The 'mean sea level' (MSL) datum used as height reference in India is based on tidal observations at:
Choose the correct answer
Mumbai (Colaba) tide gauge
Karachi harbour tide gauge (historical GTS datum) — all Survey of India RL values referenced to this datum
Chennai tide gauge
GPS ellipsoid height directly
Correct Answer
B. Karachi harbour tide gauge (historical GTS datum) — all Survey of India RL values referenced to this datum
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Indian MSL datum: Survey of India uses mean sea level determined from tidal gauge observations at Karachi harbour (pre-partition, for historical consistency) as the official height datum for Great Trigonometrical Survey (GTS) levelling network. Modern GNSS heights are ellipsoidal (WGS84 ellipsoid) which differ from MSL (geoid) by geoid undulation N (EGM2008 model for India).
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