I am analyzing a height map of a painting. The heightmap is 18cm x 18cm, and the depth of the paint is 0.5mm.
Can someone tell me what WKT and Parameter codes I might be able to use in QGIS?
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If you want to take measurements or do anything based on the units, you better not use any CRS or you might end up with misinterpretation of geographic coordinates, an ellipsoid and whatnot.
Just use the "invalid projection" that QGIS will use when you import your image and set your project to "No CRS (or unknown/non-Earth projection)". Then things should be considered Cartesian by QGIS' tools or at least lead to explicit errors instead of wrong results.
If you want to use a projection, use an engineering projection. This is the type that has been used for a coordinate systems on ships, a construction site, or images.
ENGCRS["A construction site CRS", EDATUM["P1",ANCHOR["Peg in south corner"]], CS[Cartesian,2], AXIS["site east",southWest,ORDER], AXIS["site north",southEast,ORDER], LENGTHUNIT["metre",1.0], USAGE[SCOPE["Construction"],TIMEEXTENT["date/time t1","date/time t2"]] ]
continuous (not discrete) CRS for an image.
ENGCRS["An analogue image CRS", EDATUM["Image reference point", ANCHOR["Top left corner of image = 0,0"]], CS[Cartesian,2], AXIS["Column (x)",columnPositive], AXIS["Row (y)",rowPositive], LENGTHUNIT["micrometre",1E-6]]
I pulled this from Open Geospatial. A few searches didn't turn up any other source.
You would want to change several values as below. Check the open geospatial page for what options there are. I am guessing this is close to what you want/need and I could easily be wrong. You will need to test this to be sure it is what you expect and will probably need to adjust to suit your needs.
ENGCRS["An image CRS", DATUM["Image reference point", ANCHOR["Top left corner of image = 0,0"]], CS[Cartesian,3], AXIS["(X)",columnPositive,ORDER], AXIS["(Y)",rowPositive,ORDER], AXIS["(Z)",up,ORDER], LENGTHUNIT["millimetre",0.001], USAGE[SCOPE["Painting Investigation"],] ]