GEOID99 and G99SSS: 1-arc-minute geoid models for the United States
D. A. Smith A1 and D. R. Roman A1
A1 National
Oceanic and Atmospheric Administration/National Geodetic Survey, 1315
East-West Highway, Silver Spring, MD 20910, USA, e-mails:
dru.smith@noaa.gov, dan.roman@noaa.gov; Tel.: +1-301-713-3202; Fax:
+1-301-713-4172
Abstract:
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which effectively renders GEOID96 out of date (in an absolute sense),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.
Keywords:
Key words: Geoid - GPS - Datums - Reference systems - Gravity - Deflections of the Vertical
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