How do we calculate magnification of a drawing

The actual size of something times the magnification is equal to the size of the appearance of the object in the photo.

Are you asking about a street map usage  if so here is your answer  a street map for a very limited area could simply be a bit map or a .jpg image.  but a more general map would probably store the streets as vectors.  but you shouldn't assume that there is one way to do it