GEOID99 and G99SSS: 1-arc-minute geoid models for the United States

Abstract. Two new geoid models have been computed for the United States and its territories. The first model is the purely gravimetric G99SSS model, approximating the geopotential surface W0=62636856.88 m2/s2 and referenced to the geocentric GRS-80 ellipsoid whose origin coincides with the ITRF96(1997.0) origin. The other model is the hybrid GEOID99 model, which encompasses all gravimetric information of G99SSS as well as the vertical datum information of 6169 GPS-derived NAD 83 (North American Datum 1983) ellipsoid heights co-located with spirit-leveled NAVD 88 (North American Vertical Datum 1988) Helmert orthometric heights. The coverage of both models in the conterminous United States (CONUS) is from 24°N to 58°N latitude and 230°E to 300°E longitude. Long-wavelength geoid structure was controlled by the EGM96 model coefficients, medium-scale information by 2.6 million gravity measurements, and local features through the use of 30-arc-second and (recently created) 3-arc-second digital elevation models. In addition to new elevation data, there were corrected errors in the old elevation data, better satellite altimetry data, and ellipsoidal corrections in G99SSS and GEOID99 that were not applied in G96SSS and GEOID96. The GEOID99 model replaces the GEOID96 model as the primary conversion surface between NAD 83 ellipsoid heights and NAVD 88 Helmert orthometric heights. While GEOID96 had the ability to convert absolute heights (of the 1996 GPS-on-bench-mark data set, or GPSBMs) at the ±5.3-cm level, GEOID99 works with the latest GPSBMs at the ±4.6-cm level. The improvement from 5.5 to 4.6 cm in RMS further translates into almost 2 cm of additional accuracy in single-tie differential GPS-based leveling over most short baselines. Of this 4.6 cm, some 2.0 to 2.5 cm is attributed to correlated errors in the gravimetric geoid and local GPS errors, decorrelating around 40 km. The GPS-derived ellipsoid heights in CONUS have changed since 1996 due to an increased number of observations and new adjustments of previously observed areas. This new GPS data is reflected in GEOID99, but not GEOID96, which effectively renders GEOID96 out of date (in an absolute sense), so that the absolute agreement between GEOID96 and the 1999 set of GPSBMs is only at the ±6.5-cm level. In addition to the geoid models in CONUS, gravimetric geoid models were produced in Alaska, Hawaii, and Puerto Rico and the American Virgin Islands. Surface deflection of the vertical models were also computed and show agreement with astro-geodetically determined deflections below the 1-arc-second level.