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I have a Shapefile that has properties/parcels as polygons. I am looking to select by parcels that are bigger than a certain size in acres. The problem is that the Shapefile has the properties/parcel sizes in BOTH square meters and acres. Some are in square meters and others are in acres, and there is a column that indicates whether the value is in acres (A) or if it is in square meters (S). Is there a formula that I can enter to create a new column that converts all of the numbers that are in square meters into acres?

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As the suggestion says, use one of the geometry functions in QGIS to get the area in consistent units, probably in a new column. The units will be dependent on the projection - i.e. the area will be the squared units of the projection. I think if it's in lat lon it will be approximated to metre units.

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If your attribute table is like:

  • Unit indicator field (indicator): 'A' and 'S'
  • Parcel size field (size): number

Then the expression for the new field would be:

CASE WHEN "indicator" = 'S' THEN 0.000247105 * "size" 
     WHEN "indicator" = 'A' THEN "size"
ELSE NULL
END

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