The original glass negatives survived, as did the photographer 
and the lens which he had used, Diapositives and prints were 
made from the original negatives, The Goertz 300mm objective was 
subjected to laboratory calibration and the principal distance of 
302.2mm confirmed the independent field calibration of the photo- 
graphy, Variations in radial lens distortion over the area of 
interest amounted to 80mm; there was no significant tangential 
component, 
Two photographs were chosen as the basis of measurement, One 
of these was a view from due south (Fig.1) and the other from the 
south west, The camera axes intersected at an angle of about 30° 
within the area of the dome and the scale of the pictures in the 
plane of the dome was about 1:300. Stereoscopic fusion was 
impossible but a wealth of fine detail assisted simultaneous 
monocular pointing in a Zeiss (Jena) 1818 stereocomparator, 
Control points were co-ordinated on the surviving parts of the 
south elevation though, of course, the derivation of information 
in the area of the dome involved extrapolation, It was possible 
to re-establish the positions of the original camera stations and 
to set up the projective transformation between the real photo- 
graphs and the artificial photographs (2), Thompson (1962) 
summarised: 
"The method consists of obtaining the co-ordinates (X,Y,Z) of 
four points on the existing part of the building by a ground 
survey and transforming these to accented co-ordinates (x',z') by 
«= FX. (8 f..z. 
y ^ y 
x, = f.X-Bx ! sj. Z- Ba 
— Ja y 
where By and B, are base components and f is the chosen principal 
distance, The unaccented (photo-)co-ordinates of the same points 
are measured in a suitable instrument and these data are used to 
set up four pairs of equations of the form 
x’ = Ay + A,23 + a; 
Ag Xx +A3,3 + Qgzq 
  
----- (0) 
3 s AX +4333 + 4; 
Azıx + 03,3 + Ay 
  
in which the matrix Au ^u s 
Ay Fan as 
Q5, Qs, s, 
is non-singular and from which the eight ratios of the 
coefficients to 8 are calculated, Two sets of coefficients are 
obtained, one for each picture of the pair, To obtain the space 
co-ordinates (X,Y,Z) of any point whose images appear on the two 
(2) This work appears to be an early practical demonstration of 
the application of projective transformations, 
33