41 
Initial photography of route 
Inasmuch as the route is photographed twice when the indirect procedure is 
employed, the initial photography is usually taken at a scale which is one-half 
or smaller fraction of the scale required in the second photography. As in the i 
direct procedure, each photography scale in this procedure is governed by the 1 
map scale or contour interval required, and sometimes by the relief height to | 
flight height ratio. The type of photography usable is either vertical or con- 
vergent. Vertical photography is taken where the heights of trees, buildings, Lu 
and/or relief causes critical relief height to flight height ratios in moun- WII 
tainous areas and in urban areas where buildings are tall, or where desired. I 
Convergent photography is taken where topography is only slightly rolling and/or | 
nearly level, and where buildings, timber, and other vegetation are not too 
tall, or where desired. ii 
  
First phases of procedure 
  
SA The particular phases of survey work accomplished in the first phases of 
NE: this procedure, which are comparable in detail and sequence to work done when 
JAM the direct procedure is employed, are horizontal and vertical control, plane 
coordinates, compilation of topographic maps by stereophotogrammetric methods, 
accuracy and completeness tests of maps, and correction and/or revision and 
completion of maps. In this initial map compilation, large scale, precision, H 
and minuteness are not as essential as in other preliminary survey mapping IM 
because such will be attained in the second phase of this procedure. | 
  
  
Initial engineering use of photographs and maps 
| 
| As much of the initial design work, as outlined in the direct procedure, 
| is accomplished as necessary to position properly the highway centerline and 
to compute its description. Principles given at the beginning of the discus- 
sion of the direct procedure are equally applicable throughout &ll work in this 
phase of the indirect procedure. Wherever clearing is essential before the | 
surveying and detailed engineering design work can be completed in the second MEI 
| phase, clearing limits are also established during this initial design. M 
Pic 
  
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Intervening surveys 
  
Upon completion of such intial design, intervening survey work is accom- I 
| plished. The centerline is staked on the ground. Where clearing is required, IM 
clearing limits are staked and a clearing contract is awarded. After clear- Ii 
ing is completed, the centerline is restaked. On all projects, wherever clear- IIT 
/ ing has been completed, and on all projects where clearing, although desirable, Il 
| was not done, suitable photographic targets, with adequate clearance from | 
| obscurations, are set along the staked centerline. An appropriate interval 
for placing targets on the centerline, in feet, is two times the scale, in feet 
to one inch, at which second phase photography will be taken, Or, in meters, 
1/20 the denominator of the representative fraction expressing that scale. 
Photographic targets, which will lie within the route band of topography to be I 
photographed this second time, are reset at all other places where set previ- 
ously. 
The recovery of position of permanent station markers used previously | 
and/or points of change in curvature on centerline is assured by referencing. I 
The references are set outside construction limits where they will not be dis- i 
turbed by normal use of the land. M 
  
  
  
  
  
  
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