at 1:16 PM, Lynkos <lynkosmail at gmail.com> wrote:

> Dear all, here I am back with one of my usual "newbie" questions which I'm
> sure has such a simple answer that I already feel embarassed in advance!
> Here goes anyway...
> I'm trying to calculate the area and perimeter length of a polygon
> shapefile. Using the QGIS measuring tool and clicking along the outline of
> the polygon, I get an answer of approximately 663 km area with a perimeter
> of about 110 km... and those values seem about right.
> So what's the problem? The problem is that if I do the same using the
> Field Calculator, or the "Extract Add Geometric Columns" vector tool (not
> sure if the name is right, I'm using the Italian version), I get completely
> different answers: an area of 0.072189 and a perimeter of 1.13297 (not even
> sure in what units). The layer units are set to metres and reprojection is
> disabled. Help!! What am I doing wrong!?
> I could of course simply continue using the measuring tool, but I have a
> lot of polygons to calculate and some are very complex.
> Thanks for your usual patience, Sarah.
>
>
>
> --
> Con altri occhi - parole e immagini dalla parte della natura
> www.lynkos.net
>
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