I have few problems understanding how units are managed in QGIS. According to the area column I created for a shapefile, the area of a shape is 0.7727... I just checked on Wikipedia and indeed the area of the region is 7,727 km2. So in which unit is the area column expressed? It was my understanding that because my projection was WSG84 all the units are in meters, so why my column doesn't say 7.727×10^9?

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    WSG84 is degree based. – Nathan W Feb 11 '13 at 5:25
  • It might co-ordinate system problem..try UTM projection and choose the zone according to your area wise (like 32,33 etc.) – Sunil Feb 11 '13 at 5:28
  • (Re-project your layer and then try) useful for you projection transformation in QGIS gis.stackexchange.com/questions/10087/… – Sunil Feb 11 '13 at 5:31
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    Your polygon wouldn't be located near 37 degrees latitude, would it? Around that latitude, one square degree equals 10,000 square kilometers, so 0.7727 square degrees would indeed be 7,727 square kilometers. (This is true only for small rectangular polygons, unfortunately: you cannot reliably convert square decimal degrees to areas, because the conversion depends on the polygon's shape as well as its location.) – whuber Feb 11 '13 at 17:56

If your layer's CRS is WGS84 (EPSG:4326), the layer units are in degrees. You can check this by examining the CRS definition string in the layer selection dialog.

To calculate area in meters, you have to reproject the layer to some suitable CRS for your region of interest. One default option is to go for a UTM zone that covers your area.

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    Thanks for your answers. That was clarifying. But then I don't understand the option in the 'Project Properties'-> 'Layer Units'. If I select meters there, where I have a meters output? Only in the scalebar? – Francesco Feb 11 '13 at 21:49

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