Assistant Prof. Dr. Metin Soycan
Yildiz Technical University,
One of the main purposes of geodetic aimed engineering surveying is to do an application in the shortest time and complete it economically in desired accuracy. In many surveying and engineering applications, orthometric heights are required. It is well known that, conventional leveling methods such as geometric leveling and trigonometric leveling are dominate techniques for determination of orthometric heights for years. Today Global Positioning System (GPS) is a 3D surveying and positioning technique offered by the modern technology to surveyors for the solution of the precise positioning problems. Parallel to development of GPS and methodology, there is a increase in studies aimed at obtaining orthometric heights with cm accuracy. This study proposes a cost effective GPS Leveling (GPSL) methods based on multi-reference kinematic GPS and precise geoid model for typical surveying application. The method has been probed into consideration of accuracy, practicability, measurement and computation criteria, and has been compared with other conventional leveling methods in a test area.
Since 1980s GPS have been used for the purposes of studying and monitoring the earth, locally, regionally or as a whole; exploring its unknown aspects, engineering studies, and many areas of our daily lives. In view of GPS applications that constantly develop and diversify such as earth-science research, data collection for GIS, geodetic surveying, navigation and transportation, remote sensing, environment studies, hydrology and agriculture; commercial organizations and institutions as well as scientific organizations and institutions continue research and development activities. Studies aiming to make this system a multipurpose global monitoring system that can be used more easily are made for collection and processing of GPS data in conformance to certain standards, and for delivering such data groups to users in several communication technologies with different methods.
In determination of orthometric heights with GPS, the accuracy of vertical positioning changes depending on the application type and quality of the project. The efforts and the attempts for using the system practically and rentable especially engineering aimed geodetic application have been increased for the last decades [1,2,12,13]. It is known that the surveyor especially to constitute and densification of the vertical geodetic networks, traversing and similar applications use several methods and technique. To achieve cm height accuracy, surveyors have been examined alternative methods instead of static GPS surveying methods with long session duration and professionally processing and analyzing of the collected data for long years. As the result of the development in fast static, stop and go, kinematic and other surveying methods, GPS receivers, IGS and other regional and local permanent GPS networks, it can be seen that it is possible to obtain desired accuracy in very short time interval by using single GPS receiver [5,10,14,17,18,19,24].
To the extent that, many countries throughout the world can make use of active RTK networks that have been developed in recent years for this purpose and resolve the vertical positioning problem very economically, cheaply and practically by means of multi-reference RTK approach [6,9,19,27]. GPS derived ellipsoidal heights have geometric meanings in practical engineering survey, geophysics and in other applications, and they bear no physical meanings. However, in many surveying and engineering applications, orthometric heights are required. To have ellipsoidal heights converted into orthometric heights, precise geoid heights are required. The most effective technique used in practice particularly for the determination of orthometric heights is the GPS-leveling technique. With the GPS-leveling method, it is possible to determine orthometric heights with cm accuracy. Besides, this method is one that is accepted and applied by the whole world due to its measurement, ease of computation and its economic application [3,7,8,11,15,16,23,26,28].
The purposes of this study are the determination of GPS ellipsoidal heights based on post-processing multi-reference kinematic technique with cm level accuracy, by using appropriate geoid model transformation of this heights into orthometric type and examination of its accuracy, productivity, efficiency and practicability. Strategies of the proposed model are given, and it has been applied in a test area and compared with the other conventional leveling methods. For this purposes, geometrical leveling (GL), trigonometrically leveling as unidirectional (UTL), leap-frog (LFTL) and simultaneously-reciprocally (SRTL) and GPS observation were performed on 87 test points between 16km length Beykoz and Riva highway of Istanbul.