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
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