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I would like to autocalculate area (acres) for a shape file that is in UTM zone 16 CRS.

I have tried the "Vector|Geometry Tools|Add/export geometry columns" and also the field calculator. The first gives me a warning saying

currently QGIS doesn't allow simultaneous access from threads to the same data source.
Make sure your layer's tables are closed. Continue?

When I click YES, it creates two new columns AREA & PERIMETER.

  • The AREA column is mostly 0's with a few values such as 2e-06 and so on.
  • The PERIMETER column gives mostly values along the lines of .003526 and so on.

Questions:

  • What is the warning it is giving me and why are the values so sporadic?
  • Also when I use the field calculator tool, it seems to give me values close to what the field area should be but when I click SAVE EDITS all the values turn to 0's. What am I doing wrong?

Thank you in advance for your help.

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It sounds like QGIS didn't pick up the layer's projection. If you right-click the layer > properties > General tab, in the field to the left of "Specify CRS".. is it really showing your expected coordinate system? –  elrobis Apr 19 '13 at 13:40
2  
Make sure you use a CRS in meter + try to add the columns manually using the field calculator and $area as command –  Curlew Apr 19 '13 at 13:42
    
could be wrong as i am unfamiliar with the CRS and most of the above....but using the information tool in the CRS which i work usually bring up a "derived" tab which when expanded (providing it is a polygon) tell you the area in hectares. –  Gerald Apr 19 '13 at 14:24

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