Geoid undulation modelling and interpretation at Ladak, NW Himalaya using GPS and levelling data

Fast and accurate relative positioning for baselines less than 20 km in len gth is possible using dual-frequency Global Positioning System (GPS) receiv ers. By measuring orthometric heights of a few GE'S stations by differentia l levelling techniques, the geoid undulation can be modelled, which enables GPS to be used for orthometric height determination in a much faster and m ore economical way than terrestrial methods. The geoid undulation anomaly c an be very useful for studying tectonic structure. GPS, levelling and gravi ty measurements were carried out along a 200-km-long highly undulating prof ile, at an average elevation of 4000 m, in the Ladak region of NW Himalaya, India. The geoid undulation and gravity anomaly were measured at 28 common GPS-levelling and 67 GPS-gravity stations. A regional geoid low of nearly -4 m coincident with a steep negative gravity gradient is compatible with v ery recent findings from other geophysical studies of a low-velocity layer 20-30 km thick to the north of the India-Tibet plate boundary, within the T ibetan plate. Topographic, gravity and geoid data possibly indicate that th e actual plate boundary is situated further north of what is geologically k nown as the Indus Tsangpo Suture Zone, the traditionally supposed location of the plate boundary. Comparison of the measured geoid with that computed from OSU91 and EGM96 gravity models indicates that CPS alone can be used fo r orthometric height determination over the Higher Himalaya with 1-2 m accu racy.