The greatest source of both nitrogen and phosphorus entering chesapeake bay is

Chesapeake bay is an area that  has suffered from overharvesting and eutrophication. Eutrophication is an enrichment of water by nutrients salts that causes structural changes in the ecosystem such as increased formation of algae and aquatic plants, depletion of fish species among other effects. The greatest source of nitrogen and phosphorus in a Chesapeake bay is agriculture

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