4 
camera drift sight. Looking through the drift sight, the pho 
tographer identifies the objects on which the readings are to be 
taken. The two contact lead wires from the cell are brought 
into the aircraft so that the recording head of the meter is at a 
convenient angle for reading by the photographer. Five or six 
readings are taken at the beginning of the flight line and re 
ferred to the exposure table to determine the correct f-stop 
number. Continuous observations are made along the line and 
the f-stop adjusted accordingly. Extreme care is taken to pre 
clude erroneous readings due to clouds beneath the aircraft and 
cloud shadows on the terrain. Haze conditions are compensated 
for by application of an additional correction to the aperture 
determined from the table. Thus, whereas the absence of haze 
may allow an f-stop setting of 8, the presence of haze will re 
quire a smaller f-stop number such as f-11. 
All pertinent information on meter readings, camera 
settings, terrain features, etc., is recorded in a flight log and 
returned with the exposed film to the laboratory. (2) 
Color Film Bases: The stabiltiy of film bases for color 
photography has until recently been approximately equal to the 
stability of the standard topographic bases used for panchro 
matic photography. We have found that the standard error of 
position of photograph images amounts to 14 microns for optical- 
mechanical projection instruments and 11 microns for numerical 
photogrammetry when 4 side and 4 corner camera fiducial 
marks are used to determine and compensate for film distortion. 
We have also tested the newer polyester base panchromatic 
films under standard conditions of processing, drying, and 
storage, and have found the comparable standard errors of 
position to be 12 microns and 6 microns, respectively, for 
optical-mechanical and numerical photogrammetry. We have 
recently received samples of both Ansco and Eastman Kodak 
color aerial films on polyester type bases and stability tests of 
these films are in progress. 
We think that these modern film bases, when controlled 
through the use of 8 fiducial marks for numerical photogram 
metry, will produce strip and block aerotriangulations with 
accuracies essentially equal to those produced with wide-angle 
glass plate cameras. 
Weather for Color Photography: We have had a number 
of questions as to the effect of aerial haze on color photography. 
Figure 12 at the back of this paper is one answer to the question. 
It has been our experience that when cloud and aerial haze 
conditions are satisfactory for panchromatic aerial photography 
they are also satisfactory for color photography. There is a, 
relatively rare, marginal aerial haze condition under which more 
satisfactory photography can be taken through the use of the 
standard yellow or minus-blue filter generally used for panchro 
matic photography. However, if this same filter is used for 
color photography under these same meteorological conditions, 
minus-blue or 2 color aerial photography will be produced which 
is superior to panchromatic photography both in ground reso 
lution and interpretability. 
Sun Spots: Our aerial photography for nautical charting 
purposes must be taken in such manner as to minimize the loss 
of details on the photographs because of the reflection of the 
sun from the water surfaces. The preparation of a diagram simi- 
(2) Color—A New Dimension in Photogrammetry, by John T. Smith, Jr., 
Photogrammetric Engineering, November 1963. 
lar to that shown in Figure C is a standard flight planning 
procedure with the Coast and Geodetic Survey. It is prepared 
by plotting the trace, of the image of the sun at photograph 
scale for the latitude and longitude of the area and the approx 
imate date of the planned photography. The Air Almanac and 
Sight Reduction Tables for Air Navigation are used to compute 
points on this curve. The trace of the sun’s image frequently 
passes near the center of the photograph at noonday making it 
necessary to photograph before 0800 or after 1600. The half 
ellipses, which show the probable area of sun-spot flare, are 
centered on the image of the sun at two-hour intervals begin 
ning at noon and are defined as the line of intersection between 
a horizontal surface and a circular bundle of rays which is 7 
inches in diameter. The diameter of this bundle of rays is a 
function of the smoothness of the water surface. If the water 
surface were as smooth as a mirror, the bundle would have a 
diameter of only 25 microns at photograph scale. On a calm 
day a photograph of sheltered harbor water will image a sun 
spot flare about IV2 inches in diameter; on a windy day a photo 
graph of open sea along the coastline would image a sun spot 
nearly 7 inches in diameter. After the diagram is prepared for 
a particular area, the time of day of photography and the 
layout of flight lines are arranged to reduce the sun spot to a 
minimum. 
N 
Fig. C Sun spot study for Charleston Harbor, S.C., for 
April 1962, showing the trace of the reflected image of the 
sun on the water surface during the day of photography. 
Figure C 
Laboratory Photography 
Processing Color Film: We have for some time recognized 
the need for better control of the chemistry in processing color 
film and have recently started printing a “step-wedge” onto 
each roll prior to processing. This, together with the use of a 
standard Beckman Hanes pH meter, provides the principal con 
trol for processing. 
The “step-wedge”, prepared by Eastman Kodak Company 
for use with the Kodak Model 101 sensitometer, consists of 6 
colors (red, blue, green, magenta, cyan, and yellow) and a grey 
scale combined in 21 steps, each step being equal to V2 stop 
graduation in exposure. A Wratten No. 49 and the 0.8 neutral 
density filters are used in the sensitometer to give the required 
color temperature and exposure density.