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PHOTOGRAMMETRIC ENGINEERING 
obtained from aerial photography have been 
few. Aerial photographs have been used to 
provide azimuth and length measurements in 
studies of the origin of the well-known 
Carolina bays (Melton, 1933; Johnson, 1942; 
Prouty, 1952; and Cooke, 1954). The ellipti- 
cal shape of the bays, expressed strikingly on 
aerial photographs, has been basic to most 
theories of origin. Measurements of long and 
short axes of the bays, which define the el- 
lipticity, have been made from aerial photo- 
graphs. Orientation of long axes, an impor- 
tant factor especially in Cooke's eddy theory 
of origin, also have been determined from 
aerial photographs. 
In a study of azimuth, frequency, and length 
measurements, Gross (1951) demonstrated 
the probability that in some covered areas 
of the Canadian Shield certain lineations very 
likely represent faults. Histograms were con- 
structed in which the lengths and frequency of 
topographic lineations, as seen on aerial 
photographs, were plotted against geographic 
orientation within 10° segments of the com- 
pass. Similarly histograms were plotted from 
a sampling of field data of strike of strata, 
schistosity, dikes, glacial striae, faults, or 
other features considered to have a bear- 
ing on the configuration of the topography. 
One of the histogram peaks of lineations plot- 
ted from aerial photographs did not corres- 
pond with histogram peaks for glacial striae, 
bedding, dikes, and schistosity as based on 
field sampling, and was found to represent a 
direction of prominent faulting. 
As a result of azimuth-frequency studies, 
Blanchet (1957) suggests that structural and 
stratigraphic anomalies in some areas may 
be located by a statistical analysis of frac- 
tures that are expressed as linear features on 
aerial photographs. The basis for analysis 
is a comparison of local deviations in the 
statistical mean direction of fracture sets 
with regional norms for each fracture set. 
Regional norms are established by plotting 
fracture-azimuth frequency. 
Lattman and Nickelsen (1958) have de- 
scribed azimuth-frequency | measurements 
from aerial photographs in a study of fracture 
traces in western Pennsylvania. Histogram 
plots of frequency and orientation of the frac- 
ture traces showed maxima closely correspond- 
ing to those of prevailing directions of joints 
as seen in the field, with the exception of one 
fracture-trace maximum which had no coun- 
terpart in exposed rocks of the area. This one 
maximum of frequency and orientation of 
traces, however, was found to correspond 
with joints in underlying coal beds. Despite 
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an overlying thickness of shale and sand- 
stone, the joints are expressed at the surface 
in a manner discernible on aerial photographs, 
but not in routine field mapping procedure. 
Several writers have used topographic maps 
extensively in quantitative geomorphic stud- 
ies, but the use of drainage density and re- 
lated measurements, employed for example 
by Miller (1953) in studying the effects of 
lithology and structure on the development 
of drainage basin characteristics and erosional 
landform, might best come from aerial photo- 
graphs because of the additional information 
present on them. 
In preliminary studies of the possible sig- 
nificance of drainage density (defined as to- 
tal length of streams within an area divided 
by the area) with respect to lithology, the 
authors obtained simple stream length and 
drainage area measurements from vertical 
aerial photographs. Stream lengths were meas- 
ured by chartometer on enlargements of 
tracings of drainage lines. Area measurements 
were made by polar planimeter where drain- 
age density was determined for specific 
drainage basins. It was found that circular 
sample areas appeared to give more consis- 
tent determinations of drainage density for 
any one rock type within any one given area 
than did samples of small individual drainage 
basins. Drainage density measurements were 
inconsistent where photographs of different 
scales were used, but this is understandable 
when it is considered that—factors other 
than scale being equal—the ability to see 
small drainage rills obviously decreases as 
scale becomes smaller. 
Figure 1 shows the relationship of drainage 
density to different photograph scales as 
determined for several different rock types 
and areas. The general agreement of slope of 
the lines suggests that a simple conversion 
factor may permit equating of drainage den- 
sity as determined from different scales of 
photography. The inconsistently steep slope 
shown for shale in Figure 1 may be disregard- 
ed because the high drainage density did not 
permit the plotting of all visible drainage 
rills on small-scale photographs, and hence à 
lower drainage density was calculated for the 
small-scale photographs than was known to 
to exist. 
The authors are currently testing different 
drainage density sampling methods in the 
hope that a line-intercept or other simple 
technique may be applied directly to the 
to the aerial photograph. 
Figure 2 shows several sample plots of 
drainage density determined from vertical