Büyük Ölçekli Harita Üretiminde GPS ile Ortometrik Yükseklik Be- lirlemeye Yönelik Jeoid Modelleme ve Uygulamalar

The use of GPS in all kinds of engineer- ing applications for determining precise coordi- nates, especially in the last ten years, has made fundamental changes in Turkey as in the other countries. 3-D precise coordinates, easily obtained by GPS, are widely used in such applications as large-scale map production and Geographical In- formation System. The latitude and longitude ob- tained by GPS are used directly as they are or after suitable transformations, but ellipsoidal heights cannot be used in applications. For the transforma- tion from ellipsoidal heights to orthometric heights, which are used in applications, geoid heights must be known with required accuracy. In Turkey, Turk- ish Geoid- 1999A is used at the moment and the computation of the new Turkish geoid is about to be finished by the end of 2003. The bylaws for large scale map production is re- quired to be changed and allow to GPS applica- tions, as parallel to the increasement in application of the GPS technique. The new draft bylaw con- tains observation and calculation methods for ob- taining orthometric heights by GPS, as well. For transforming ellipsoidal heights into or- thometric heights, four general methods are given in the new draft bylaw. These methods are, (a) us- ing existing TG99A geoid directly, (b) local updat- ing existing TG99A geoid by local GPS/Levelling points, (c) adjustment of the orthometric height dif- ferences on the base vectors, (d) determination of a local GPS/Levelling geoid. In this study, explanations and applications are given briefly about the existing geoid models and orthometric height computation from ellipsoidal heights obtained by GPS. Methods were applied in chosen area and it was seen that (b) and (c) meth- ods gave appropriate results.