Future trends
Looking into the immediate future, the
French authors expect that both projection and
optical-mechanical plotters will be equipped
with digital acquisition systems. Plotters will
be grouped in series and work in a time-sharing
mode. For long-term development, analytical
plotters will be used more and more, and the
tendency will be toward completely digital
mapping systems, undoubtedly increasing car-
tographic productivity. However, managers
will have to find and train personnel to man
the automated systems.
JAPAN
Digital photomap
In an article by a Japanese author (Kamiya,
1972) the concept of the digital photomap was
introduced, consisting of a pair of photographs
on which planimetric distances and elevations
on the ground are indicated digitally. It was
proposed as a substitute topographic map for
civil engineering design. The steps in produ-
cing a digital photomap are :
1. Comparator and computer are used for
aerotriangulation to obtain the orientation ele-
ments.
2. Photogrammetric plotter is used to record
a grid of point elevations.
3. Grid coordinates are projected on the pho-
tograph for measuring photocoordinates.
4. Automatic plotter is used to plot the pho-
tocoordinates of the grid points.
5. A rectifier printer is used to superimpose
the plotted points on the photographs.
The author emphasizes the point that by
putting the digital information directly on the
photographs the need for a contour map is eli-
minated.
NETHERLANDS
Optimal production of digital terrain models
Several significant investigations in the Ne-
therlands have been reported in the I'TC Jour-
nal. One interesting development (Makarovic,
1973) is a proposed procedure called « progres-
sive sampling » for optimized production of di-
gital terrain models. The procedure is started
by. measuring heights. on a low-density point
grid in a stereomodel. The data are analyzed by
an on-line minicomputer, which in turn gene-
rates the locations of new points to be sampled
in the next run. Then the computer stations
the tracking device accordingly ; the differen-
tial height setting on each point is controlled
by either the operator (manual) or the corre-
lator (automated). The procedure is repeated
in successive sampling runs until the local den-
sity of the point grid is matched with the ter-
rain roughness.
'The requirements for storage capacity of the
computer can be substantially reduced by ope-
rating patchwise. The size of a patch can be
adapted to the terrain roughness and the speci-
fied accuracy. It is usually selected to be smal-
ler than the field of view of the instrument eye-
piece. In a preliminary test case, coherent sam-
pling data (regular grid) was shown to be less
adequate than progressive sampling, too high
in some areas and too low in others ; 54 unne-
cessary points were observed, and 113 points
were missed. In progressive sampling of the
same area, only one significant point was mis-
sed.
Off-line orthophoto system
Another proposal (Makarovic, 1973b) con-
cerns a conceptual off-line electronic orthopho-
to system. The system would operate in two
phases, preparation and printing, both of them
automatable. The choice between optical and
electronic image transformation is related to
the model of printing. In the continuous prin-
ting mode, image transformation need not be
rigorous because the upper accuracy limit is
imposed by the dynamic errors. Thus transfor-
mations, such as simple scale change,. similari-
ty, and affine, have been used in optical ortho-
printers. To maintain accuracy during conti-
nuous scanning, strip widths should be adapta-
ble. The stationary printing mode, on the other
hand, entails no dynamic errors and allows for
larger areas to be printed with more rigorous
transformation. This is done more effectively
by electronics because optical means are not sui-
table for nonlinear transformations.
In the first phase (preparation) of the pro-
posed system, the computer generates data to
control the second phase (printing). From
DTM data, the elevation of the center of a
