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I started a QGis project with adding information layers from OpenStreetMap such as the streets and railways for a city. This data has WGS 84 CRS. After that, I made a shapefile with urbanized bodies of land using the OpenLayers plugin.

Problem: I want to calculate the areas for the shapes I made. When I use Open Field Calculator or Export/add Geometry columns the values I get are like this "0.0000099" which doesn't seem right.

Any idea how I can solve this?

My version (which didn't work): I added some data with Stereo 70 CRS which did overlap due to Enabled on the fly CRS. However when I tried to convert my WGS 84 data to stereo 70, my shapes got moved away and the calculated areas are still with "0.0000099".

So is there any workaround for such a problem or do I have to import my areas into AutoCad, scale them can calculate manually?

John Powell
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Stadler Reini
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Few things:

  • Starting with the precision of measuring an computing areas and distances based on OSM basemaps. OSM tiles are using a geographic coordinate system which is pseudo-wgs84. That is mainly aimed to speed-up display on computer resolution, while distorting the shape and size of map element, i.e features. Hence using those tiles-base maps to digitize features for the sake of areas calculation is not the best practice. However if your data is not a base map, and is indeed uses WGS-84, it should be fine. See this thread
  • Getting the calculation in projected units, i.e. squared meters or squared miles, needs conversion, as you did, or a mathematical conversion of the results which might be a bit complicated. Sticking to your work flow, I might suggest that after conversion you re-calculate the ares in your new, projected layer. That is since the attribute table is not dynamic. It meas that after conversion of the data, the geometry of the data was reprojected, hence the shift and distortion in your data. Yet other attributes are not automatically being altered.

To sum-up: If you are indeed using OSM base map (tiles) as a source for digitizing, I encourge you to find other data sources;

More specific avout area calculation: try re-computing the area after conversion.

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dof1985
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  • Thank you for the quick reply. Indeed i did save the shapefile for stereo 70 imported it into a new Qgis project and it gave much better results. However the measured areas are still 10% higher than they should be. I'm guessing that this error is due to the Openlayers plugin which i used for support to draw the shapes (CRS - WG 84). Great thanks for the help :) If you however have any solution for that 10% error after transforming from WGS84 to Stereo 70 that would be outrageous. – Stadler Reini Apr 12 '15 at 07:52