Comment by defrost
14 days ago
Each ellipsoid is rigidly defined (well, some historic ones are sloppy), so WGS84 won't drift .. (that's a bold statement, is it true down to the micron and if so what are the absolute* datums to reference against?).
That said, there are literally hundreds of historic pre WGS84 ellipsoid|datum pairings, each with a somewhat different "survey map pole".
Historically geodectic poles have shifted as a function of datums.
The main point here, such as it is, was to poke at the infomation free aspect of "polar drift" as a comment .. which pole and what does that have to do with climate change? etc.
We still use many of those old ellipsis and datum’s today. When you’re doing human things, like surveying land, and defining property boundaries. It’s nice to work with a coordinate system which remains fixed relative to the area you’re surveying, and doesn’t drift due to annoying things like tectonic movement, or your entire country slowly tipping into the ocean.
FWiW I'm old enough to have navigated via LORAN and travelled through over two thirds of the 190+ countries on the planet tying in multitudes of old datums to the "new" WGS84 standard as part of a career in geophysical surveying (Gravimetrics, tides, magnetics, radiometrics, EM, etc.)
I'm not old enough to have seen Great Britain and relate isles pressed down by the weight of kilometres of ice though .. that'd be a great great great grand something that saw that.
Sure everyone these day maps their local references back to WGS84, but the local references are still tied to local datum.
Plate tectonics can result in some parts of the world moving at up 10cm a year, which over 10s to 100s of years can add up to something pretty significant. Funnily enough an OSM April Fools joke is a good place to learn more[1].
Talking of the UK. Ordinance survey still maintain their own master geodesics, and geographic references, which allows them to tie the OS grid (which what the land registry uses to locate property) back to WGS84, as both WGS84 and the UK slowly drift around due to various reasons (such as improved tech to refine the definition of WGS84, tectonic drift etc). You joke about the ice age and glaciers, but the UK is still “recovering” from all that ice, resulting in vertical movement of about 1m every 100 years. Which given how long property rights can last (Oxford University is almost 1000 years old), can actually turn into a material difference, and real land disputes, over time, if not properly corrected for.
Each of these adjustments may seem insignificant on their own, but they accumulate over time, and it gets complicated when these adjustments are forced to interact with humans, our somewhat fuzzy perception of reality, and general disregard for well defined coordinate systems which don’t align well with our “intuitive” understanding of the world.
None of this is any different to how we deal with issues that are thrown up by our increasing ability to measure time accurately. We track International Atomic Time (IAT), which is time as tracked by a set of atomic clocks, but then we apply various adjustments to get UTC, which is human time. Those adjustments exists purely to keep UTC aligned to what humans expect, because the earths orbit wobbles enough that the absolute time produced by IAT doesn’t match up perfectly with how we’ve historically measured time. All of this seems a little silly, but we now live in a world where everyday systems depend on measures accurate enough that all this minor drift becomes important.