Abstract
The present thesis, consisting of three parts, deals with the methods used today, for the determination of height differences, appropriate for engineering works. The necessary corrections and reductions for the application of each method as well as for their combination are examined and classified, so that according to the case one may select the procedure that will allow for optimum results concerning accuracy as well as cost in labour and instrumentation. The thesis comprises theoretical documentation of the methods as well as a variety of case studies. In the first part of the thesis (chapters 1, 2, 3, 4 and 5) the theoretical documentation is presented. Chapter 1 deals with the definition of each kind of height and its respective reference surface. Chapter 2 deals with the method of geometric levelling (GL) and its evolution due to modern digital levels. Chapter 3 deals with the method of trigonometric levelling (TL) as applied today using modern Total Stations. The effect of geode ...
The present thesis, consisting of three parts, deals with the methods used today, for the determination of height differences, appropriate for engineering works. The necessary corrections and reductions for the application of each method as well as for their combination are examined and classified, so that according to the case one may select the procedure that will allow for optimum results concerning accuracy as well as cost in labour and instrumentation. The thesis comprises theoretical documentation of the methods as well as a variety of case studies. In the first part of the thesis (chapters 1, 2, 3, 4 and 5) the theoretical documentation is presented. Chapter 1 deals with the definition of each kind of height and its respective reference surface. Chapter 2 deals with the method of geometric levelling (GL) and its evolution due to modern digital levels. Chapter 3 deals with the method of trigonometric levelling (TL) as applied today using modern Total Stations. The effect of geodetic refraction and deviation of the vertical on height differences determined by TL is examined. Also results obtained by GL and TL are compared. Finally, the method of Special Trigonometric Levelling (STL) is presented. STL being a combination of TL and GL allows for the elimination of errors in TL due to instrument and target heights and minimises errors due to refraction. Chapter 4 comprises a brief presentation of 3D positioning by the GPS with emphasis on the kind of height provided. Chapter 5 deals, in brief, with methods applied for the determination of deviations of the vertical and Geoidal Undulations (N and ΔΝ), aiming at the conversion of geometric heights and height differences to the respective orthometric in order to allow for the combination of results from terrestrial and GPS measurements. In the second part, consisting of one chapter (chapter 6) case studies concerning five vertical control networks, with different characteristics, are presented and the methods applied for each case are evaluated with respect to accuracy and productivity. The networks considered are the following: ❖ A "small" (300 m x 300 m) vertical control network on the site of line B of the Athens Metro with some of the control points located on ground level and some underground. The network was measured in 5 epochs, during the excavation of the tunnel. ❖ A vertical control network connecting the islands of Milos and Kimolos which is a case of height difference determination between two shores at a distance of the order of 2 km, where terrestrial and satellite methods were applied. ❖ A vertical control network in the town of Metsovo with difficulties due to steep slopes, rough terrain and the density of the urban fabric. A large part of this network was measured in two epochs, during May 1999 applying the method of STL and during June 2006 by a combination of STL and GPS. All the results are presented as well as the procedure followed for converting geometric height differences measured by GPS to orthometric ones, in order to compare them with those measured by STL. ❖ A vertical control network in the area of Aliveri with distances between control points varying from 3 to 10 km (length of leveling traverses from 4 to 17 km). Due to the size of this network the application of terrestrial methods may be considered quite inconvenient. The purpose of the establishment of this network was to: - examine the results obtained by STL when applied to a large network (accuracy and productivity). - compare orthometric heights derived by STL with orthometric heights derived after the appropriate conversion of geometric heights given by GPS. In order to convert GH to OH a local model of the geoid was produced while global geodynamic models such as PGM2000A were also considered. ❖ A vertical control network in the site of the Rion-Antirrion Bridge, established in order to allow for the monitoring of the vertical positions of characteristic points on the bridge during and after its construction. The distances between the sea shores varied from 2 to 3.5 km. Measurements were carried out during 4 epochs (May 1999, April 2001, November 2002 and July 2004). The measuring procedure as well as the comparisons between the results obtained during the 4 above mentioned epochs are presented and discussed. The third part of the thesis consisting of two chapters (chapters 7 and 8), deals with the conclusions derived from the theoretical analysis and the case studies while some proposals for further research regarding vertical positioning in engineering applications are suggested. The conclusions may be summarised as: → Measurements with modern digital instruments can become easy and very accurate provided that the necessary calibrations are carried out before and after the measurements. → Geometric Leveling still remains the most accurate method for measuring height differences (up to 0.5 mm/km), especially for small networks. Digital levels have improved the method, giving accurate results in a short period of time, with a significant reduction of systematic errors in reading and registering of measurements. → In cases of rough terrain and long distances, if there is a need for high accuracy, Special Trigonometric Leveling, using modern total stations, is considered as the optimum terrestrial solution. If the distance between the 2 points is over 500-600 m, in order to minimize errors due to refraction and earth curvature, the procedure of reciprocal and simultaneous observations must be applied. Also, zenith distance errors caused by the difference of deviations of the vertical between the two participating points must be examined with care. → GPS, using 2 double frequency receivers, gives similar accuracies in vertical positioning with those given from STL. The two methods may be combined with no special problems, provided the network is designed according to the needs of both methods. → Converting geometric height differences measured by GPS to orthometric ones is not always a simple procedure, and not of equal accuracy. → In areas of a few km², the geometric determination of a local model of the geoid, using points of known horizontal position as well as geometric and orthometric height, is the optimum solution for the conversion of geometric heights and height differences measured by GPS to orthometric ones. This procedure gives an accuracy of 1 - 2 cm, which is enough for most surveying applications, but not enough for applications demanding higher accuracies (monitoring vertical displacements and deformations, high accuracy settings out, etc). In these cases terrestrial methods and specially Geometric Leveling may still be considered as the optimum solution.
show more
