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I created isochrones using Iso-Area algorithm as polygons in QGIS. Is there any tool in QGIS that helps with calculating areas covered by created polygons?

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    E.g. $area with field calculator?
    – Babel
    Jan 30 at 14:10
  • I have polygons of four time intervals. The attribute table of created polygons has only an id field and a cost level which shows the time intervals. I think it is not possible to calculate are using $area or area functions of field calculator.
    – Elham
    Jan 30 at 14:26
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    Why not? $area calculates the area based on the geometry, not based on attributes.
    – Babel
    Jan 30 at 14:27
  • @Babel Thanks for your help. There are 16 rows in the attribute table with the IDs from 0 to 4. I guess each id is related to one polygon. So to calculate the area of each polygon I need to sum up the areas that are related to the same id, right? one more question, are areas in square centimeters?
    – Elham
    Jan 30 at 14:48
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You can create the area using field calculator: $area or area ($geometry) will calculate the area for each feature. If you want to get the sum for each group with the same id, you can use the aggregate function sum, so your expression will probably look like: sum ($area, group_by:="id").

The unit of measurements depends on the CRS of your layer, have a look in the layer properties. For the difference between $area and area($geometry) see the help in the expression editor or here: https://qgis.org/api/2.14/qgsexpression__texts_8cpp_source.html

$area:

Returns the area of the current feature. The area calculated by this function respects both the current project's ellipsoid setting and area unit settings. For example, if an ellipsoid has been set for the project then the calculated area will be ellipsoidal, and if no ellipsoid is set then the calculated area will be planimetric.

area ($geometry):

Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned area will match the units for the SRS.

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