How should I populate city/state fields based on the zip?

Question

I'm aware there are databases for zip codes, but how would I grab the city/state fields based on that? Do these databases contain the city/states or do I have to do some sort of lookup to a webservice?

Answer 1

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Important realization: There is not a one-to-one mapping between cities/counties and ZIP codes. A ZIP code is not based on a political area but instead a distribution area as defined for the USPS's internal use. It doesn't make sense to look up a city based on a ZIP code unless you have the +4 or the entire street address to match a record in the USPS address database; otherwise, you won't know if it's RICHMOND or HENRICO, DALLAS or FORT WORTH, there's just not enough information to tell.

This is why, for example, many e-commerce vendors find dealing with New York state sales tax frustrating, since that tax scheme is based on county, e-commerce systems typically don't ask for the county, and ZIP codes (the only information they provide instead) in New York can span county lines.

The USPS updates its address database every month and costs real money, so pretty much any list that you find freely available on the Internet is going to be out of date, especially with the USPS closing post offices to save money.

One ZIP code may span multiple place names, and one city often uses several (but not necessarily whole) ZIP codes. Finally, the city name listed in the ZIP code file may not actually be representative of the place in which the addressee actually lives; instead, it represents the location of their post office. Our office mail is addressed to ASHLAND, but we work about 7 miles from the town's actual political limits. ASHLAND just happens to be where our carrier's route originates from.

For guesstimating someone's location, such as for a search of nearby points of interest, these sources and City/State/ZIP sets are probably fine, they don't need to be exact. But for address validation in a data entry scenario? Absolutely not--validate the whole address or don't bother at all.

Just a friendly reminder to take a step back and remember the data source's intended use!

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