I'm trying to compute a CHM (canopy height model) by using the rastercellcalc transformer by having it subtract a DEM from a DSM.

The process works fine when only one DEM and DSM are passed through to the transformer at a time however I have these DEM/DSM's clipped to multiple smaller files. The CHM produced should have the same boundaries as the inputted DEM and DSM.

I've tried using the group by parameter in the transformer however it doesn't work for some reason and still tells me that the raster rows and columns don't match. See images below:

The attribute manager and trimmer transformers just create a shared attribute between the correct DSM and DEM tiffs tiffs so that the rastercell transformer can do group by "Tile_name). See below: