I am a Civil Engineer with limited survey knowledge and have been reviewing geodetic concepts and the ellipsoid datums. I came across confusion regarding the origin of GRS80 thinking it was earth centered, earth fixed. But several articles later, I found it was actually tied to the tectonic plates by permanent monuments. But it still remains unclear if the NAD83 datum is actually assumed to be centered on earth or anchored to the ground by monuments / base stations more like the Clarke 1866 datum anchored at meads ranch.
This Content Originally Published by a land surveyor to Land Surveyors United Network
Dear Mr. turlington,
GRS80 is an ellipsoid of revolution. (Not meaning to sound too trite) its origin is where ever one wishes it to be.
It is the shape adopted most by those wishing to establish geodetic horizontal (3-D actually) datums. For example the horizontal datum of the U.S. is named NAD83 (North American Datum of 1983). For that putpose the GRS80 ellipsooid was centered at the place thought to be the center of mass of Earth with the Z axis aligned with the Earth's axis of rotaton and with an assigned zero azimuth aligned with Greenwich, England.
As time passed, the ability of locating Earth's center of mass has been refined many fold. where there was once a doubt in the position of 5 or so metres, today it is expressed smaller than a few centimetres. That refinement also resulted in the most recent expressions of the location of the COM are about 2-1/3 m away from the estimated origin in the 1980s.
A convenient way to think of an ellipsoid such as GRS80 is as some 3_D graph paper. It is very simple (only 2 parameters) and we orient it any way we like, ; much like usual 2-D graph paper with which we are familiar. As we know from experience we may orient the graph paper any way we wish. So it is with geodetic ellipsoids. It functions as the framework on which we hang coordinates such as latitude and longitude.
I hope this is helpful.
GRS 80 is a reference ellipsoid in use by IAG. When a TRS is defined , the ellipsoid origin should be w.r.t. earth's mass centre and orientation should be w.r.t. earth axis of rotation. The size of ellipse will be defined by a and e. This ellipsoid is to best fit the geoid over the whole earth. This is how a TRS is defined and then it can be realised by the assignment of cords to a number of points on earth ( in theory 3, usually more used). The TRS is then improved and refined over time taking into account any movements.
Hope this helps.
So when a tectonic plate shifts enough for a permanent reference monument to have a different lat-long reading, is the datum shifted to maintain its original lat-long reading, or is the origin of the datum re-calculated based on the new lat-long reading ?
That is a very astute question; one whose answer is somewhat dependent upon the body politic. Consider two general styles of reference systems, one is called inertial and the other classical.
An inertial system is more like the ideal situations presented in a Physics class, frictionless surfaces and no air drag on a falling object. An inertial geodetic reference system is defined as having its origin at the COM and its Z axis aligned with the mean axis of rotation and it rotates at the mean rotational velocity of Earth.
A classical system is more pragmatic in nature. The technique dates before the availability of artificial satellites, which being in orbit, provide a direct means to estimate the COM. A classical system may be defined by surveying monuments presumed to be stable and determining the shape of an (any) ellipsoid whose surface may be made to fit the observed data (actually the assumed zero elevation of the data observed). An aphorism is that the classical datums is "pinned to the monuments on the ground."
In your example, after a monument has moved within the whole frame of the inertial system, its current location is represented by its new coordinates within the reference frame. The Latitude and Longitude and perhaps height assigned to it change.
In the classical system things are more local in nature. If something anomalous occurs to a point, its coordinates change as in the inertial system. If the whole area moves upon the tectonic plate, then the coordinates do not change since they are dependent upon the monuments that moved along with the plate.
Geodetic authorities are sometimes loathe to allow the coordinates of monuments to change for fear of the public being upset that may expect them to be permanently affixed. Remember local surveying doesn't require global geodetic coordinate systems but bureaucrats and database makers dearly love keeping their work simpler by requiring everyone to make reports in a single system.