Hello,

What do the parameters sigma a, sigma b, and sigma c mean?

These parameters have different meanings depending on the type of observation.

As far as I understand, if the accuracy of the instrument we use is 1mm, sigma b should have a value of 1. Sigma c is always zero, but what is sigma a?

Both, σb and σc, depend on the distance between the start-point and the end-point. Contrary, σa is independent from the distance and can be interpreted as some random background noise of the measurement process having the same unit as the measured itself. Moreover, σa cannot be set to zero by convention. In a leveling network, σa refers to the uncertainty of a single measurement setup.

All the best
Micha