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.
