In other cases the advantage of stereophotogrammetric plotting at the first order
instrument is in seeing the distortions of architectural and photogrammetric geometry directly
revealed during instrument movements in the coordinate system established for the optical model.
Often - even after prolonged orientàtion of the instrument - these distortions are not insignificant.
They may disappear within the standard error acceptable in the drawing of large historical
structures at relatively small scale, but they have been seen, and error has become a geometrical
reality rather than a mathematical abstraction. It is a condition to attack, for there are situations
which demand distinguishing between the deformations of architectural and photogrammetric
geometry.
The bowing outwards of the top of a wall in an architectural optical model - with minor
influence on the vertical dimensioning of a facade - may indicate uncorrected divergence of two
camera axes in the exterior orientation, the bowing of a photogrammetric plate, or the actual
progressive inclination outwards of a wall pushed by expanding and contracting roof trusses which
can ultimately drop within the displaced wall structure. An actual deformation of architectural
geometry may have serious meaning; it is always of interest to the architect, who must find
photogrammetry inadequate if it leaves him in doubt of the actuality of architectural deformations
he has seen.
Figure 1. Camera Test Field, 1960, Professor Hallert's Institute, Royal Institute of Technology, Stockholm, Sweden.
The deformations of the test wall in stereoscopic view are due to lack of film flatness in wide angle photography.
Figure 2. Gothic Cathedral at Linkóping, Sweden, 1964. The actual outward lean of the piers and wall may be caused
oy the thrusts of the vaults and the original yielding of tension members of the roof truss before the addition of exterior
uttresses.