GNSS in Practical Determination of Regional Heights - Global Navigation Satellite Systems: Signal,Theory and Applications - page 127

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GNSS in Practical
Determination of Regional Heights
Bihter Erol and Serdar Erol
Istanbul Technical University, Civil Engineering Faculty
Department of Geomatics Engineering
Turkey
1. Introduction
Describing the position of a point in space, basically relies on determining three coordinate
components: the Cartesian coordinates (X, Y, Z) in rectangular coordinate system or latitude,
longitude and ellipsoidal height (ϕ, λ and h ) in ellipsoidal coordinate system, referred to any
given reference ellipsoid. Today, of course, global navigation satellite systems (GNSS) is the
best and most popular method for determining ϕ, λ and h , directly. The instantaneous
determination of position and velocity on a continuous base, and the precise coordination of
time are included in the objectives of GNSS, and positioning with GNSS base on ranging
from known positions of satellites in space to the unknown positions on the earth or in
space. Besides the geometrically described coordinates however, the natural coordinates, the
astrogeodetic latitude, longitude and orthometric height (, Λ, H), which directly refer to
the gravity field of the earth, are preferable to take for many special purposes. In particular
the orthometric heights above the geoid are required in many applications, not only in all
earth sciences, but also in other disciplines such as; cartography, oceanography, civil
engineering, hydraulics, high-precision surveys, and last but not least geographical
information systems. Traditionally, these heights are determined by combining geometric
levelling and gravity observations with millimetre precision in smaller regions. This
technique, however, is very time consuming, expensive and makes providing vertical
control difficult, especially in mountainous areas which are hard to access. Another
disadvantage is the loss of precision over longer distances since each height system (regional
vertical datum) usually refers to a benchmark point close to the sea level, which is connected
to a tide gauge station representing the mean sea level (Hofmann-Wellenhof & Moritz,
2006).
In order to counteract these drawbacks of levelling, GNSS introduces a revolution also in the
practical determination of the heights in regional vertical datum depending on the basic
relation H = h - N among the heights. This equation relates the orthometric height H (above
the geoid), the ellipsoidal height h (above the ellipsoid), and the geoidal undulation N , as
such, when the h is provided by GNSS and N exists from a reliable and precise digital geoid
map, the orthometric height H can then be obtained immediately. This alternative technique
for the practical determination of H is called GNSS levelling. In the recent decades the wide
and increasing use of GNSS in all kinds of geodetic and surveying applications demands