Report of GPS-99 Session 13
GPS for gravity field and geoid determination
The session highlighted the developments in geoid determination methods for
height determination by GPS techniques. The current status of geoid
determination includes both extensive gravimetric computations utilizing
updated earth reference models ? notably EGM96 ? as well as the tayloring
of geoids to GPS/levelling results. Satellite, airborne and shipborne
gravity field determination techniques are developing rapidly to aid in the
determination of improved geoid models, and presentations on these topics
were included as well.
The developments of large-scale, state-of-the art high-resolution
continental geoid models were presented by an invited paper of Dru Smith,
who presented the new US geoid model ?Geoid99?, which represents a new
high-resolution (1?) geoid of the US, taylored to more than 6000 GPS points
on levelling benchmarks, and by a last-minute addition of a presentation of
Heiner Denker, Germany, on the current European quasigeoid model EGG97.
Both of these geoid models indicates that results of height determination
with GPS is possible at an accuracy level of a few cm, and that the geoid
accuracy in many cases are controlled by the accuracy of the available
GPS/levelling data. H. Denker presented results of GPS surveys from Germany
(covering the entire former East Germany), where an r.m.s. fit between GPS
and levelling were below 2 cm, indicating that the old dream of a 1 cm
geoid over extended regions are close to becoming a reality.
A number of papers on regional geoids were presented, illustrating partly
some of the problems encountered in more rough areas with highly varying
gravity fields. The progress of geoid model determination in Nepal was
presented by T. Baral, local geoid determination experiments in Brasil and
Korea presented by M. Stewart and U. Han, and results of a new,
comprehensive geoid determination of Japan presented by Y. Kuroishi. The
latter model was based on more than 0.5 million gravity values, and the
introduction of a new levelling system with proper orthometric heights gave
geoid shifts of more than 20 cm, highlighting the need for the use of a
good and comprehensive implementation of geoid computations, taking into
account the necessary theoretical refinements.
The progress in gravity field data collection for improved geoid
determination were illustrated by talks on improvement of marine gravimetry
through use of differential GPS (presented by M. Satomura), and by the
prospects of using satellite missions to improve the long-wavelength
gravity field using new satellite missions (the geoid utilization of the
Taiwan ROCSAT-3/COSMIC mission presented by B. Chao, USA). Airborne
gravimetry will be an effective method in the future for filling the ?mid
wavelength? band in geoid determination: Two papers addressed the issue,
one by the European AGMASCO group (presented by R. Forsberg), showing how
airborne gravity can yield accuracies of 2 mgal, transforming into geoid
accuracies of 5-10 cm regionally, and one paper on recent test flights of
current Japanese helicopter-based developments of new accelerometer
hardware, with a potentially very high resolution and accuracy, presented
by J. Segawa.
The session took place on Tuesday afternoon, and attracted an audience of
30-35 participants.
Rene Forsberg