> 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?
Fixed points actually move relative to each other. This is measurable even locally if you are doing high-precision localization e.g. with LIDAR. The geometry of relationships between objects is in constant motion but below the threshold of what a human can sense. There are many identifiable causes of this motion that vary with locality (tidal, thermal, hydrodynamic, tectonic, geophysical, et al). Additionally, there are local time dilation effects, both static and transient, that influence measurement but aren’t actually motion.
This comes up concretely when doing long-baseline interferometry. Lasers are used to precisely measure the distance between receivers in adjacent structures for use in time-of-flight calculations. Over the course of a day, the distance between those structures as measured may vary by multiple centimeters, which is why they measure it.
The air medium does add noise to the measurement depending on wavelength but it’s also small things adding up like the repeatability of the angle the satellite is at when it measures that same point. An arc-second of error at 400km is over a meter so even a fraction of an arc-second is enough to introduce a lot of noise between measurements.
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?