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# ZIP Code Utility

, 2 Jan 2005 CPOL
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This article provides an easy method to lookup a U.S. City/State by ZIP Code, or one or more ZIP Codes by City/State. It also describes a method to calculate the distance between two ZIP Codes and find all other ZIP Codes within a radius of X miles of a specified ZIP Code.

## Introduction

Intrigued by Ben Fry's zipdecode [^] applet, I decided to write a little ZIP Code utility that allows lookups of U.S. locations by ZIP Code, City/State, or all three. Since the data were already in the database in the form of latitude/longitude pairs, I added the capability to find the distance between two points, and to find all other ZIP Codes within a radius of X miles from the original location.

## Background

### Database

The MS Access database contains the following fields:

Field Name Description
ZIP The ZIP Code
LATITUDE Latitude coordinate (decimal degrees)
LONGITUDE Longitude coordinate (decimal degrees)
CITY City name
STATE State abbreviation
COUNTY County name
ZIP_CLASS ZIP Code class

### ZIP Code — City/State lookups

The lookups are straightforward database queries using the `OleDb*` classes.

### Distance calculation

To calculate the distance between two points, I used the Haversine Formula, which I found on the Ask Dr. Math web site.

### ZIP Codes within a radius of X miles

Most ZIP Codes in the database contain latitude/longitude coordinates. To make the SQL query as simple as possible, I used a square of size 2Rx2R (where R is the radius of the circle) to encompass the search area as shown in the figure below.

This has the unfortunate side effect of searching an area ~22% larger than needed, but these "outliers" are filtered out of the result set on the client side before being returned to the calling application. I could have added a stored procedure to perform the distance calculation, but I didn't want to modify the database in any way. That way, if the author decides to update the data, (hopefully) all the users of this library will have to replace the old database file with the new one.

Now, using this approximation, the SQL query becomes as simple as this:

```SELECT *
FROM ZIP_CODES
WHERE
LATITUDE >= <Southern Latitude Line> AND
LATITUDE <= <Northern Latitude Line> AND
LONGITUDE >= <Western Longitude Line> AND
LONGITUDE <= <Eastern Longitude Line>```

To calculate the Northern/Southern Latitude and Western/Eastern Longitude lines, I again turned to Ask Dr. Math.

### Important classes

Class Name Description
`ZipCodeUtil` The `ZipCodeUtil` class provides methods to lookup City/State by ZIP Code, or ZIP Code by City/State.
`Location` A `Location` represents a City, State, ZIP Code, County, Latitude, Longitude, and ZIP Class. This just so happens to correspond to the columns of the ZIP_CODES table.
`LocationInRadius` Derives from `Location`, and adds the `DistanceToCenter` property.
`Distance` The `Distance` class' `static` `GetDistance` method takes two `Location` objects and uses their Latitudes and Longitudes to determine the distance between them.
`Radius` Provides a `static` method that takes a `Location` and a radius (in miles), and returns the `LocationInRadius`es that fall within that radius.

## Using the code

Using the code is very straightforward.

2. Compile the ZipCodeUtil library in VS.NET.
3. Add a reference to the new DLL (SagaraSoftware.ZipCodeUtil.dll) to your application.
4. Add the following `appSettings` to your application's config file (you'll need to add a config file if one doesn't already exist):
`<add key="ZipCodeProviderType" value="Access" />`
```<add key="ZipCodeConnString" value=
"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=
D:\Example\Path\To\Database\zipbase.mdb" />```
5. Add code to use the ZIP Code Utility library.

Here is the sample code from the example application:

(Note that in order to run the sample application, you'll first need to download the database and modify the config file to point to the database on your hard disk.)

```//    Location by ZIP Code.
Location location = ZipCodeUtil.LookupByZipCode ("93275");
if (null != location)
Console.WriteLine (location.ToString ());

//    Location(s) by City/State.
Location[] locs = ZipCodeUtil.LookupByCityState ("Tulare", "CA");
if (null != locs && locs.Length > 0)
{
foreach (Location loc in locs)
{
Console.WriteLine (loc.ToString ());
}
}

//    Location by City/State/Zip
location = ZipCodeUtil.LookupByCityStateZip ("Tulare", "CA", "93275");
if (null != location)
Console.WriteLine (location.ToString ());

//    Distance between two locations.
Location sf = ZipCodeUtil.LookupByZipCode ("94175");
Location la = ZipCodeUtil.LookupByZipCode ("90185");
Double dDistance = sf.DistanceFrom (la);
Console.WriteLine ("{0} is {1} miles from {2}", sf.City, dDistance, la.City);

//    Other Locations within an X-mile radius of a specific location.
if (null != locs && locs.Length > 0)
{
foreach (Location loc in locs)
{
Console.WriteLine (loc.ToString ());
}
}```

## Limitations

This library relies on data from a free database that doesn't look like it has been updated since September 2001. I cannot vouch for the accuracy of this data. If you plan on using this in a production environment, you may want to invest in a commercial ZIP Codes database that is guaranteed by its maker and that is updated regularly.

## To do List

• Pending approval from the creator of the database, provide MS SQL and MySQL versions.

## History

• 2nd Jan 2005 - Version 1.0.0
• Initial release.

## Share

Software Developer (Senior) Sagara Software, Inc.
United States
Jon graduated from Cal Poly with a B.S. Computer Engineering. He is currently building a Silverlight-based report scheduling interface for a pharmaceutical reporting company.

When he's not fooling around with computers or reading, he's busy spending time with his super wife, Kelly, his two boys, and their rambunctious dog, Homer.

Visit my blog

 FirstPrev Next
 Missing dlls mtone1-Feb-06 12:13 mtone 1-Feb-06 12:13
 Re: Missing dlls Jon Sagara1-Feb-06 12:17 Jon Sagara 1-Feb-06 12:17
 Re: Missing dlls mtone1-Feb-06 12:26 mtone 1-Feb-06 12:26
 Filtering "Outliers" eanderson12-Jan-05 11:04 eanderson 12-Jan-05 11:04
 Trust me. Keep the outlier filter in your client code. I tried doing the necessary trig on MSSQL and it literally took thousands of times longer than we could tolerate. Lesson: MSSQL does not do trig very well. once I moved the filter code to VB.NET it sped up some obscene amount. Then I reworked the algorithm and sped it up again by as little as 5x and as much as 500 times, depending on the set of zips to filter. I don't know how other DB engines handle trig, but don't be surprised if you end up with a dog. In our case we had to filter down thousands of jobs according to their zips, and it had to be a web-usuable response time. eg: less than 5 seconds at worst.
 I knew I shoulda written! eanderson12-Jan-05 9:46 eanderson 12-Jan-05 9:46
 Re: I knew I shoulda written! DavidCrow12-Jan-05 10:55 DavidCrow 12-Jan-05 10:55
 Re: I knew I shoulda written! Jon Sagara12-Jan-05 10:57 Jon Sagara 12-Jan-05 10:57
 Re: I knew I shoulda written! eanderson12-Jan-05 11:00 eanderson 12-Jan-05 11:00
 Re: I knew I shoulda written! ethanselzer10-Feb-05 15:45 ethanselzer 10-Feb-05 15:45
 Re: I knew I shoulda written! renzea1-May-06 12:59 renzea 1-May-06 12:59
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