I would like to refine an precision estimate of local gravity based upon current GPS position. Ideally, similar to the National Weather Service API, I would like to interrogate a server with a REST interface, providing location and accepting gravimetric data in return. If no such service is available, I would consider hosting mgal data and referencing its value by location, possibly with interpolation.
Together with other natural measurements, this gravity data would be an input to a developing Android application. (No, I don't deem the device's native sensors sufficiently accurate for this; thus, I am seeking a benchmark reference.)