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.
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:
- 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.
- 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.
- Use the field calculator to add a new field for area in square meters. Use the expression
$areato calculate area in the units of the layer's projection.
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.