Thank You.
Since the GPSr always selects the 'best positioned' SV's to calculate the users position, I can not imagine any scenario where having less SV's for the GPSr to choose from would be beneficial. Just because the GPSr can see a couple dozen SV's does not mean it is using them all to calculate the devices location...
This brings up an interesting point.
Certainly the Earths surface is in a constant state of movement, most of which is imperceivable to the human eye, but not always (for example, large magnitude earth quakes). Once upon a time the Earth had a single super-continent that broke up and separated over a very long period of time into the pattern we are currently familiar with. Any selected location (or waypoint) on Earth today would be found in an entirely different location a billion years ago (by a time traveling surveyor, for example).
Wikipedia states: "Surveying (or land surveying) is the technique, profession, art, and science of determining the terrestrial or three-dimensional positions of points and the distances and angles between them."
So, one can imagine how plate tectonics would play a part in why a surveyors results might change over a period of time, as they are measuring the relationship of multiple points on the Earths surface against each other.
A simple definition for 'datum' is "1. a piece of information." or "2. a fixed starting point of a scale or operation."
There are many different datums used for mapping the Earths surface, some of the more common being 'NAD 27', 'NAD 83' and 'WGS 84'.
Garmin GNSS receivers calculate their position using trilateration, where they reference their relationship only to the known locations of a number of SV's. At no time does the GPSr reference any location on Earth to determine it's position.
Map Datums only come into play when translating a GPSr position into a usable map location. Legacy GPSr (and still some today) do not have internal mapping capabilities, and provide the user only with coordinates for their current location. Those coordinates will be translated into the Map Datum specified on the device, allowing the user to plot their location on a paper map that was created using the same map datum.
A mapping GPSr does this in real time for a digital map specified by the user.
I might argue that the coordinates do not change position, but rather the surface of the Earth under them.
The GPSr still finds itself in the same location as when the 'waypoint' was originally marked, however, the 'landmark' found at that location may be different due to movement of the Earths crust.
Here the GPSr reported location is still correct. It is the 'Map' that has changed, and it is the map makers responsibility to integrate these changes into newer versions of their maps.
So, was this 1.8m of plate movement uniform and consistent throughout the entire surface of the Earth?
No, of course it wasn't.
This is a perfect example of why changes in the Earths surface due to plate tectonics can not possibly be integrated into any uniform coordinate system, and why it is the maps that must be updated.
The GPSr estimated accuracy here is not misleading, as it has no direct relationship to any map chosen by the end user.
The GPSr estimated accuracy refers to the expected level of error for position calculations in relationship to the SV's used for determining its location.
To summarize,
1. A GPSr calculates its position based on the spatial relationship between itself and multiple SV's orbiting the Earth.
2. The coordinates reported by the GPSr are a translation based on the datum and format specified by the user.
3. Map datums use a static grid system to reference locations on the Earths surface (latitude and longitude, for example).
Thus,
1. The relationship between specific landmarks on the Earths surface are never static.
2. The coordinates for any and all landmarks on the Earths surface will change over time.
3. Waypoints marked using a GPSr are a static record of the devices location, not the position of the Earths surface.
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