A ZIP code (Zone Improvement Plan) is a five or nine digit number used by the US Postal Service to improve mail distribution. In MapViewer, five-digit ZIP code base maps use text five-digit ZIP codes. If your data file contains numeric five-digit ZIP codes, add an apostrophe in front of the number to make it text before making a thematic map (hatch, pie, bar, density, symbol, and prism). (i.e. '00401)