vertical lines converge to a vanishing point upon this line. There are three vanishing 
points for systems of lines parallel to the three coordinate axes. 
It is now possible - having the fiducial markings of the photogrammetric plate 
to establish the camera axis and having the seeming convergence of the parallel lines 
of the building to establish the horizon and the line of sight normal to the facade - to 
visually make a quick and close approximation of the absolute orientation of the 
elements o, ¢ and « for single photographic plates in the A 7 Autograph. 
The most useful elements of survey control upon a building should now be 
evident. First is to establish one or more unquestionably vertical lines intersected 
or nearly intersected by the camera axes. In the case of highly irregular or deformed 
structures - and when there is no wind - a weighted line can be lowered over the 
eaves from a parapet or cupola to establish a vertical line in the photography. A 
dimensioned vertical line can be established by lowering a steel measuring tape with 
prominent marking of major dimensions. For such structures as the cliff dwellings 
of Colorado, carved into and built up under an overhanging rock face, the vertical 
dimensioned lines of three measuring tapes lowered over the cliff, with measurement 
of horizontal distances between them, could provide effective survey control. 
Second is to establish the horizon. This may be done either by verifying the 
level of the horizontal joints of the building or by sighting through the levelling tele- 
scope of the phototheodolite, preferably to a corner and two facades of the building, 
and marking the height of the camera horizon with chalk at three or more points on