A STUDY ON WGS-TM MAP PROJECTION OF THE THAI-CHINESE HIGH-SPEED RAIL PROJECT

Abstract

During the Thai-Chinese High-speed Rail Project designer had opted Transverse Mercator (TM) Map Projection with WGS84 datum (WGS-TM) for the project. The Transverse Mercator (TM) is known as one of popular choices for Low Distortion Map Projection (LDP) for construction. Unfortunately during project implementation in Thailand the concrete information namely scale factor  for datum plane is missed. Since Thai engineer need exact TM definition for calculation of the control point network extension, for construction surveying. The study has discovered that the definition of the given TM map projection especially the geodetic height of projection plane  is inadequate and not practicable. To remedy this we have tried to recover the TM definition by expanding size of the ellipsoid datum using Geodetic height of projection plane . After applying this TM definition to 81 control points in the project and comparing to given WGS-UTM coordinates results horizontal difference by ± 3 millimeter, which is negligibly small and quite satisfactory. Nevertheless the study has tried to recover the actual scale factor by trial-and-error method. The latter try has revealed three new TM scale factors  namely 1.00000000, 1.000030770 and 1.000025247 for each of “Construction Coordinate System” within the project. Using these scale factors for TM definitions and reprojecting the 81 control points then comparing with given WGS-UTM coordinates now results horizontal differences of ± 6 millimeter which is again considered to be small and negligible. The reversed computed scale factors has fulfilled the WGS-TM map projection definitions in project and engineer can now use them conveniently with familiar GIS and CAD software. We summarize that any LDP application for construction, the map projection should be defined with some de facto standard e.g. PROJ4, EPSG, WKT or OGC, which it always includes the defined scale factor .