This is something you'll have to solve using Math, and calculate what the distance is between the two points. The formula to do this is pretty complex, and uses the sinus and inverse tangent. Those aren't functions we currently have in microlfow.
You'll probably want to make a Java action to do this, see this post for the Java function to do the calculation. http://stackoverflow.com/questions/837872/calculate-distance-in-meters-when-you-know-longitude-and-latitude-in-java
Maybe best to look for a service that already provides these kind of lookups.