2h 
The only ground surveying done is that necessary to establish basic and sup- 
plemental control, both horizontal and vertical, for the photogrammetric por- 
tions of the work. 
Photographic targets 
  
Whenever attempt is made to use only images of existing objects on or of 
the ground for relating details on the aerial photographs to ground surveyed 
positions during the accomplishment of supplemental control surveys, compi- 
lation of the maps, and subsequent use of the maps, especially in transfer of 
designed alinement from map to ground, frustrations and uncertainties are very 
likely to occur, causing delays and errors. To decrease such frustrations and 
to reduce systematic errors by eliminating uncertainties in relating photo- 
graphic details to ground surveyed positions, and to improve efficiency and 
effectiveness in making control surveys and in performing photogrammetric oper- 
ations, photographic targets are essential in all large scale mapping by photo- 
grammetric methods, especially in the preliminary survey stage. 
These photographic targets are placed on the ground within the boundaries 
of the route band to be photographed before any photography is undertaken. 
Some of these targets are centered on each permanent station marker, which is 
a metal pin or concrete monument centered by a metal tablet, in the basic net- 
work of existing horizontal control surveys, and on each station marker of sim- 
ilar character set in the basic survey project control tied to the network. For 
PLACEMENT OF PHOTOGRAPHIC TARGETS 
  
O.5 TO LO MILE 
0.5 TO 1.0 MILE 
TE 
HY 
PHOTOGRAP 
  
+ __ LENGTHWISE DIRECTION 
OF PHOTOGRAPHY ZONE 
  
Targets within mapping zone for control of stereoscopic models. These should be 
placed where they will be visible from any possible camera position. 
+ 
u 
Targets outside the mapping zone for correlation between separate stages of 
mapping. These must be placed in visible position within the photography zone. 
- Permanent station markers in basic control surveys for project. These station 
markers should be targeted wherever their positions are visible from any possible 
camera position. 
X = Spacing along lengthwise direction of route photography zone. This spacing in 
feet should be two times the scale of photography expressed in feet to one inch. 
For example, if the photography scale is to be 500 feet to one inch, place tar- 
gets at interval of 1,000 feet. 
Y = Spacing normal to centerline of mapping zone. This spacing should be random. 
Placement on alternate sides of the centerline of mapping zone is recommended. 
Other targets should be placed, as desirable, for certainty in photogrammetric- 
ally positioning property corners or lines, ground control station markers 
(both basic and project), and other points essential for both mapping and engi- 
neering uses of the maps.