Why Search By Radius?
The design of the search is based on the premise that most people that are searching for something don’t care what state (where) your store is located. They only care that it is 3 miles away not 30 miles away.
I understand this is not a perfect solution for everyone. Sometimes you need to refer someone to the proper sales agent based on sales territories which are typically based on state/city/country. Discrete search can help with that, however territory support is not part of the basic Search add-on unless your territories are divided by city, state, or country alone. Directory Builder add-on can be used for territory search of that nature if no Map is included. Territories is now included in the Premier Add-on
The search algorithm converts the latitude and longitude of the starting search address and each store by using a spherical cosine formula. The short version of this is that the latitude and longitude are subtracted from each other and multiplied by 6371 kilometers or 3959 miles. This is the radius of the earth and is used to determine how far apart two points are on a perfect sphere.
As you know, the Earth is not a perfect sphere and thus this formula is an approximation. The closer your points are to one of the poles, the more varied the terrain, and other factors will dictate how accurate the results are. However, in most cases this is the accepted methodology and formula for finding the distance between two points when given the latitude and longitude on Earth.
Looking Under The Covers
Here is the response where you can see the SQL query used in the location database, the version of SLP, and the results. Here the count of 0 shows now results. If you click the response tab you can see the list of locations (there are none).