cases where the geometric regularity of the building is in question or the photo- 
grammetric problem is to record building movements, this characteristic of archi- 
tecture can be exploited in an appropriate system of orientation. The system is 
based upon the geometric principles of perspective drawing which architects use for 
the graphic presentation of their designs and projects. 
Perspective drawing is the reverse of the photogrammetric process. Starting 
with orthographic drawings of plan and elevation of the building the architect arrives 
at a perspective drawing which is an anticipation of a photograph of the completed 
building. His geometry assumes no distortion of the perspective rays in an imperfect 
photographic objective. On the drawing which contains the plan of his building the 
architect chooses a station point, corresponding to a camera station, or, more 
precisely, to the principal point in a photogrammetric objective, and a picture plane, 
corresponding to the photographic plate. The line from the station point perpendicular 
to the picture plane corresponds to the camera axis of a phototheodolite. 
There are two systems of architectural perspective drawing. One is known as 
two-point perspective. This system assumes a vertical picture plane upon which all 
vertical lines of the building appear vertical and parallel while systems of horizontal 
lines converge to vanishing points on a horizon of the same height as the station point. 
Three-point perspective assumes a picture plane inclined from the vertical. Systems 
of horizontal lines still converge to vanishing points on the horizon, but now only the 
vertical line which is intersected by the camera axis remains vertical while all other