EEE f 
27 
These clearances are necessary to assure that each photographic target 
is not obscured by the "perspective layover" on the photographs of the nearest 
and tallest object, regardless of the position in which the photographic tar- 
get might appear on each of the respective photographs in the applicable flight 
lines. 
Route photography 
  
Route photography must be suitable for fulfillment of photographic inter- 
pretation and engineering mapping requirements. In the direct procedure, the 
route is photographed only once. The photography scale is governed by map 
compilation scale, when that scale, in feet to one inch, divided by the con- 
tour interval, in feet, yields a quotient less than 40, or the denominator of 
the representative fraction expressing that scale divided by the contour inter- 
val, in meters, yields a quotient less than 1,575. The contour interval gov- 
erns when the interval, in feet, divided into the map scale, in feet to one 
inch, yields a quotient larger than lO, or the interval, in meters, divided 
into the denominator of the representative fraction expressing map scale yields 
a quotient larger than 1,575. The photography scale is governed by the ratio 
of relief height to flight height, when the usual double projection instru- 
ments are used and buildings are tall and/or topography is rugged. The ratio 
of relief height to flight height is the difference in elevation between the 
lowest and highest points within the area of coverage of a stereoscopic pair 
of photographs, divided by the flight height above the lowest point in that 
area. For more details concerning the relief height to flight height ratio, 
refer to table 5. 
Vertical and convergent photographs are best suited for engineering map- 
ping requirements. Vertical photography is usable throughout all types of 
topography and land use, and especially where the heights of trees, buildings, 
and/or relief cause critical relief height to flight height ratios in moun- 
tainous areas, and in urban areas where the buildings are tall. Convergent 
photography is used where topography is only slightly rolling and/or nearly 
level, and where buildings, timber, and other vegetation are not too tall. 
Horizontal and vertical control 
  
Wherever possible and practicable, station markers in the National net- 
work of basic control surveys, as established by the U. S. Coast and Geodetic 
Survey or other competent control surveying organizations, are used as origin 
and closure for all basic project control surveys. The accuracy of basic con- 
trol for each survey project is second order or better in urban areas and in 
rural areas where topography is not rugged. In rural, rugged areas, the accu- 
racy is third order or better. The supplemental control is measured to an 
accuracy sufficient for accomplishing the mapping and other essential photo- 
grammetric measurements. Permanent station markers are used in basic control 
surveys on the project for the preservation of ground surveyed position.  Semi- 
permanent station markers are used for temporary preservation of supplemental 
control positions and photogrammetrically measured details, such as property 
boundaries, and other specific features, as required. All station markers 
serve as bench marks, particularly the permanent station markers. 
Plane coordinates 
  
In the United States, the Coast and Geodetic Survey has compiled and made 
available plane coordinate projection tables containing scale correction fac- 
tors for each of the established State plane coordinate systems.