Question:
Which one of the following is NOT a geodetic reference frame?
Which one of the following is NOT a geodetic reference frame?
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Ask whether the item defines coordinates and a datum origin, or whether it only describes the Earth's gravity field/geoid.
Updated On: Jul 20, 2026
- EGM96
- WGS84
- ITRF14
- NAD83
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The Correct Option is A
Solution and Explanation
Step 1: Recall what a geodetic reference frame (datum) is.
A geodetic reference frame, commonly called a datum, is a defined coordinate system tied to a specific reference ellipsoid (or, for a dynamic frame, a set of tracking station coordinates and velocities) that lets every point on or near the Earth be assigned unique, consistent coordinates such as latitude, longitude, and height, or 3D Cartesian X, Y, Z. It answers the question of coordinates with respect to what surface and origin.
Step 2: Check each option against this definition.
WGS84 (World Geodetic System 1984, option B) is the reference frame and ellipsoid used globally by GPS, it defines an origin, an ellipsoid, and a realization through tracking station coordinates, it is a true geodetic reference frame. ITRF14 (International Terrestrial Reference Frame 2014, option C) is the highly precise, regularly updated global reference frame maintained by the IERS using VLBI, SLR, GNSS, and DORIS observations, again a true geodetic reference frame. NAD83 (North American Datum 1983, option D) is the official geodetic datum for North America, tied to the GRS80 ellipsoid, again a genuine reference frame.
Step 3: Examine EGM96.
EGM96 (Earth Gravitational Model 1996, option A) is not a coordinate reference frame at all. It is a spherical harmonic model of the Earth's gravity field, used to compute geoid undulations (the separation between the geoid and a reference ellipsoid such as WGS84) so that GPS-derived ellipsoidal heights can be converted to orthometric (mean sea level) heights. It describes the shape of the geoid/gravity field, it does not itself define an origin, an ellipsoid, or a coordinate system in which positions are expressed.
Step 4: Conclude.
Because WGS84, ITRF14, and NAD83 are all genuine geodetic reference frames (datums) while EGM96 is a gravity/geoid model used alongside a datum, EGM96 is the one that is NOT a geodetic reference frame.
\[ \boxed{\text{Option (A): EGM96}} \]
A geodetic reference frame, commonly called a datum, is a defined coordinate system tied to a specific reference ellipsoid (or, for a dynamic frame, a set of tracking station coordinates and velocities) that lets every point on or near the Earth be assigned unique, consistent coordinates such as latitude, longitude, and height, or 3D Cartesian X, Y, Z. It answers the question of coordinates with respect to what surface and origin.
Step 2: Check each option against this definition.
WGS84 (World Geodetic System 1984, option B) is the reference frame and ellipsoid used globally by GPS, it defines an origin, an ellipsoid, and a realization through tracking station coordinates, it is a true geodetic reference frame. ITRF14 (International Terrestrial Reference Frame 2014, option C) is the highly precise, regularly updated global reference frame maintained by the IERS using VLBI, SLR, GNSS, and DORIS observations, again a true geodetic reference frame. NAD83 (North American Datum 1983, option D) is the official geodetic datum for North America, tied to the GRS80 ellipsoid, again a genuine reference frame.
Step 3: Examine EGM96.
EGM96 (Earth Gravitational Model 1996, option A) is not a coordinate reference frame at all. It is a spherical harmonic model of the Earth's gravity field, used to compute geoid undulations (the separation between the geoid and a reference ellipsoid such as WGS84) so that GPS-derived ellipsoidal heights can be converted to orthometric (mean sea level) heights. It describes the shape of the geoid/gravity field, it does not itself define an origin, an ellipsoid, or a coordinate system in which positions are expressed.
Step 4: Conclude.
Because WGS84, ITRF14, and NAD83 are all genuine geodetic reference frames (datums) while EGM96 is a gravity/geoid model used alongside a datum, EGM96 is the one that is NOT a geodetic reference frame.
\[ \boxed{\text{Option (A): EGM96}} \]
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