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I am using qgis version 1.8.0. My project contains tiff and several "shape" layers. The CRS is WGS 84 (EPSG:4326). I am attempting to measure the areas of the layer polygons. However if I click on the symbol "Measure area", I am getting a message. It keeps telling me the measurements are problably wrong. (Testing this statement by measuring leads to enormous but wrong values) It suggests to define a coordinate system in project properties. I have been trying this serveral times - with no results. Furthermore I read "Working with Projections" and "Measuring" in the User guide (http://docs.qgis.org/user_guide/html/en/user_manual/introduction/general_tools.html and http://docs.qgis.org/user_guide/html/en/user_manual/working_with_projections/working_with_projections.html). Also after adjusting all other CRS settings the problem stays.

What should I do ?

Thank you for your advice

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1 Answer 1

A little theory: When using a geographic (non projected) coordinate system, such as WGS84, all measurements which are made using planar geometry (calculations on a plane) are wrong. The bigger the area, the bigger your error. To have precise measurements on the ellipsoid, qgis would need to implement great arcs, etc...

QGis is suggesting that you project your data into a projected coordinate system (from an ellipsoid to a projection plane). If the areas you measure are big, lets say, half a europe big, you should probably consider using CRS with epsg code 3857 (Pseudo mercator, based on wgs84).

As for what you should do with your data, you could transform on the fly (im not sure this works for rasters) or you could create datasets that are projected into 3857. For vector: right click on a shapefile -> save as -> select shapefile format and select 'selected CRS' from combo box, than select EPSG:3857. Saving your file will project the data in the new shapefile. For raster: Go to Raster->Projections->Warp and select the raster(s) and respective source and destination crs, in your case 4326 as source and 3857 as destionation.

When you add the new datasets, your measurements should work ok.

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I would suggest to use a real projected CRS like UTM or Gauss-Krüger. EPSG:3857 is not useful for measurement the more to the north you come. Just look at the size of Greenland on Openstreetmap... –  Andre Joost Oct 9 '12 at 4:12
    
Good point. But since he didn't specify which area he's working on, i suggested probably the most general projection. –  U2ros Oct 9 '12 at 6:51
    
Most of my areas are less than 1 km² big. Transforming the layers from WGS 84 to Pseudo mercator, by "Save as" failed. I'm getting another error message. However the layers' (vector and raster) CRS with the properties dialogue leads to a partial success: When the CRS spezified in the "Project Properties" differs from the one of the layers (e.g. layer : Mercator , Project Properties : WGS 84), I can measure distances appearing to be more realistic than those 10⁹ m I measured before. However checking google earth tells me these distances are about 1/4 to small. –  Leah König Oct 9 '12 at 20:17
    
Whats the error message? –  U2ros Oct 9 '12 at 20:30
    
Original german :"Export in Vektordatei schlug fehl. Fehler: Transformation eines Punkts schlug beim Zeichnen eines Objekts vom Typ 'wue_wohngebiet' fehl. Schreiben beendet (Ausnahme Vorwärtstransformation von (~numbers~) PROJ.4: +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs Fehler: latitude or longitude exceeded limits)" Meaning translated: Transformation failed ! Transformation of a point failed after drawing of an object (type 'wue_wohngebiet '[name my layers]) –  Leah König Oct 9 '12 at 21:08

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