2 
id 
il It will be appreciated that by the method so far described 
i it was possible only to determine the depths of points below the 
li horizontal datum when they lay directly under the aircraft, This, 
il however, was possible along the tie Strips as well as along the 
Il directions of the main Strips; thus at those places where the main 
I strips were intersected by the tie strips, depths of points to the 
i ground were available on the lateral edges of the main Strips. 
lH This enabled the lateral tilt of the main strips to be removed 
| and depths to be measured down from the elevated horizontal datum 
| to all points all over the area covered by the photography. Heights 
| above sea level were then obtained by utilising one height control 
| to establish a connection between sea level and the elevated datum 
jn and subtracting this constant from all depths. 
E Observational Procedure 
| | Throughout the work the two Multiplex projectors used were 
| maintained at heights which gave rise to precisely the same magnifi- 
| cation on the plotting surface, This proved to be the same height 
| for both projectors. The observational procedure followed was to 
erect each model Scparately in correspondence without altering the 
| heights of the projectors, At this stage the scale and lateral 
tilt of the models was not considered important; but to keep subse- 
  
  
  
| quent corrections small the scale was carried forward from model to 
| model by adjusting the machine base to make the height of the back 
i principal point the same as the forward principal point of the 
| previous model, Lateral tilt on the initial model was adjusted 
approximately to topographical indications and similarly carried 
forward from model to model by reference to observed heights on 
orthagonals through the principal points. On each model heights 
above the plotting surface were observed. to selected points where 
the ground was locally flat where observations of radar alti- 
metry were available fron the A.P.R. record, In addition heights 
  
Wette tet 
  
were observed to all pass points and check control points, 
| All heighted points were plotted on Separate working sheets for 
| each model, The heights observed were then converted to depths 
| below the machine air base by subtracting them from the known height 
| of the projectors above the plotting surface. These depths were 
f ^ 
then written against the plotted points on the Working sheets. 
Scaling the models and the determination of the scaled depths 
  
Í The rest of the work was performed analytically, The first 
i | procedure was to scale the models, This was undertaken by dividing 
nM the Multiplex depths to those points where radar altimeter depths 
I | were available, into the recorded radar altineter depths, Before 
| this was done however, it was necessary to apply a small correction 
| to the radar altimeter depths as these were observed from the alr- 
  
Fn craft, The depths required were those  fror the air base, The 
I| hypsometer record provided the means for making this correction, 
Md Having determined the scale of each model by this means the multip 
| multiplex measured depths to all points in each model were 
| converted to true depths below the air base, 
ihe correction of the depths below the oir bose into depths below 
2n _clevated horizon 
i | The next step was to correct the hypsometer record by applica= 
8 tion of Henry's formula so as to provide a record of the distance of