graphs and perhaps extending across into photographs of adjoining flights. 
Large linear features lying radially are wonderful clues in mass identifica- 
tion. Highway tangents and relatively straight sections of large streams, for 
example, are more easily identified than shorter objects of equal width. This 
is true many times when smaller or narrower features are concerned; a path 
will be visible whereas a building nearby will be hidden even though the 
building presents a much wider frontage. This technique of identification is 
especially useful on obliques where the scale falls off very rapidly as the dis- 
tant horizon is approached, and wherein the cartographer must be able to 
visualize the existing perspective and first recognize the large group of fea- 
tures. At times, two opposite perspectives are involved and the visualization 
factor is doubly depended upon to identify common areas and to select from 
them common features as control stations. The problem requires special 
photo-interpretation ability in the application of the mass-area technique, but 
efforts are being made to ease the burden through instrumentation. 
7. There is another special problem in the use of tri-metrogon photo- 
graphy calling for a degree of photo-interpretation ability in its solution. I 
refer to horizon identification in both obliques of a photographic unit which 
is necessary to ensuing calculations for cross-tilt determination. In effect, 
the apparent horizon is identified and the true horizon is then computed and 
plotted on the oblique in proper relation to it. Horizon areas appear in many 
forms on the photographs due to variations in atmospheric conditions and in 
geologic formations. Mountain peaks near the horizon, for example, pose a 
problem, but more often it is the presence of haze or low lying clouds that 
must be surmounted. This "pseudo" horizon may be at various levels above 
it, but in one or more photographs of a flight a clue will be found whereby 
the apparent horizon can be positively identified. It is then a matter of 
carrying it through the flight, both sides. Not overlooked is the balancing 
effect derived from computations through the interlocking angles of each 
composite set of exposures; the true position on one side can be computed 
from the photo-identified position on the other. This serves as a double check 
on both identifieations, and the flight strip is left in balance, so to speak. 
Longitudinal tilt has not been included in this discussion since positive iden- 
tification of the horizon is not necessary in its determination; it can be meas- 
ured directly on any of the parallel lines in the vicinity of the imaged horizon. 
However, the location of the horizon is definitely required for cross-tilt de- 
termination and photo-interpretation definitely has a hand in the technique 
involved. 
8. Background knowledge of a given area is valuable in most photo-in- 
terpretation problems, anywhere, in any mapping or charting endeavor. This 
is especially true in aeronautical charting where it exists to a greater degree 
perhaps, than in the work of many other organizations. This statement is 
made for two reasons; first, because of the extreme size of our area of chart- 
ing endeavor, and second, because charts are meant to portray the earth’s 
surface for use in the air rather than for use from a ground point of obser- 
vation. In the first instance, our cartographer cannot be expected to be 
acquainted with the living and working habits of all the peoples of the world, 
nor can he be expected to be familiar with the varying geography of the 
world through environment and association. Secondly, background informa- 
tion is not only valuable in feature identification, it is valuable in feature 
presentation on charts where it must appear as near as possible like the fea- 
ture it represents. Cartographers in volcanic countries have understood the 
peculiar terrain forms there since childhood, and the same is probably true 
in the desert lands, for example. The Air Force cartographer must be famil- 
iar with all the existing variations in landforms and natural or manmade 
phenomena; he must make a thorough study of the subject in order to be 
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