CIVIL ENGINEERING FORMULAS

184 CHAPTER SEVEN

whereRintercept on rod between two sighting wires, ft (m)
ffocal length of telescope, ft (m) (constant for specific instrument)
idistance between stadia wires, ft (m)

Cfc (7.28)

cdistance from center of spindle to center of objective lens, ft (m)

Cis called the stadia constant, although candCvary 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.

PHOTOGRAMMETRY

Photogrammetry is the art and science of obtaining reliable measurements by
photography (metric photogrammetry) and qualitative evaluation of image data
(photo interpretation). Itincludes use of terrestrial, close-range, aerial, vertical,
oblique,strip, and space photographs along with their interpretation.
Scale formulas are as follows:

(7.29)

(7.30)

where ffocal length of lens, in (m)
Hflying height of airplane above datum (usually mean sea level), ft (m)
h 1 elevation of point, line, or area with respect to datum, ft (m)

Photo scale

ab

AB



f

Hh 1

Photo scale

Map scale



photo distance

map distance