Hi there,
I'm up in Canada and we have two commonly used geoid models. A screenshot is attached which explains the two, taken from the NRCan website.
I have seen the terms geoid/geoid model and vertical datums used interchangeably, but I know they aren't the exact same thing. Can someone explain how they relate to eachother?
For example in that screenshot, the geoid model CGG2013a is described as "the realization of the vertical datum CGVD2013". What does this mean - the realization of?
My understanding is that a geoid model is a specific model of the earth's surface, and at any given point, my elevation will be derived from it. How is this different than a vertical datum?
Thank you for any feedback!
IO
A geoid is a model of the earth's surface
It can be used as a vertical datum
A plane can also be used as a vertical datum
Thank you for the simple answer! Perfect.
A geoid is an equipotential surface that is a best-fit approximation of mean sea level, if the oceans were to stand still.
Modelled geoid heights (that is, the difference between the geoid and the ellipsoid) are used to convert ellipsoidal heights to orthometric datums. Some are more accurate than others.
Geoids may be derived from gravity measurements, or GNSS + levelling, or a combination of both (hybrid).
A geoid is an equipotential surface. Period. Nothing to do with mean sea level.
At least, that is what I remember from my Geodesy class in college. The NOAA website declares it otherwise.
https://oceanservice.noaa.gov/facts/geoid.html
Mean sea level was historically considered a best guess as to an equipotential surface, lacking the instruments to measure gravity at the time, while knowing that other factors (wave harmonics, shapes and restrictions of flow in bays, etc.) introduced errors into the sea level equivalency to an equipotential surface.
If a geoid were merely "an equipotential surface", there would be an infinity of geoids.
But the geoid is a very specific equipotential surface.
From Thomas Meyer's excellent article series "What Does Height Really Mean?" (Part II):
The direction of the Earth’s gravity field stems from the Earth’s rotation and the mass distribution of the planet. The inhomogeneous distribution of that mass causes what are known as geoid undulations, the geoid being defined by the National Geodetic Survey (1986) as “The equipotential surface of the Earth’s gravity field which best fits, in a least squares sense, global mean sea level.” The geoid is also called the “figure of the Earth.” Quoting Shalowitz (1938, p. 10), “The true figure of the Earth, as distin-guished from its topographic surface, is taken to be that surface which is everywhere perpen-dicular to the direction of the force of gravity and which coincides with the mean surface of the oceans.”