3

I would like to autocalculate area (acres) for a shapefile 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?
nmtoken
  • 13,355
  • 5
  • 38
  • 87
Dave
  • 41
  • 2
  • 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. – Ger Apr 19 '13 at 14:24
  • The comments to this question http://gis.stackexchange.com/questions/66113/how-to-measure-area-of-polygons-generated-from-gps-data-in-qgis may lead you to the solution. – AndreJ Jul 17 '15 at 08:21

0 Answers0