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I would like to use QGIS for the documentation of generic flat surfaces (so I don't need coordinates relative to a sphere but to a cartesian flat plane) but just overlays some images of surfaces with known dimensions and adding information to them. I would like to open a raster file and set the origin to 0 and the corresponding points to a chosen reference systems so that the units of measurement are right and I can perform some estimation of area and length in the new reference system.

I'm trying to set a user-defined projection system using proj4. I'm looking for a suitable CRS for a 2D surface.

At the moment I am opening the raster using the georeferenced and adding some controls to place it on the qgis project. Is there any better approach for using QGIS for documenting generic flat surfaces?

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    Any of the Projected Coordinate Systems or "None" should be adequate for a 2D flat surface. The particular CRS you use matters when you convert between reference systems. If you are working in meters or feet, you might one to choose one with a matching unit. If you are working in mm or some other unit, you could define a +proj=tmerc +units=mm etc.. You might look into the gvellut.github.io/FreehandRasterGeoreferencer Or you might write or add a world file (gdal.org/drivers/raster/wld.html) to specify the pixel->map units before adding the raster.
    – Dave X
    Commented Nov 10, 2020 at 19:40

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