Investigating the Geometrical Problems Related to the Redefinition of the Egyptian Geodetic Datum using the High Accuracy Reference Network (HARN)

Abstract

Ain Shams University proposed, on 2006, the DMT software package which can transform any digital map from the Egyptian Geodetic Datum (EGD30) to WGS84point by point in few seconds. This software has some limitations that should be overcome to convey the current operational requirements. Furthermore, the transformation of these maps from EGD30 into WGS84 will certainly produce some geometrical problems in the features existing in the maps. From these geometrical problems, which has great importance, is the deformation in the geometric area of the features and the effect of the deformation in the length on the scale of the connected traverses. Hence, the purpose of this investigation is to update the Digital Map Transformer (DMT) solution for transforming the digital maps from the old datum into the new one. Moreover, the area deformation and also the effect of the length distortion on the connected traverses, associated with this transformation, were investigated. To achieve these goals, new software, named M-DMT (Modified-Digital Map Transformer), has been developed using the Visual Basic.Net programming language which can transform the map existing in a digital DXF format from any mapping system into another accurately in few seconds. It can transform the map between Egyptian Transverse Mercator (ETM) and Universal Transverse Mercator (UTM) for all involved zones, between ETM and Modified Transverse Mercator (MTM) for all included zones, and between UTM and MTM for all included zones as well. The transformation in this software is achieved based upon many datum transformation approaches such as Bursa, Molodensky, 2-D poly-zone, and 3-D surface polynomial. In addition, the deformation in the area property of the polygon features constituting the map, due to the transformation from ETM into UTM and from ETM into MTM based on the poly-zone and ESA transformation parameters, is to be studied. The behavior of this deformation is investigated taking into mind three factors which are the location, area, and the geometry of the polygon. The study involves the deformation behavior all over Egypt. Also, the effect of transforming the mapping system on the connected traverse scale was investigated. The obtained results indicated that the area deformation occurs due to the difference in the scale factors of the two projection systems between which the transformation is carried out.The area deformation is mainly dependent on the position and is independent of both the area magnitude of the polygon and its geometry. The area deformation varies fast in the East-West direction and very slowly in the North-South direction throughout Egypt. There are slight differences in the area deformation values when using the poly-zone or the ESA transformation parameters. The deformation variation with position is mainly dependent upon the difference in the central meridian of the projection systems included in the transformation. Moreover, the scale of connected traverses is distorted introducing new closing error when changing the coordinate system used. In general, the deformations resulting from the transformation into MTM are less than those resulting from the transformation into UTM. Finally, new reliable software, named the DEFORMATION CALC program, is developed by the author. The program can estimate the deformation in area encountered during transforming the map from ETM into UTM or from ETM into MTM anywhere in Egypt. The results indicated that the MTM system is recommended for use in Egypt over the UTM one and the combined effect is recommended to be taken into mind when constructing the traverse using both GPS and total station. Keywords: Map transformer software ; (M-DMT) software ; Area deformation behavior ; Area deformation ; Coordinate system transformation; Area distortion calculator software ;Coordinate system transformation software ; Redefinition of the Egyptian Datum ; deformation of connected traverses ; combined effect.