Question

Which of the following statement(s) is/are CORRECT?

• A. WGS84 ellipsoid is an oblate ellipsoid
• B. GPS positioning gives the orthometric height of a place
• C. Height of a point above the geoid is its ellipsoidal height
• D. Shape of geoid changes with time

Hint:

Keep the trio straight: {Ellipsoid} (smooth reference) $⇒$ ellipsoidal height $h$; {Geoid} (MSL) $⇒$ orthometric height $H$; relation: $h=H+N$ where $N$ is geoid undulation.

Answer 1

The Correct Option is A

(A) True. WGS84 defines a geocentric, oblate reference ellipsoid (flattened at the poles: \( a>b \)).
(B) False. Stand-alone GPS provides ellipsoidal height \( h \) above the WGS84 ellipsoid. Orthometric height \( H \) (height above the geoid/mean sea level) requires \( H = h - N \) using a geoid undulation \( N \).
(C) False. “Height above the geoid” is the orthometric height, not ellipsoidal. Ellipsoidal height is above the reference ellipsoid.
(D) True. The geoid is an equipotential surface of Earth’s gravity field; mass redistribution (hydrology, ice, oceans) makes the gravity field—and thus the geoid—time-variable at the centimeter scale.
\[ \boxed{\text{Correct: (A), (D)}} \]