The 'ellipsoidal height' from GPS differs from the orthometric height (MSL) because:
Choose the correct answer
They are identical (GPS gives MSL directly)
GPS gives ellipsoidal height h; orthometric H = h − N (N = geoid undulation from MSL to ellipsoid, varies ±70 m in India)
GPS gives depth below sea, not height
Ellipsoidal height is always 100 m less than MSL height
Correct Answer
B. GPS gives ellipsoidal height h; orthometric H = h − N (N = geoid undulation from MSL to ellipsoid, varies ±70 m in India)
Why is this the answer?
GPS measures ellipsoidal height h (above WGS84 reference ellipsoid). Orthometric (levelling) height H = height above geoid (MSL). Relationship: h = H + N (N = geoid undulation). In India: N ranges from −65 m (south) to −50 m (north) approximately. EGM2008 geoid model gives N with ±10–30 cm accuracy. Civil engineering needs orthometric heights (water flows downhill based on geoid, not ellipsoid).
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