Assessment of Orthometric Height Derived from Levelling, GNSS and EGM2008 Geoid Model in Bangladesh

Abstract

The use of GNSS to determine the orthometric height requires a geoid model which is used as a transformation linkage between ellipsoidal height and orthometric height. GNSS derived ellipsoidal height is purely mathematical, having no physical meaning. So, in most of the geodetic applications, it is necessary to use physically defined orthometric height. Orthometric height can be obtained accurately from traditional spirit levelling which is labor-intensive, tedious and costly. Conversion of GNSS-derived ellipsoidal height into orthometric height is the popular surveying technique in GNSS user community. This approach is required to integrate a geoid model which provides separation of geoid with reference ellipsoid. The EGM2008, a global geoid model is widely employed for the purpose yielding sub-meter accuracy. This paper evaluated the orthometric height derived from GNSS measurements and Global Geo-potential Model, EGM2008 in Bangladesh. GNSS Survey as well as spirit levelling was carried out connecting fifteen points distributed in the area. GNSS observations were taken using dual frequency geodetic receivers in differential static mode and levelling works were carried out using digital levels. The geoidal undulation for the corresponding points were obtained using the EGM2008 Geoid model. The orthometric height of these points are computed using ellipsoidal height and the geoid undulation. These heights are compared with the orthometric heights obtained from levelling. The difference in these two, was used to compute the root mean square error, mean and standard deviation which are found 17.57 cm, 17.18 cm and 3.76 cm, respectively. The results show that EGM2008 model is appropriate with root mean square errors. It is proven that orthometric height can be obtained at centimeter level accuracy by this method for any Engineering, Surveying or Mapping works where rapid heights are required.