> if the “fixed point” you are measuring doesn’t sit still relative to any spatial reference system you care to use.

But do those points actually move or the air medium changes the measurements?

I ask because I saw a very interesting documentary once about how they started accurate mapping in England with fixed points and measuring the angles between those points to a high degrees of precision.

My mental model has always been that those points are all fixed, but now that you mention it, why should they be fixed?

After all, my 7 grade teacher clearly demonstrated the thermal deformation or copper rods and all bridges have gaps that allow for thermal deformation, so indeed, this would apply to soil on the scale of tens of kms?