Survey Touch
Cartographer Survey doesn't care about absolute frequency values and what Z height that corresponds to. Survey is all based around the change of signal, specifically it detects when the rate of change of the frequency... changes.
As the Cartographer gets closer to the bed, the frequency value increases at a predictable rate, which we can see as a slope on a graph. When Cartographer stops (because the nozzle has now touched the bed) this slope changes angle and flattens out. The Cartographer Survey software detects this change and stops and now knows that the nozzle has touched the bed. Note that the cartographer is still inducing the electric field into the closes metal body, usually the spring steel sheet or the bed itself. For Cartographer Survey to work it needs to be able to induce this field which means it needs metal beneath it, within a few mm.
The real neat thing about this is that because we are just looking at this change of slope, we don't care that this happens at the exact same frequency every time we run a nozzle probing action. This means that even if the frequency values are different (because of different coil temperatures) we can still detect when the slope changes and therefore know when the nozzle touches the bed. So we don't even need any temperature compensation as we don't care about absolute numbers. Neat!
Finally, to tie all this together. We do a SCAN Z homing, which triggers at a certain frequency (FREQ). Then we can do a SCAN bed mesh which will generate a mesh based around the frequency (and therefore distance) variation from the first trigger FREQ. Same with QGL/Z-Tilt, it's based off of the FREQ. At this point we may not know exactly how far this FREQ trigger distance is to the nozzle, so we do a Survey nozzle probing action and now the system knows the Z difference between the FREQ height, and the nozzle height. The bed mesh will be based off of this new "nozzle is here" value and everything works.
Now, because we work in the real world and each printer is different it is common that there is still an additional fixed offset applied even after the nozzle probing action figures out the bed location. This is due to many factors including thermal expansion between nozzle probing temp and printing temp, flex in the toolhead, flex in the bed, inherent noise, etc. The good news is this offset should be a representation of the "system" as a whole and therefore you can change nozzles/sheets and this offset won't change.
