Insurance bubble
The insurance vial (also height circle , coincidence vial ) is used in older theodolites to precisely align the vertical circle (height circle) according to the plumb line . Its name comes from the fact that it prevents systematic errors in altitude measurement .
It is a tubular level , the ends of which are mirrored to increase accuracy. It is played in by pressing a knurled wheel on the side of the telescope support and thereby rotates the height circle. With modern theodolites this function is performed by an automatic height compensator .
In both designs, the remaining amount is called an index error . It can be eliminated by measuring in both circular positions .