QGIS – Understanding ‘Current Feature’ in Field Calculator and Expression Builder

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I am curious about field calculator and expression builder language in QGIS.
For example, $area returns the area of the 'current feature'. Does 'current feature' refer to all features of a shapefile that is loaded in the workspace?

Best Answer

The current feature is the one you're doing the calculation for. If you e.g. selected a single feature on the layer and choose to only updated selected features, the area is only calculated for that feature. If you run the calculation for the whole layer, the area is calculated for each feature and attached as an attribute to that feature. Meaning, the value in the column area of feature 5 is the actual area of said feature, and not the area of e.g. feature 23.

Different from that, some expressions, e.g. sum(), calculate something with is not only represented by a single feature.

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$area is the area of the current feature. It's a numeric variable.

area is a function which takes a geometry, and returns its area. This geometry could be any geometry, not just the current feature.

You could say that $area is the same thing as area($geometry)

Some examples - you could use area (the function) to get the area of the bounding box of the feature, for example...

area(bounds($geometry))

.. or the area of the feature after applying buffering

area(buffer($geometry,0.01))

You could replace $geometry in those examples with a different geometry completely, for example you could lift gml or wkt from a field and use that as the geometry.

QGIS – Measure Distance Using Field Calculator

This should work:

length(make_line(make_point(25.906543, -80.546800),$geometry))

you create a line (make_line), starting at the point with your fixed coordinates (created with the command make_point(25.906543, -80.546800)) and ending at the second point, defined with $geometry as the current point of your layer. You then measure the length with the expression length() If you create a new field with the field-calculator, you get a new field containing the distances for each point of your layer to the point at 25.906543, -80.546800

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QGIS – Measure Distance Using Field Calculator

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