Why is still water an ideal environment for the formation of mold and cast fossils? Sediment builds up over organisms.
Sediment turns organisms into rock.
Hard tissue decays under sediment.
Soft tissue is preserved under sediment.

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