Print
Major improvements have been made over the past quarter-century or so in mapping the geoid on both national and global scales. And because there are large complex variations in the geoid related to both the density and relief of the earth, geoid models and interpolation software have been developed to support the conversion of GPS elevations to orthometric elevations. For example, in early 1991, NGS presented a program known as GEOID90. This program allowed a user to find N, the geoidal height, in meters for any NAD83 latitude and longitude in the United States.
The GEOID90 model was computed at the end of 1990, using over a million gravity observations. It was followed by the GEOID93 model. It was computed at the beginning of 1993, using more than 5 times the number of gravity values used to create GEOID90. Both provided a grid of geoid height values in 3 minutes of latitude by 3 minutes of longitude grid with an accuracy of about 10 cm. Next, the GEOID96 model resulted in a gravimetric geoid height grid in a 2 minutes of latitude by 2 minutes of longitude grid.
GEOID99 covered the conterminous United States, and it includes U.S. Virgin Islands, Puerto Rico, Hawaii, and Alaska. The grid is 1 degree of latitude by 1 degree of longitude, and it is the first to combine gravity values with GPS ellipsoid heights on previously leveled benchmarks. According to NGS, “When comparing the GEOID99 model with GPS ellipsoid heights in the NAD 83 reference frame and leveling in the NAVD 88 datum, it is seen that GEOID99 has roughly a 4.6 cm absolute accuracy (one sigma) in the region of GPS on benchmark coverage. GEOID03 superseded the previous models for the continental United States. It was followed by GEOID09, and the current geoid model in place is known as GEOID18.
NGS has an ongoing project known gravity mapping project known as GRAV-D, "The gravity-based vertical datum resulting from this project will be accurate at the 2 cm level where possible for much of the country. The proposal is official policy for NGS and is included in the NGS 10 year plan. The project is currently underway and actively collecting gravity data across the United States and its holdings." https://geodesy.noaa.gov/GRAV-D/