The general and basic principles of underground surveys meant for mines and tunnel setting out are almost the same as for surface except for the fact that the conditions under which the work is to be done are entirely different. The special conditions confronted in the underground surveys make the following changes:
- Due to limitations of space, small instruments of special designs with extension tripod legs or suspension rods are used.
- Due to very short sights, and sometimes very steep or vertical, special methods of observations are necessary with particular care to avoid the accumulation of excessive errors in measurements.
- Usually the transit station is in the roof (i.e. above the transit) and hence the procedure of traversing is modified.
- Due to darkness, special arrangements for illumination of both the instrument as well as the target are necessary.
- Usually the distances are measured on slopes, the traverse measurements include vertical angles and hence it is necessary to determine the three rectangular co-ordinates of all instrument stations.