Thanks for the snippet. Turns out I found the same database before googling here and found the same resource for distance calcs. Your citation is meridianworlddata.com/Distance-Calculation.asp.

Not a programmer - a pretty fair engineer-mathematician 30 years ago but my comments for what they are worth:

* I think using zip_gcd may be overkill in zip_radius given that it uses a more approximate formula to filter. You know better than I whether it will make a perceptible difference in performance.
* I believe that the left side of the filter in zip_radius will always return statute miles while a user may be seduced into entering radius in the units specified by type

I've found alot of applications on the web where people are attempting a radius search, so the work you did is pertinent. I haven't found any that require accuracy greater than +-10%, so my approach will be to use the simplest approximation.

I ran an error check on the distance approximations using Census Bureau data from Autauga County, AL to all other counties in the 48 contiguous states + AK. Average error was 3.5% for the simplest, 2% for the more accurate approximation. Results were as bad as 30% off to points close to the north pole, but fine everywhere else.

I did clean up the database and replaced records where the 2000 Census defined a Zip Code Tabulation Area. Still a work in progress and, of course in Excel, but if I figure out how to make it available I will.

Thanks again.

Should leave the programming to programmers, after playing with your snippet, crashing the server, balooning execution time, and finally understanding a little more SQL, I see why the overkill was ok.

I pared down your snippet to just a simple radius search in an 'unworldly' way and included it below. Application is joining with an address table to find out who lives within x miles. Any hint how to do that?

As long as you have some way of getting the latitude and longitude for your zip code, you can easily use these procedures and functions. My code is written to exploit a single table that holds nothing but city, state, and latitude/longitude information for a zip code. You can then in turn get your zip code from your address table and get the zips in range using zip_radius().

If you need further help, my username for gmail is dohpaz.

Gives you the ability to implement the Distance and Radius API atZIP Code Download with PHP to get a list of ZIP Codes in a radius. You would need the API and any ZIP Code database with lat/long coordinates to implement it fully.