Issue with Field Calculator in ModelBuilder (continued)

[geguill]

Hello,
I would like to revisit a point from this discussion: https://github.com/opengeos/GeoLibre/discussions/2334
In a processing model, how do I add a field that performs the following calculation: my_field_A * area calculation ($area?)
I tried using the Field Calculator, but I get an error message: expression evaluation failed: execution error: expression evaluation failed: Function identifier is not bound to anything by context: "$area".

Thanks,
Géguil

[giswqs]

Thanks for following up, and sorry the model part of #2334 went unanswered.

Area in a model. Inside a model, the Field Calculator is Whitebox's Field Calculator, not the one in the attribute table, and it uses a different syntax: $area is a variable, not a function, so it takes no parentheses. $area("hectares") and $area() both produce exactly the error you're seeing. This works:

my_field_A * $area

However, $area is measured in the layer's coordinate units, and GeoLibre layers are in longitude/latitude (EPSG:4326), so it returns square degrees, which isn't useful. The reliable approach is to add one step before the Field Calculator:

1. Add Add Geometry Attributes to the model and connect your polygon layer to it. It adds an AREA_M2 field with each polygon's area in square metres, computed correctly from longitude/latitude.
2. Connect its output to the Field Calculator and use:

   my_field_A * AREA_M2 / 10000
   

   (drop the / 10000 if you want square metres rather than hectares).

I checked this in a model on test parcels near Paris: AREA_M2 came out at the expected area, and calc_ha matched my_field_A times the area in hectares. Avoid Polygon Area for this: on longitude/latitude data it also returns square degrees.

Join fields in a model. You were right, and it was a bug on our side (#2710): field pickers in the model editor were never filled with the input layer's fields, so a join could not be configured there. It's fixed in #2711. A field parameter in a model now lists the columns of the layer connected to that input, and it keeps a text box too, so you can also type a column that an earlier step in the model creates (such as AREA_M2 above). The fix will be on https://web.geolibre.app shortly and in the next desktop release.