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I've just succeeded in getting the area of a polygon using field calculator. The polygon was drawn on a Google hybrid aerial photo. There are no units given in the calculation and I assume the area figure is based on latitude / longitude. How do I go about changing the projection in order to get an area figure in hectares?

  • what software are you using? – Nathan W Aug 6 '13 at 9:53
  • QGIS - sorry for omission! – otis_b_flywheel Aug 6 '13 at 9:59
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You have to look up what CRS is used for the polygon layer with Rightclick -> Set CRS for layer.

If it is in EPSG:3857 (the same as for openlayers plugin background), it has no real unit. Google mercator units (although called "metres") fit only at the equator to real metres.

To get a real metres layer, choose the UTM projection for your part of the world, and

  • reproject your layer into it with Rightclick -> Save as ...
  • create a new layer in that CRS and draw the polygon again.

Note that simply changing the CRS with Set CRS for layer does not reproject your data to the desired CRS.

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    why someone need to redraw a polygon when it is possible just to reproject the layer? O_o – SS_Rebelious Aug 6 '13 at 10:42
  • @SS_Rebelious: For a beginner, it might be quicker to redraw than to reproject ;-) – AndreJ Aug 6 '13 at 10:58
  • I sincerely doubt that reprojection is slower for the beginner and that such approach is valid in general. See, this question and answer have a huge chance to pop up on top of google search for a relevant query, so it will be better at least to mention reprojection possibility. – SS_Rebelious Aug 6 '13 at 13:46
  • UTM is not a great choice for area calculations and will be invalid for any regions with considerable longitudinal extent. Any equal-area projection that covers the region of interest would do fine. Note, too, that the area in the end has to be converted from square meters to hectares (divide the value by 100^2=10,000). – whuber Aug 6 '13 at 14:58
  • Writing as a beginner, I did find it easier to re-draw the polygon than to re-project the layer. Agree that UTM did not help for area calculations - I was wanting to work with British Ordnance Survey backdrop and found out that the appropriate CRS for that is OSGB 1936 - British National Grid. – otis_b_flywheel Aug 7 '13 at 12:10

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