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?