----------------------------Original message---------------------------- Art L. is correct, there is little correlation between ZIP Codes and population density. You can however, develop reasonable estimates by using polygon overlay with Block Group polygons and BG population with ZIP Code polygons. Since a ZIP+4 commonly represents only one side of one city block, ZIP+4 polygons aren't feasible. ZIP+2 polygons are possible, but they will be oddly shaped and are inappropriate for most applications. Why not simply go with Block Groups and be done with it? GO>----------------------------Original message---------------------------- >>I know that zip code boundary files are not in the public domain, but >>they just keep asking... Has anyone ever seen the area (as in sq. miles) >>of zip codes? Five digit would be fine now, but you know he'll be back for >>+4. This is for a population density question and the user has been advise >>that zips are not an appropriate statistical area... --- * OLX 2.2 TD * ~ GISnet BBS 303-447-0927 Boulder, Colorado