I think Zipdy be producing inaccurate results. Or, why don zipdy calculations match up with US Postal Service calculations?
Zipdy uses GIS points associates with zip codes from data provided by the US Census. The zip code long/lat points provided by the census are at the population center of the zip code rather than the true center of the zip code polygon. In data provided by the US Postal Service, which is used by many other commercial applications, the long/lat zip code points are the geographical center of the zip code rather than the population center. This may result in varying degrees of geographic inaccuracy. However, for the use Zipdy was origionally intended for, the use of population zip code centers will produce more accurate results than the use of geographic zip code centers. I think one of the data files might be corrupt. Please check the MD5 checksum of the file against the MD5 checksums posted on the Zipdy page both before and after decompressing. If the MD5 checksums are the same and you still believe the file to be corrupt, please let me know. Please include a description of the corruption
