THE MOSAIC
(FIGS. 13 AND 22)
In its simplest form, the mosaic is made by mounting overlapping prints
so that the corresponding details coincide. This type of mosaic is quite
adequate for relatively small areas or where a high degree of accuracy
is not required. For larger areas and greater accuracy, an accurate
outline map is used as a base upon which the prints are mounted so that
recognizable features coincide with their location on the map. When the
prints are properly arranged, the better print of each overlapping pair
is selected, the excess paper removed, and the whole mounted and
photographed. Figure 13 is left untrimmed to illustrate the method of
matching the overlapping prints. The differences in shade are due to
difference in printing and developing the pictures which make up the
mosaic. The slight offsetting of line at the junction of the prints may
be due to errors in mounting, shrinking, or stretching of the
photographic paper, tilting of the camera at the time of exposure, or
other cause. Such errors and imperfections illustrate the difficulty of
using these photographs in the making of maps.
A skillful manipulation of both airplane and camera is necessary to the
success of the mosaic. To prevent distortion and variation of scale, the
camera must be maintained at the same altitude at all times and pointed
directly downward. This can be accomplished by flying with an even keel
at a uniform altitude. Mechanical devices are also being perfected to
accomplish the same result. Still greater skill is necessary when
consecutive rows of exposures are made for the purpose of placing strips
of photographs side by side to cover a large area. It is difficult under
the varying conditions of wind and weather to fly so evenly and so
nearly at the same level that distortions and differences in scale are
not noticeable. Strong objection to the mosaic is frequently raised
because of inaccuracies due to difference in scale in neighboring
prints. Until these defects are overcome, such a group of matched
photographs cannot take the place of an accurate map. Much, however, is
being done to correct these defects, and, even in photographs where
inaccuracies in scale are many, the value of the photograph for the
portrayal of detail cannot be denied.