V and H Coordinates

The V and H coordinate system is used in many telephone databases to geographic location.

Jay K. Donald of AT&T created the V&H coordinate system in 1957 to simplify the calculation of distance between telephone rate centers. The system is based on Donald Elliptic Projection. It is a two-point equidistant projection for the continental United States and Canada. It uses units of the square-root of one-tenth of a mile. The system was designed allow calculations in the field using a slide-rule.

To calculate the distances between a point 1 and 2 each with V and H coordinates use this formula:

Distance in miles = square_root( ( (V1-V2)**2 + (H1-H2)**2 ) /10 )

Converting from V and H to or from Latitude and Longitude appears not to be simple.
Here are some links that have source code and commentary on the conversion process

Telcordia has a 190 page document (for sale) on the history and mathematics underlying the V and H system titled: Telcordia® Notes on V&H Coordinates: The Mystery Unveiled
Geo::Coordinates::VandH Perl code is available from CPAN or downloadable from here
PHP functions for VandH Calculate the distance between two VH coordinates, or convert them to lat long.

There are also a number of products you can purchase that have functions to do the conversion.