In QGIS 3, the identify feature tool automatically calculates area. However, there are two areas: Cartesian and ellipsoid. The first is being calculated in degrees (not wanted) and the second in hectares (as set in settings).

How can I set the units for the first one (Cartesian) in hectares? I have already set hectares in project settings.

  • Sorry, not sure what you mean. Qgis calculates the ellipsoidal and cartesian areas automatically in degrees and hectares. I want to know how to change the units. In other projects, both are in hectares but I can't figure out why. – user127267 Feb 26 '19 at 20:01
  • Ok. So, Qgis has a tool under the view menu called "Identify Features". When using, you select a feature and you see a bunch of information on the panel. Some of the information shown are: coordinates, closest x, closest y, feature ID, number of vertices, name, etc. Two of the fields are Area (Cartesian) and Area (Ellipsoidal, WGS84). The datum changes according to the one used. The number for the first area is shown in degrees (hectares in other projects) and the number for the second is shown in hectares. My question is how can I define the units for these calculations. – user127267 Feb 26 '19 at 20:10

The Cartesian area is dependant on the layer's CRS. It's a "raw" value, testing the coordinates as 2d numbers only. So for geographic coordinates systems with coordinates as degree values, it's impossible to convert these to a planar meters/hectares/etc value.

  • Thanks. If I convert the WGS84 decimal degrees to WGS84 UTM, the features remain the same and the cartesian area is calculated in hecatares. Importantly, the cartesian area is slightly different from the ellipsoidal, which leads one to understand that there is a cartesian area to be calculated. This seems strange. Why can't QGis do this automatically? – user127267 Feb 26 '19 at 20:17
  • @user127267, QGIS don't know in which projection do you want the cartesian calculation until you define it as the layer CRS. – Gabriel De Luca Feb 26 '19 at 20:51
  • There's no way to convert degrees to meters directly -- this conversion varies depending on the latitude and ellipsoid used. Only approximate conversion factors taken at a single fixed latitude could be used, but they'd be misleading at best – ndawson Feb 26 '19 at 22:28

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