57° 
templet 1s of course, unnecessary in areas 
which do not displav a grid pattern, it has 
been found to materially affect the results 
in areas where these patterns do exist. 
After completion of stereoscopic map- 
ping of fracture traces on each photograph, 
it is good practice to observe the photo- 
graph without stereoscope or magnification 
to ascertain if any obvious fracture traces 
have been overlooked. 
One of the reasons for stereoscopic study 
is to allow, as much as possible, the recog- 
nition of man-made linear features, bed- 
rock schistosity and lineation, outcrop- 
ping edges of dipping beds, and other fea- 
tures which should not be mapped as 
fracture traces. If there is doubt as to the 
origin of a linear feature on an aerial photo- 
graph it should not be mapped, although 
the inclusion of a few linear features which 
are not truly fracture traces will probably 
not materially affect the study, 
Several suggestions concerning the map- 
ping of fracture traces are listed below: 
1) It is not advisable to map these 
features for more than two consecutive 
hours. It has been found that fatigue (4) 
reduces the number of fracture traces 
mapped in the same area, (B) reduces the 
objectivity of the observer in mapping 
fracture traces in various directions (the 
diagonal directions are apparently favored 
by the tired observer), and (C) reduces 
the ability to distinguish between fracture 
traces and other linear features 
2) Several geologists claim that they 
can map fracture traces more efficiently 
by transmitted rather than reflected light. 
This is accomplished by placing the 
photographs on a light table. Difficulty 
may arise due to overlap, which doubles 
the photograph thickness. If it is desired 
to use transmitted light, the photographs 
will probably have to be cut. 
3) Particularly in areas where soil tone is 
an important method of mapping fracture 
traces, it has been found advantageous to 
use aerial photographs made from infra- 
red film exposed through a minus-blue fil- 
ter. These photographs emphasize soil 
moisture differences, and may thereby 
emphasize fracture traces expressed by 
soil tone. Photographs made from infra- 
red film exposed through a red filter are 
not recommended for general fracture 
trace mapping, although they may have 
some advantages when mapping fracture 
traces expressed by linear differences in 
vegetation types. 
4 PHOTOGRAMMETRIC ENGINEERING 
4) In areas of low to moderate relief, the 
effect of photogrammetric errors on frac- 
ture trace directions has not been found 
to be serious if mapping is confined to the 
central part of the photograph (an area 
whose sides are at one-half the distance 
from the photograph center to the photo- 
graph edges). However, where accurate 
trends of fracture traces are required such 
as for statistical studies, or in high-relief 
areas, it is necessary to remove the plani- 
metric errors from each photograph 
when compiling the final map of an area. 
This may be accomplished by locating the 
true position of the end points of each 
fracture trace by use of a simple (or high- 
order) plotting machine or by radial tri- 
angulation by the graphical, radial-arm or 
slotted-templet method. Because of the 
profusion of fracture traces on a photo- 
graph, a plotting machine, such as KEK 
or Mahan plotter is far more efficient than 
simple radial triangulation. The use of an 
instrument such as the Zeiss Stereostop 
allows direct mapping of fracture traces in 
correct orientation. Soll tonal fracture 
traces may be difficult to map directly with 
instruments using the anaglvph method of 
stereoscopic viewing. 
A photogeologist experienced in mapping 
fracture traces can map about ten to 
twenty-five fracture traces per :square 
mile on 1 to 20,000 photography in about 
twenty minutes. A geologist without ex- 
tensive experience in viewing aerial photo- 
graphs will map about five to fifteen frac- 
ture traces in the same area in about forty 
minutes. 
TECHNIQUE OF MAPPING LLINEAMENTS 
In addition to the illustration on Plate 
2, lineaments are well shown by Blanchet 
(1957), Mollard (1957 a, b) and Kupsch 
and Wild (1958). Lineaments, being at 
least one mile long, and most commonly 
many miles long, are best studied on 
mosaics. "Staple" mosaics, photo-index 
sheets or controlled-mosaics may be used. 
Photo-index sheets and uncontrolled ‘“lay- 
downs' may cause difficulty by obscuring 
lineaments which trend parallel to the 
photograph edges; those lineaments which 
closely coincide with the photograph edges 
are particularly obscured. On the other 
hand, care must be taken when using un- 
controlled or controlled mosaics in which 
the edges of the individual photographs 
have been ‘‘feathered.” These junctions 
between photographs may be hard to de- 
  
  
        
  
  
   
   
  
   
  
  
    
   
   
   
  
   
   
  
    
   
  
   
  
    
  
   
  
  
  
  
    
  
  
  
  
  
  
   
  
  
  
  
   
  
  
  
   
  
  
  
   
  
  
  
    
   
  
  
  
  
  
   
  
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