I am writing a program that uses GPS lat and long coordinates and a time change to determine speed with the output in units of m/s (precise to 0.1 m if feasible, but 1 m is sufficient). When researching the distances associated with a change in latitude and longitude, however, I saw just how variable it was. When continuing my research to find the distances associated with the general location I'm interested in, I could not find such a resource.
I know what area this program will be used in (Boston, Massachusetts) and the distance travelled during each time step is short enough that I can assume it moved across a plane. As such, I am not looking for a formula to use in the general case over long distances but a look-up table or data base that has the relevant distances I need.
Where should I look online (or elsewhere) for this information?