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PHOTOGRAMMETRIC ENGINEERING
Sor
Rock unit A
‘’ Rock unit B
REFLECTANCE
%
o
L 1 l 1 J
400 450 500 550 600 650 700
| l
WAVE LENGTH IN MILLIMICRONS
FrG. 3. Spectral reflectance curves of weathered samples of red, thin-
bedded shaly siltstone (4) and gray limestone (B).
landform characteristics that may be geo-
logically significant.
SPECTROPHOTOMETRY
The objective of spectrophotometric stud-
ies undertaken to date in geologic work is to
provide data that would allow photographing
the terrain so that different features, gen-
erally rock formations, would be emphasized
by tonal contrasts and hence easily measured
or mapped on aerial photographs. A method
of photographic differentiation of rock types
would speed up basic geologic mapping, which
is fundamental in much geologic research.
Spectrophotometric studies yield data that
may permit the selection of film and filter
combinations for differentiating mappable
rock units on the resultant photographs.
Spectral reflectance curves for several rock
types in New Mexico are shown in Figures 3
and 4. Results were obtained using a Bausch
and Lomb Spectronic 20 colorimeter with
color analyzer reflectance attachment. Re-
flectance readings were referenced to a white
porcelain standard calibrated in turn by ref-
erence to a white magnesium carbonate
block.
In Figure 3 it is clearly evident that strong
tonal differences between rock types should
exist if only the short or long end of the spec-
4
trum is recorded in photographing these rock
types. It is interesting to note that the wave-
length range of 500 to 675 millimicrons,
which represents the ‘normal’ vertical aerial
photograph, will not permit strong tonal dif-
ferences to be visible because light-reflectance
differences will be balanced out.
Figure 4 shows spectral reflectance data for
four rock types. Here the curves for rock
units A (light-brown sandstone), B (gray
limestone), and D (gray sandstone) are close
together at the short (blue) end of the spec-
trum; these rocks reflect more light than rock
unit C (red shaly siltstone) they thus will
photograph light in tone as contrasted to
rock unit C—if only the short end of the spec-
trum is recorded. Similarly, rock units B, C,
and D will photograph dark in tone in con-
trast to rock unit À if only the long end of the
spectrum is recorded. On the normal aerial
photograph little tonal distinction of the
various rocks can be expected.
Figures 5 and 6 show samples of rock units
A (oldest), B, C, and D (youngest) as photo-
graphed at the short and long ends of the
spectrum, respectively, and demonstrate that
for rocks in the area tested it would be be pos-
sible to design film-filter systems that would
permit easy differentiation of rock types.
Figure 5 was taken with a combination of