To work with a coordinate system that does not exist in the Pix4D database, a .prj file needs to be imported into PIX4Dmapper. The .prj can be either downloaded online or manually created.
The following link has .prj files for almost all common coordinate systems: https://jjimenezshaw.github.io/crs-explorer/.
To obtain the .prj file:
1. Open https://jjimenezshaw.github.io/crs-explorer/.
2. On the top right, on the search text box, type the desired system and click Search.
3. Click the desired coordinate system (if it is not listed, navigate via the Next page link).
4. Click the link .PRJ File, the browser starts downloading the file.
Manually create a .prj file:
If a local coordinate system is used:
1. Create with notepad a file using the format:
LOCAL_CS["Non-Earth (Meter)",LOCAL_DATUM["Local Datum",0],UNIT["m",1.0],AXIS["X",NORTH],AXIS["Geocentric Y",EAST],AUTHORITY["EPSG","262148"]]
2. Save it as .prj.