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HYDROGRAPHIC APPLICATION OF PHOTOGRAMMETRY U.S.A.-3
methods. Even if topographic maps are made from nine-lens photographs,
horizontal aerial triangulation is performed by the use of transparent, vinylite,
line templets; also planimetric maps are nearly always compiled graphically,
although a steadily-increasing number are being compiled by instrument. Single-
lens photographs are usually compiled by stereo instrument, whether for topo-
graphic or planimetric mapping, although the latter, also, are sometimes com-
piled graphically.
Two Stereoplanigraphs, two Reading nine-lens contour plotters, two Kelsh
plotters, and one Stereotop are used in the Washington Office. In a nearby
field office are two more Kelsh plotters and five Multiplex units. Two other
field compilation offices each have a Kelsh plotter. Seven Focalmatic desk ratio
projectors are used, some in chart revision work, and others by field units.
Prior to the extensive use of photogrammetry, shoreline data and control
stations for hydrography were surveyed and located by planetable [3]. These
surveys were usually made by a combined operations party as the work in-
cluded geodetic triangulation, planetable surveying and hydrography. Today,
photogrammetric mapping usually precedes hydrography and supplies both the
shoreline data and control for the hydrography. Hydrographic signals are
identified on photographs and located photogrammetrically on board the hydro-
graphic ship. A special set of photographs is used and the method of location is
by the radial line technique using a transparent copy of the photogrammetric
map provided beforehand for this purpose.
Coastal mapping and inshore hydrographic surveys are nearly always co-
ordinated and planned so that aerial photography is obtained the season before
the hydrographic survey is scheduled. Geodetic control is assumed to follow
aerial photography. In any event, the control is identified on the photographs,
after which a map of the shoreline is compiled in an office. All possible detail
which might be of assistance to the hydrographer is compiled. The high-water
line is shown, with the aid of notes made here and there on the photographs
during a field inspection. Contours are plotted if the topography is helpful to
navigation. All landmarks are shown, such as towers, steeples, tanks and chim-
neys. All rocks, reefs, sand bars, and channels visible on the photographs are
denoted. And, finally, of possibly greatest importance, a series of identifiable
objects along the shore are located in latitude and longitude by photogrammetry
with all possible accuracy. The hydrographer can then find these objects, erect
signals at their sites, and use the signals for sextant sighting, or he may select
other sites, identify them on the photographs and determine their positions
photogrammetrically. Thus, the hydrographic operations can proceed without
delay and need not wait for the progress of ground surveys.
As an interesting alternative in this connection, the geodetic work can be
delayed if time is an important factor, for a planimetric map at some consistent
scale can be compiled without control. The hydrography can also be performed
at this unknown scale. Later, when the geodetic work is completed, a final,
accurate chart can be compiled showing all planimetry, hydrography and topog-
raphy in their proper relationship. In a few instances, the photography was ob-
tained, the preliminary map compiled and sent via air to the ship, the hydrog-
raphy survey was made, and geodetic work completed, all in the same season.
The Photogrammetry Division cooperates in every way in these instances.
Sometimes a photogrammetrist is assigned on board the ship to help coordinate
the work. Frequently, a photogrammetric shore party operates ahead of, or in
conjunction with, the ship building signals and adding notes to the photo-
graphs to aid the compiler in the correct interpretation of rocks and high-water
line.
Another phase of this work consists of the presence of one or more photo-
