Flatten curved surface

I want to flatten this sofa back surface to take accurate dimensions of it, how to do this ?

Mennah, can you image trying to bend this in thin cardboard? If it isn’t possible doing that, then it is not that great to flatten out. —-Mark
Maybe you could do it in sections.

As was said that surface by itself can’t actually be accurately “flattened” so it would either need to be reworked into sections that can be or…what dimensions do you actually need, could you pull them off the 3D?