Global Positioning System Reference
In-Depth Information
5. Conclusion
Numerous advantages of GNSS techniques from a practical perspective and its high
precision in geodetic positioning make this satellite based positioning systems on service in
a very large spectrum of applications, ranging from routine engineering surveys to scientific
researches. On the other hand, the reference system definition of GNSS coordinates separates
the geometry from the Earth gravity field, and therefore developing a solution for transition
between the ellipsoidal and natural coordinates, especially in heights, constitutes a challenge
for geodesists to be solved by a combination of terrestrial and GNSS data in the recent years.
As a reflection of advances in computation techniques and improved data resolutions and
accuracies, the precisions of geoid models increase and hence GNSS levelling, as a new
concept in vertical control, become a consideration as a viable alternative for practical height
determination. All these developments lead modernization of geodetic infrastructures in the
national and consequently global scale, and cause leaving the traditional onerous surveying
techniques aside as a means for obtaining heights. Today, in many countries, the new
vertical datum definition is based solely on the geoid and vertical control is provided via
GNSS levelling with a precise geoid model (see e.g. Rangelova et al., 2010).
In the light of recent developments on GNSS techniques and their tremendous impacts on
definitions of the reference systems and hence geodetic infrastructures, this chapter
reviewed the principle geoid models and widely used methodologies for practical
determination of regional heights using GNSS. With this purpose, the evaluations on global
models validated the improvement of the long and medium wavelength information of the
gravity field, as a result of the current state of technologies with modernized GNSS, as well
as new LEO missions for dedicated gravity field research (i.e., CHAMP, GRACE, GOCE).
The improvements on global models as well as the terrestrial data qualities contribute also
to the regional geoid models by reducing their errors in the total budget of hybrid geoid
representation. However, according to results drawn from this study, the accuracy of
regional geoid model of Turkey is insufficient yet for deriving regional orthometric heights
with centimetre precision from GNSS levelling, and therefore local solutions such as
modelling local geoid with geometric approach or improving the regional geoid model with
local terrestrial data are still required for providing heights with an accuracy under 5
centimetres. Although the local geoids provide high accuracies, there are handicaps related
to their determination and use. The determination of local geoid models requires specifically
acquired reference data, having good quality and adequate distribution representing the
topography well, and an appropriate modelling algorithm, fitting the data. One of the
disadvantages related with the use of local geoid models is that they can be applied only in
the limited area with high precision and so are not suitable for extrapolation. These local
solutions do not contribute to a unified vertical datum definition in the country. In this
manner the importance of a precise and reliable regional geoid model in the concept of
GNSS levelling for practical determination of precise regional heights is obvious. In Turkey,
geoid modelling efforts as a part of modernization of geodetic infrastructure continue, and
with the enhanced data qualities, a precise regional geoid model with its time dependent
variations for GNSS levelling purposes will be possible in the near future.
6. Acknowledgment
GNSS/levelling data, used in local geoid modelling, were provided by Istanbul GPS
Levelling Network-2005 and Geodetic Infrastructure of Marmara Earthquake Region Land
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