along or near the main roads, it was hoped that the control parties could 
commence putting in the level control immediately this early photography was 
available, thus completing the essential data for controlling the APR runs in 
advance of the main coverage and APR flying, which would not be possible 
until September/October. Levelling along clear roads would also be faster than 
cross country, while the road itself would help considerably in the identification 
of photo- points. 
Subsequent experience proved that the country was not so difficult of access 
as had been feared at first and that it would have saved some photogrammetric 
work,with perhaps increased accuracy, if the ideal theoretical locations of the 
tie strips had been adhered to, and if the level lines had been driven straight 
across country. Against this may be set the fact that by using the roads a large 
number of additional spot levels and bench-marks were included which have 
Since proved of value in later stages of the work by providing a close check on 
the accuracy of the APR control. 
After adjustment of the APR profiles the intention was to select only those 
points along each profile which coincided with the principal points of the 35mm 
recording camera wherever these fell on even terrain and in areas of minimum 
record 'noise'. In this way it was hoped to obtain at least nine selected control 
points for the levelling of each overlap and to adopt a best mean fit in setting the 
overlap on these nine points. A further check on the accuracy would automatically 
result from comparison of the values obtained from adjoining overlaps. 
This procedure had the advantage that once the APR strips had been reduced 
the detail mapping and contouring could be distributed immediately between a 
number of plotting machines without having to wait for block adjustment of the 
Strips. As a precaution it was planned to start with the overlaps which included 
any level points established on the ground, where an absolute check was possible 
between the value of the photogrammetric height based on the setting from APR 
control and that of the ground spot height value. Subject to these checks being 
within the required tolerance the plotting of the remaining overlaps could then 
proceed more or less independently on the assumption that any serious error in 
levelling an overlap, were such to occur, would be immediately apparent when 
passing to the next overlap. It could hardly be coincidence if the error distribution 
between points obtained from three profiles run at different times (and often on 
different days) should lie within the limits of tolerance of the contouring, and 
if moreover every overlap, when set up independently, should give a satisfactory 
edge comparison with its neighbours.