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I have now used my entire evening, but I can't figure out how to make areal calculations (eg. in km2). I have tried the 'field calculator', I have tried the 'measure area' function on the "attributes" toolbar, and i have tried fTools in the "Vector"-menu.

How I do:
1. Open Qgis
2. Add Google Hybrid Layer (OpenLayers Plugin)
3. Find tenniscourt (should be 200-260m2)
4. Measure
- attribute toolbar->measure area - result: 65.060 m² hmm?
- layer->new shapefile layer->polygon, save, "add feature" and drawing polygon. Then field calculator $area - result: 118.48828125
- repeat with new shapefile - result: 4.05503328693158e-08
- vector->geometric tools->export/add geometric colums = another random result

I can't figure out what I am doing wrong! For me it seems like there is some error in the program? I have tried saving the shapefile and opening it again in a "blank" project - same result - and new result everytime?

I do not understand how this can be so difficult, for me is seems like a pretty basic function, but I am completely lost here.

looking forward to hear from you!

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1  
Related: gis.stackexchange.com/questions/20054/… –  underdark Oct 18 '12 at 19:44
    
when using either of the methods, area is (furthermore) relative to zoom-level!??!?!?? :( –  Lasse Jesper Pedersen Oct 18 '12 at 19:54
5  
Cannot duplicate - I measured the area of a tennis court to be 260m^2. What projection are you using? –  L_Holcombe Oct 18 '12 at 21:25

1 Answer 1

For the Project coordinate reference system, you have to choose a projected CRS, which uses real metres as units. Openlayers Plugin uses Google mercator, which is only corresponding to real meters at the aequator. The further north you come, the more distorted the length units are (look at Greenland in Openstreetmap, it is not so big in reality).

By creating a blank project, you have most probably WGS84 as CRS, wich will give you distances (and areas) in degrees, not meters. Any UTM CRS will do, as long as you are within the 3-degree zone of the CRS. Many countries have their own projected CRS to enable most precise surveying.

Qgis seems to measure distances in most projected CRS correct, but this might not be the case with area measuring.

EDIT: If you check "Ellipsoid" in the measurement toolbox, the area is always computed in metres. At least for Qgis Lisboa.

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But can I then still use the OpenLayers plugin - I mean; if I change the CRS? –  Lasse Jesper Pedersen Oct 19 '12 at 15:02
    
No, the Openlayers plugin forces the project CRS to 3857. This is because Google maps or Openstreetmap comes in tiles, and reprojected tiles look ugly. Unless the program puzzles them together before reprojecting, but this needs too much time. You can draw a line around your objects of interest in a seperate layer, delete the OL layer, and change the project CRS. –  Andre Joost Oct 20 '12 at 17:43

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