In stadia surveying, a transit having horizontal stadia crosshairs above and below the central horizontal crosshair is used. The difference in the rod readings at the stadia cross hairs is termed the rod intercept. The intercept may be converted to the horizontal and vertical distances between the instrument and the rod by the following formulas:

H= Ki(cos a)² ( f+c) cos a

V=0.5Ki(sin 2a)+( f +c) sin a

where
H= horizontal distance between center of transit and rod, ft (m)

V =vertical distance between center of transit and point on rod intersected by middle horizontal crosshair, ft (m)

K =stadia factor (usually 100)

i =rod intercept, ft (m)

a =vertical inclination of line of sight, measured from the horizontal, degree

f+c =instrument constant, ft (m) (usually taken as 1 ft) (0.3048 m)

For horizontal sights,the stadia distance ft(m) can be calculated as:

D=Rf/i+C

where
R= intercept on rod between two sighting wires, ft (m)

f =focal length of telescope, ft (m) (constant for specific instrument)

i distance between stadia wires, ft (m)

C= f+c

c= distance from center of spindle to center of objective lens, ft (m)

C is called the stadia constant, although c and C vary slightly. The value of f/i, the stadia factor, is set by the manufacturer to be about 100, but it is not necessarily 100.00. The value should be checked before use on important work,
or when the wires or reticle are damaged and replaced.