Geoscience Reference
In-Depth Information
does not work over land areas. The classical method for a detailed geoid
determination on the continents has been the gravimetric method, in spite
of the fact that it is severely handicapped by lack of an adequate gravity
coverage (or lack of information on such a coverage). Thus, we have the
paradoxical situation that on the oceans, long a stepchild of geodesy, the
geoid is now in general known much better than on the continents.
Still, the gravimetric method has continued to fascinate theoreticians
because it gives rise to very interesting and deep mathematical problems,
related to the geodetic boundary-value problem discussed above in this chap-
ter.
These enormous practical and theoretical developments concerning global
satellite and gravimetric gravity field determination have somewhat over-
shadowed the determination of detailed geoids in smaller areas, in partic-
ular, astrogeodetic geoids. Especially in mountainous regions, local geoid
determinations are di
cult. The gravimetric method does not work very
well in high mountains. The astrogeodetic method, using astronomical ob-
servations of latitude and longitude, does work well there but is considered
time-consuming and somewhat old-fashioned, perhaps also because work-
ing during the night is not very popular nowadays. An appropriate use
of gravity and astrogeodetic data in high mountains must involve some
topographic-isostatic reduction. Furthermore, the theory behind the astro-
geodetic method is not nearly as attractively dicult as the theory of Molo-
densky's problem. Last but not least, high-mountain areas are exceptional
and, apart from such countries as Switzerland and Austria, are frequently
regions of little economic interest. For these and similar reasons, the main-
stream of geodetic practice and theory has flown with grand indifference
around high mountains, ignoring such trivial obstacles.
Still, a country such as Switzerland has made a virtue out of necessity
and has traditionally been very active in local astrogeodetic geoid determi-
nation (Elmiger 1969, Gurtner 1978, Gurtner and Elmiger 1983). Austria
has followed up ( Osterreichische Kommission fur die Internationale Erdmes-
sung 1983). It has been found that, even besides the problem of getting the
required observations, the underlying theory is not so trivial as one might
think and shows quite interesting features.
Concerning measurements, astronomical observations have again proved
very feasible in mountains; see the articles by Erker, Bretterbauer and Gerst-
bach, Lichtenegger and Chesi in Chap. 2 of Osterreichische Kommission fur
die Internationale Erdmessung (1983), followed by Sunkel et al. (1987). The
main advantages of astrogeodetic versus gravimetric data for local geoid
determination in mountainous regions may be summarized as follows:
1. It is sucient to have astrogeodetic deflections of the vertical in the
