[aloer]

Fun facts I remember from looking into this in university some 9 years ago:

It’s not only the sideways movement of plates that causes a shift.

WGS 84 and other reference systems work with an underlying ellipsoid reference model. This ellipsoid is chosen to approximate the earth surface but it’s a simple shape and as such can’t account well for things like mountains and other irregularities in the shape of the earth.

Not only does that mean that your position on a mountain is less accurate, it also means that it becomes less accurate over time due to mountains growing.

These of course are tiny numbers so compared to a few cm shift of the plates it is nothing.

Another fun fact is that the ellipsoid also grows more inaccurate over time due to the earth rotating around a fixed axis.

The rotational force slowly causes the earth to flatten at the top and bottom (where the poles are) and to widen in the middle.

This too I assume is negligible

Speaking of ellipsoid, today I think it is measured via satellite. But in the past things had to be done by hand and were more local in nature. There’s a lot of history here as well since mapping has always been a very important task for governments.

So today in a lot of places a lot of data is still based on systems other than wgs84. Either because it’s historical data (e.g. property boundaries before gps was invented), or because they use a more localized reference system including a more localized ellipsoid that better matches specific country or state needs.

Perhaps one day we will use a big world wide look up table instead of a mathematical representation of the earth shape.

What I did at university was to compare a Germany-wide lookup grid with the mathematical approach of a Germany-wide reference system.

This lookup grid was created by a state agency and was more accurate because it could include more localized reference systems. In technical terms think of this as a pre calculation where each state can choose its most accurate method and then the results are put together into a country wide lookup table.

Last fun fact: The result of this comparison showed that the difference of a German-wide reference system compared to this collection of state-wide reference systems is up to four meters.

Edit: removed word inaccuracy, added difference. It’s all relative. What it mostly shows is that when working with geo data you need to know what source and what target reference systems to use. Otherwise things break