Measuring distances optically with the level

 the reticle carries two stadia lines arranged symmetrically to the crosshair. their spacing is such that the distance can be derived by multiplying the corresponding staff section by 100.(This diagram is a schematic representation). Accuracy of distance measurement :10-30 cm. 

:Example

Reading on upper stadia line  B=1.829 Reading on lower stadia line A=1.603 staff section   l =B-A = 0.226 Distance =100 l = 22.6 m