How accurate are the orthometric (mean-sea-level)heights produced by my GPS?
GPS receivers can only compute latitude, longitude and ellipsoidal height. Because most GPS users want orthometric (or mean-sea-level) heights, GPS receivers often incorporate a lookup table of geoid height values (also called geoid separation or geoid undulation). Unfortunately due to storage constraints in GPS receivers these values may not be very accurate. For example many receivers use the DMA 10×10 geoid model which stores one geoid height value per 10° x 10° area. In fact geoid heights can vary a great deal within a 10° x 10° area. It is not uncommon for a GPS receiver’s geoid heights to be erroneous by several metres.
