A
photo theodolite is a form of ground camera. It is a combination of
camera and theodolite and is used for taking photographs and measuring
the angles which the rival plane of collimation makes with base line.
Both the theodolite and the camera rotate about a common vertical axis.
The instrument is used for terrestrial photogrammetry

It
should be noted that the pointing of the theodolite is completely
independent of that of the camera, but the horizontal circle, which is
located on the top of the camera housing, is fixed in such a way that
when the circle reading is zero, the optical axes of the theodolite and
camera lie in the same vertical plane. This means that all horizontal
directions observed with the theodolite can be easily related to the
principal point of the photograph.

Not
only is the terrestrial camera useful for mapping construction sites at
scales as large as 5 ft. to 1 in., but the photographs can be utilised
in a suitable instrument for taking off quantities for earthworks or
stock-piles and for directly plotting tunnel profiles and other varied
uses. At the other extreme the photo-theodolite can be employed for
mapping at small scales and even for extending control.