How to measure areas of features in OpenLayers plugin background maps correctly?

Question

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 columns = 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 every time?

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.