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I imported an ESRI personal geodatabase vector file (.mdb file) into QGIS, when I open the attribute table, it seems the the shape_area unit is in square degree. I changed the project properties to a projection rather than WGS84. But the units remain in square degree. Is there a method that can convert all the data shape_area unit to square meters, then I can save it easily as csv file with square meters instead of square degree? And should one be clever to choose the correct projection to a country to get the true areas or is there a universal projection? The file contains administrative divisions.

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Attribute table fields are static. This means that your shape_area field won't automatically update when you change the projection, or if you edit a polygon and its actual area changes.

Here's how to add a new area field in square meters:

  1. Choose an equal-area projection that uses meters as units. Selecting the best projection depends on the size of the area of interest. ESRI provides some helpful information on this topic.
  2. Save your vector layer as a new file with the projection you chose in step 1. QGIS can't create personal geodatabases, so you'll have to choose a different file type. Shapefile is a fairly safe choice if you're not sure.
  3. Use the field calculator to add a new field for area in square meters. Use the expression $area to calculate area in the units of the layer's projection.

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If you want a live-updating field, check the box next to "virtual field." A virtual field is only saved in the current QGIS project, so if you load the layer into a new project the virtual field won't be present. However, a regular field is good for most purposes.

NOTE: Different versions of QGIS handle unit selection for the $area calculation differently. As you can see in the screenshot here, in QGIS 2.18 the $area function uses the units set in project properties for the current project. Older versions of QGIS use the units of the layer's projection. It's a good idea to verify the calculated area of at least one polygon by comparing it to the known area. For example, if you have a country polygon, look up the area of that country on wikipedia.

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  • Updating the existing field containing the degree area would work too. – AndreJ Mar 24 '18 at 15:19

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