5
The data needed to bring the individual orthophotographic segments
that make up the orthophotomosaic to the desired common scale and to
place them in their correct positional relationship are obtained by
stereotemplet triangulation. As the images on each orthonegative are,
within the limits of the process, registered orthographically and at
uniform scale, the information needed for the preparation of the stereo
templets can be derived directly from the orthonegatives. This approach
eliminates the need to reorient stereomodels to obtain data for aero-
triangulation.
The scanning operation is planned so that adjacent orthonegatives
will have overlapping areas. After the common areas are superimposed,
artificial pass points can be established by pricking through the ortho
negatives. The artifical marks need not coincide with discrete images.
Although the orthonegatives differ slightly in scale, this difference is
insufficient to prevent the superimposition of images in a limited area
containing the location of an intended pass point. Because many images
are observed in the superimposed area, this procedure minimizes the
detrimental effect of any local scan error in the vicinity of the pass
point. When a significant reduction from the scale of the orthonegative
to the final scale of the orthophotomosaic is necessary, a precision
pantograph is used to transfer the pass point data from the orthonegative
to the templet material.
The positions of the pass points, as established by the stereo
templet assembly, define make-line distances for use in the copy
camera, in photographing the individual sections of the mosaic at the
desired scale. The pass points, as recorded on the base sheet, furnish
the information needed to position and orient each orthophotographic
segment of the mosaic.
Mapping Applications
Orthophotographic materials have been used to great advantage by
geologists and engineers to resolve problems resulting from inadequate
map coverage in their areas of interest.3 How orthophotographs can be
used most effectively in a mapping program remains a subject for study.
Perhaps the geometric advantages of orthophotographs are not significant
to the mapmaker because the stereoscopic model is available to him. How
ever, other characteristics of orthophotographs are decidedly different
from those of stereoscopic models formed in precise plotters. For example,
an orthophotograph is a permanent record and can be reproduced on a variety
of materials. It is re&dily portable and easily stored.
Orthophotographs have been applied in map revision experiments to
evaluate maps of doubtful horizontal accuracy and to determine whether
the existing map is sufficiently reliable to serve as a base for adding
new map data. Orthophotographs have also served as a source from which
to extract new map data and as a guide to its positioning.
The feasibility of compiling the planimetry of an urban area from
a 1:24,000-scale orthophotomosaic was the objective of a recent research
project. After various possible compilation materials and several printing
techniques were investigated, a continuous-tone diazo rendition of the
orthophotomosaic printed on a white, scribe-coated, scale-stable plastic
was selected as most suitable for the purpose. The experiment established
that the direct scribing of the planimetric information on this base is
^Southard, R. B., 1958, Orthophotography--Its Techniques and Applica
tions, Photogrammetric Engineering, Vol. XXIV, No. 3, p. 443-451.