TITLE:
Computing Local Geoid Model Using DTM and GPS Geodetic Points. Case Study: Mejez El Bab-Tunisia
AUTHORS:
Noamen Rebaï, Olfa Zenned, Hichem Trabelsi, Hammadi Achour
KEYWORDS:
Geoid Model, “Géoide Soft”, GPS, Digital Terrain Model, Height, Vertical Deflection, Mejez El Bab-Tunisia
JOURNAL NAME:
International Journal of Geosciences,
Vol.9 No.3,
March
13,
2018
ABSTRACT:
Different methods have been deployed to compute the geoid, the altimetry
reference for surveying applications. One of their main goals is to allow the
use of GPS (Global Positioning System) or GNSS heights, which are related to
an ellipsoid and therefore must be corrected. Some of these methods are accurate
but quite heavy as developed by [1], but one of them is easy to use
while giving very good results in a local system: some mm for a 10 × 10 km2 area developed by [2] [3]. In our study, we have used software called “Géoide
Program”, previously used at the CERN in Switzerland and set up by [4],
which they complete this software allowing a parameterization of general data
to provide results in a general system. Then, tests have shown the way to optimize
computations without any loss of accuracy. For our computations we
use gridded of geodetic heights, from Lambert or WGS 84 datum’s, DTM
(Digital Terrain Model) and leveled GPS points. To obtain these results,
components of the vertical deflection are computed for every point on the
grid, deduced from the attraction exerted by the mass Model. Then, geodetic
heights are computed by an incremental way from an arbitrary reference.
Once the calculation is performed, the geodetic height of any point located in
the modelled area can be interpolated. The variations of parameters (mainly
size and increments of the DTM and of the modeled area, and ground density)
have shown that they do not play a significant role although DTM must be
large enough to take into account an important area around a selected zone.
However, the choice of the levelled GPS points is primordial. We have performed
tests with real data concerning Mejez El Bab zone, in north of Tunisia.
Nevertheless, for a few hundreds of square kilometers area, and just by using a
DTM and a few levelled GPS points, this method provides results that look extremely promising, at least for surveying activities, as it shows a good possibility
to use GPS for coarse precision levelling, and as DTM are now widely
available in many countries.