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# Calculating Distance Between Zip Codes

## Problem

I've seen several web sites where one can enter a zip code and a mileage amount, and then a list of stores, doctors, and other information within the defined region is returned. How is this done?

## Solution

The trick is to use longitude and latitude settings. Each zip code is centered around a particular longitude and latitude intersection, and you'll need a table containing these values. Lists are available on the Web. Some cost a bit, but others are free—run a search with "zip," "longitude," and "latitude" as keywords, and you're bound to find many choices.

Figure 9-27 shows a section of a table that contains the necessary information.

Now, let's consider the process. When a user enters a zip code on the web page, it has to be found in the zip code table to access its longitude and latitude values. A second scan of the zip table is then run to identify zip codes near the entered zip code. How near is "near" depends on the selected mileage setting. There is a formula to calculate what other zips are within the mileage parameter.

Longitude and latitude are measured in degrees, minutes, and seconds. Roughly, a degree is 69 miles, a minute is one-sixtieth of a degree (or 1.15 miles), and a second is about 100 feet.

These are approximations—nautical miles are measured differently than land miles, and there are other ...

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