Effect of changing the datum and the three-dimensional transformation on updating Iraqi topographic maps

Iraq’s national topographic maps from the 1960s and 1970s are commonly used as official references in engineering and geological projects. These fieldwork-based maps were referred to Clarke’s 1880 ellipsoid. As a result, the datum is frequently transformed into modern world geodetic systems such WGS_1984 in order to be compatible with the most widely used maps. However, due to the complexities of converting contour line datum, currently used conversion method is based on re-georeferencing Easting and Northing coordinates and includes an index elevation correction value for each map tile. This study aims to explore the efficiency of using a standard approach to convert contour line data using 3D conformal transformation. First, contour lines were vectorized and re-defined as polylines vertices. Second, the conversion parameters were calculated using DGPS observations of provided points on Clarke’s 1880 ellipsoid inside the study area. The obtained RMSE was 5mm. Then the contour line vertices were converted to WGS_1984 and were used to produce a Digital Terrain Model (DTM). The last step was to produce a new contour line map from the DTM. The comparison of the results of current study and conventional methods revealed a significant difference in the elevation index adjustment within each map tile of up to 7cm. Hence, the 3D conformal transformation is recommended.