Silver spring cant be more than a couple hundred feet above sea level... I would bet that the error from the calibration of your hydrometer contributes much more to the potential error due to elevation.
A hydrometer actually measures the ratio of specific gravity refernced to water. It has nothing to do with pressure or pressure altitude. Now gravity is ever slightly less at elevation, but even brewing on Pikes Peak (14,110 ft), the difference would be invisible to the human eye on a hydrometer.