Geodetic aspects of engineering surveys requiring high accuracy
Abstract
Many of today’s engineering surveys require relative positional accuracies in the order of 1/100 000 or better. This means that positional observations must be very accurate, and that a rigorous geodetic approach must be followed.
This thesis is directed toward the geodetic aspect. Chapter 2 reviews the geodetic models and coordinate systems available. For an engineering survey requiring high relative positional accuracy a local plane coordinate system and a geodetic height system, both based on the classical geodetic model, is the appropriate choice. Chapter 3 reviews the well known geometric and gravimetric effects in a local coordinate system.
Special emphasis is placed on methods to determine deflections if the vertical in chapter 4. It was felt that a contribution could be made if a simple method could be developed to determine deflections, which describe variations in the gravity field. (Very often the effect of variations in the gravity field on survey observations are neglected only because they are difficult to determine.) Such a method was developed by the author by applying a difference method to the usual astrogeodetic determination. The method is very simple and practical, and field test results indicate it is accurate to 1” to 2”. Extensive field work associated with the use of trigonometric levelling to determine local deflections led to inconclusive results because the effect of vertical refraction could not be isolated.
Chapter 5 shows the application of the material presented in the first four chapters, with emphasis on the effect of deflection of the vertical. The two problems considered show that often, even for engineering surveys requiring high accuracy, the effect of variations in the earth’s gravity field can be safely neglected. This however can only be determine by analyzing each problem using accurate deflection components to estimate the effect in the horizontal and a small number of gravity values to estimate the effect on heights.
Being able to easily define the local gravity field a priori by the astrogeodetic difference method will probably have its best application in situations in which local variations have their greatest effect, for example in the determination of heights in a three dimensional coordinate system and in the determination of horizontal positions with inertial surveying systems.