Geoscience Reference
In-Depth Information
region of geoid determination; no data are needed outside that region
as they would be in the gravimetric method.
2. Errors in the topographic height have significantly less influence on
deflections than on gravity data. Thus, a relatively crude terrain model
will be sucient for the use of astrogeodetic data.
As a matter of fact, the two types of data are not mutually exclusive;
an optimal geoid determination will combine astrogeodetic deflections of
the vertical, gravity anomalies, and possibly data of other type. A suitable
technique for this purpose is least-squares collocation to be discussed in
Chap. 10.
From the observational point of view it is interesting to note that
inertial
surveying techniques
will be able to furnish deflections of the vertical and
gravity anomalies rapidly and with sucient accuracy for many purposes.
Let us finally try to give a list of various methods of geoid determination:
•
conventional satellite techniques (Doppler, laser, etc.),
•
satellite-to-satellite tracking,
•
satellite gradiometry,
•
satellite altimetry,
•
gravimetry,
•
astrogeodesy, and,
•
most directly, GPS leveling (Sect. 4.6).
As a general rule, these methods are listed in such a way as to start with
the most global and end up with the most local method, that is, according
to decreasing globality or increasing locality. In general, going down the list
also corresponds to increasing resolution and accuracy.
Again it should be stressed that these methods complement each other
and should be combined for best results.
New satellite gravity missions have been discussed in Sect. 7.6.
Astrogeodetic method according to Molodensky
The remaining part of this chapter deals primarily with the lower end of the
list, providing a detailed theory of astrogeodetic local geoid determination
in areas with dicult topography. The role (and necessity) of topographic-
isostatic reduction is investigated in detail. The computations for Austria
give concrete numerical results for questions which have been much dis-
cussed theoretically, such as the difference between geoidal heights and height
anomalies according to Molodensky (quasigeoidal heights), or the numeri-
cal effect of analytical continuation from the earth's surface to sea level
( Osterreichische Kommission fur die Internationale Erdmessung 1983).
