Page 250 - Civil Engineering Formulas

184 CHAPTER SEVEN

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 (7.28)
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.

PHOTOGRAMMETRY

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

Photo scale photo distance
(7.29)
Map scale map distance
ab f
Photo scale (7.30)
AB H h 1
where f focal length of lens, in (m)
H flying height of airplane above datum (usually mean sea level), ft (m)
h elevation of point, line, or area with respect to datum, ft (m)
1