A study of the systematic errors inherent in a high precision geodetic network incorporating GPS, gravimetric, and astronomic observations

Szu-Pyng Kao, Purdue University

Abstract

The reduction of terrestrial observations to a chosen reference ellipsoid, a good distribution of astro-geodetic (deflection and Laplace) stations, a good calibration of distance measuring instruments, an adequate modeling of atmospheric refraction, an insignificant personal equation in astronomical observations, and an accurate geoidal undulation map are all needed to insure a high quality geodetic control network. These conditions are not always fulfilled in some of the countries of the world. In these countries, the established local geodetic datum and primary geodetic networks may lack the detailed information of geoid undulations and deflections of the vertical due to the low density and often inferior accuracy of astro-geodetic stations. As a result of these deficiencies, of occasional instrument calibration problems, and of other error sources, many high precision geodetic networks suffer from shape and scale distortions which may accumulate into significant systematic errors. Failure to account for these will lead not only to incorrectly adjusted geodetic coordinates, but also to over-optimistic estimates of a posteriori accuracies of the local high precision geodetic networks. The influence of the systematic errors considered in this study are on the scale and on the orientation of the network. The resulting network deformations will cause inconsistencies with newer high precision GPS observations. These inconsistencies can also appear to be "network systematic errors" and will be referred to as such hereafter. A few examples of the analysis of a high precision geodetic network are used as illustration. The first order triangulation network of Taiwan Geodetic Datum of 1980 (TGD80) is the source of the data used to investigate these systematic errors. For addressing these problems, a comprehensive description of procedures is given to determine and partially correct the systematic errors. By combining satellite (TRANSIT and GPS), gravimetric and astro-geodetic observations, errors of scale and orientation can be detected, and to some extent, corrected within the local high precision geodetic network. The proposed approach seeks to detect and, where possible, reduce the significant systematic errors before dealing with the network least squares adjustment. Network distortions remaining after adjustment are also investigated using independently derived GPS station positions as a reference standard. Errors can be detected and controlled more efficiently and better with GPS in the near future. This could enhance the utility of primary high precision geodetic networks which may otherwise be degraded by the presence of significant systematic errors.

Degree

Ph.D.

Advisors

Bethel, Purdue University.

Subject Area

Civil engineering|Remote sensing

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