21 
as necessary, to attain suitable and uniform gradients wherever tilt in the 
aerial photographs affected the initial reconnaissance survey and route loca- 
tion work, so that the route will best fit the topography and land use, which 
includes the existing travelways. 
Wherever the maximum permissible grade is 
exceeded because of tilt, appropriate adjustments are made in route position 
to compensate for the initial deviation in gradients. 
This is done by changing 
route delineation on the tilted photographs and connecting changed sections 
with sections not requiring change. 
Sufficient qualitative information and 
quantitative data from field checks and stereoscopic examination of the photo- 
graphs are procured to compile adequate estimates of costs of rights-of-way, 
construction, and maintenance. 
Critical topography and land use 
  
Wherever topography and land use cause especially critical gradient and 
horizontal alinement problems, and wherever demonstration of the solution of 
the problems is vital, regardless of character of topography and/or land use, 
adequate topographic mapping is required. 
Existing basic control is utilized 
and such additional control, as necessary, is established by surveys on the 
ground to serve as origin and closure for subsequent control surveys work. 
Supplemental horizontal control is bridged by use of precision double pro- 
jection or optical train instruments. 
When double projection instruments are 
employed, a fairly accurate but approximate scale is first established for each 
stereoscopic model. 
This is done by assembling radial plot templets, prepared 
at contact print size, to basic control plotted by plane coordinates at an 
appropriate scale. 
Using such control, each stereoscopic model is then scaled 
and leveled in the double projection instrument and stereoscopic, radial plot 
templets are prepared for use at the scale at which the maps will be compiled. 
Plane coordinates for each radial pass point are determined by assembling the 
stereoscopic templets over a plane coordinate grid layout on which the basic 
horizontal control had been plotted at that scale. 
When optical train instru- 
ments are employed, horizontal bridging is accomplished stereoscopically and 
adjusted to the plane coordinates of the basic project control. 
The adjustment 
is done mathematically or graphically, as desired, for use at the map compi- 
lation scale. 
Sufficient vertical control for the mapping is bridged photo- 
grammetrically, as feasible, or is obtained by use of precise altimeters, or 
trigonometric and spirit levels, as necessary. 
The topographic maps &re compiled using precision photogrammetric instru- 
ments of the double projection and/or optical train types. 
maps are compiled is the smallest practicable for accomplishing the location 
and quantitative comparison of the route alternatives. 
Scale at which the 
The smallest contour 
interval feasible is governed by the scale of the map and the largest by the 
Generally the scales and separate contour inter- 
val utilized for such mapping are as shown in table 1. 
ruggedness of the topography. 
Table 1.—3Scales and contours related to topography 
  
  
  
  
  
  
  
  
  
  
  
Seale 
Character of Land use feet to representative Contour Interval 
topography one inch fraction feet meters 
rugged little or 1,000 or 1:12,500 or 25 or 8 or 
none 800 1:10,000 20 6 
rolling rural 800 or 1:10,000 or 20 or 6 or 
400 1:5, 000 10 3 
slightly intense 400 or 1:5,000 or 10 or 3 or 
rolling or rural and 200 1:2,500 5 1.5 
nearly level urban