Calculating area of clipped shapefile in QGIS

Question

I am relatively new to GIS in general, and to QGIS in particular. I have what I assume to be a pretty elementary question so I will give a simple bullet point work flow that will help demonstrate my question.

• I create a polygon shapefile (A) and add an $area attribute (_Area), and set it to use the automatic default value if changed.
• I create (or import) another polygon shapefile (B) as an overlay to be used to clip the previously created shapefile A.
• I clip shape A with shape B.
• The new "temporary" layer has the same value for _Area as the original shapefile A and not the actual $area of the new clipped poygon.
• In order to get the actual value of _Area for the clipped shapefile, I have to edit the field _Area and add $area, then update.

This seems like a lot of work to get the intersecting areas between to shapefiles and prone to errors if there are a lot of clipped areas to measure. Am I missing something?

Is there a way to make a shapefile created from clipping another shapefile use the default value $area of the original shapefile?

Answer 1

You can:

• in the polygon shapefile (A), create a virtual field with the $area expression
• use the "Processing in-place layer modifier" functionality and the "Vector overlay - Clip" processing algorithm to clip the the polygon shapefile (A) by the polygon shapefile (B)

The virtual field will contain the actual ellipsoidal area of the polygon geometries contained in the polygon shapefile (A).