When mixed with water, phospholipids spontaneously form membranes because they

It is because they have hydrophilic phosphate groups that are attracted to water and hydrophobic fatty acid tails that avoid water. 

Hope this helps!

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