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I have been trying to create a grid, or raster, from point data which has a variety of classes (ie, I am trying to grid a categorical attribute).

Strangely (to me) using Interpolate (SAGA, cubic spline) creates a continuous numerical variable when I use my categorical attribute (no idea how it is converting a string to a float).

'Multilevel b-spline interpolation for categories' (SAGA) seems like the function I want, but it produces an error (Unknown option: SHAPES).

What would be a good way to produce what I want?

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Convert your categories to numbers, you can do this with creating new field in attribute table with field calculator. Example expression:

CASE 
WHEN "classes"='forest' THEN 1 
WHEN "classes"='grassland' THEN 5
END

Then just Rasterize your point layer, using values from new field. There are few algorithms for rasterizing in Processing Toolbox.

As result you will get raster with values of your categories (numbers) and rest of the pixels will be nodata.

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