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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