[Qgis-user] geometry attribute units

[Andre Joost]

Am 22.08.2014 23:27, schrieb john polo:
> Hi,
> I have a polygon that I am trying to measure the perimeter for. I used
> the Export/Add geometry columns function and a column was thus added.
> The value in the column has a value that I don't understand, mostly
> because I don't know what unit it used for the calculation. The
> perimeter would probably be between .7 and 1 mile or the equivalent in
> metric. The value given in the column is 0.0133.
>
> How do I tell what the unit is supposed to be for these calculations?
> (And how would I set/change it?) I've looked in Settings>Options on the
> main QGIS window and the layer and haven't found anything. I did find
> where one sets the units for the measure tool and that is set to meters,
> but I am assuming the units selected for the measure tool aren't
> necessarily the same as for the calculation for the geometry.
>

The perimeter is calculated in units of the layer. If that has degrees 
(like EPSG:4326), the perimeter result is surely nonsense.
You have to reproject your datasource to a projected CRS that uses 
metres, feet or miles to get a useful result.

Note that the measurement tool behaves different in this point.

HTH,
André Joost