Use input parameter as variable in expression

Question

In my project I have a vector layer and a raster layer. I want to remove all features of the vector layer where the feature's location has a height value of 0 in the same location in the raster layer.

This works fine by using "Select by expression" and using the below expression:

raster_value('layerid' ,1,make_point(x($geometry),y($geometry))) = 0

and then replacing 'layerid' with the ID of the raster layer.

I want to automate a lengthy workflow with the graphical modeler, where this is one of the tasks.

It currently looks like this:

I'm trying to use "Extract by expression", with the same expression as above, and it works as long as the layerid parameter is changed prior to running the model, however I want to use the raster layer input heightmap to get the ID. Like so:

This isn't working. The expression returns null every time. I've tried using eval(@heightmap), as well, but no luck.

As an attempt of a workaround I have tried to use "Load layer into project" algorithm, and then using it's output variable in the expression, but this did the same.

How can I put the layerid of the heightmap input into the expression properly?

Answer 1

I have not found a solution to the issue of referring to a model input in expression, however I did find a solution which solved what I was trying to do.

The algorithm "Zonal Statistics" can add attributes to each feature of the vector layer which can contain the mean value of the raster layer for the polygon area.

This allowed me to use that field as the comparison value in the "Extract by expression" algorithm

To ensure that "Zonal statistics" runs before "Extract by expression", "Zonal Statistics" can be set as a dependency.

Note that you can also use "Extract by attribute" instead of "expression".

Answer 2

(In 3.28.3) in Expression Dialog inside Model Designer there are special variables which represent layer id's for input map layers.

This works at least for calculating feature count using layer property function like this, where inp is the Description for input Parameter:

layer_property( @inp,'feature_count')

Sample project:

Model setup:

Result of executing the model: