Validation of GGMplus and CHGeo2004 Using Observed Deflection of the
Vertical Data from QDaedalus and CODIAC Systems in the Mountainous
Terrain of the Surses Region, Switzerland
Abstract
State-of-the-art astrogeodetic measurement systems—Digital Zenith
Camera Systems (DZCSs) and the tachymeter-based QDaedalus systems—are
currently used to obtain astrogeodetic deflection of the vertical (DoV)
components. In this study, we use the CODIAC DZCS (accuracy
~0.05″) and two QDaedalus systems (accuracy
~0.20″) developed by ETH Zurich to collect DoV data in
the Surses Region (Canton of Grisons), Switzerland. Based on these data,
we assess the quality of the DoV predicted by the Global Gravity Model
GGMplus and the Swiss Geoid model CHGeo2004. DoV data previously
observed with the QDaedalus system in coastal (Istanbul) and smooth
(Munich region) terrain areas were compared with GGMplus to determine
the accuracy of the model for both types of terrain. The Surses Region
was chosen for these observations because of its mountainous nature to
examine the quality of the GGMplus model in mountainous terrain, as well
as the GGMplus’ quality with the CHGeo2004. Astrogeodetic observations
were conducted at 15 benchmarks in the astrogeodetic profile, at
elevations ranging from 1185-1800 m and a station spacing of
~1.5 km. At one benchmark (BM12), observations were done
with all three systems (eccentric BMs), while at 13 benchmarks,
observations were only completed with the QDaedalus systems.
Observations at the final benchmark were completed only with the CODIAC
(0⁰ zenith angle) due to line of sight obstacles. The standard
deviations of the QDaedalus data for each session are 0.04″-0.22″ and
0.01″-0.20″ for the N-S and E-W components, respectively. These
high-quality data were compared to DoV data derived from GGMplus and
CHGeo2004. The residuals from GGMplus are: The N-S component exhibit
large residuals ranging from -2.31″ to 1.75″, while the E-W component
residuals are -0.27″ to 1.80″. The residuals from CHGeo2004 range from
-0.60 to 1.21 for the N-S components, and -1.01 to 0.32 for the E-W
components. The results show that the predicted DoV data from CHGeo2004
are closer to the observed DoV, and more accurate than GGMplus. We
conclude from these results that GGMplus is of significantly lower
quality in mountainous terrain than in the smooth terrain of the Munich
region (~0.2″, and reaching a maximum of 0.3″ and 0.4″
for the N-S and E-W components).