"Ce. ) as well as
ly, using image
Lid Bac erecmet,
pped lines, or
‘tour lines
precision
height errors
he human
ing floating
th the ground,
er of 6.3 ro
, Hobbie 1971).
ling, the
periments with
nal of Paris
n using the
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een used when
way.
form of dropped
3 no height
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ie dropped
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cal consic
rors-are negli-
the horizontal
equal to the
rain slope).
from dropped
ying height)
stereoplotted
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i
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a
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de
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aps), since the
om the photo-
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1:1,000.to
For the basic
ining height
story at all
instead by
©
conventional stereoplotting from the photography also used for the
orthophoto production (at air photography scale 1:13,000), or even
from special photography at larger scale (1:8,000). Also for the
South African 1:10,000 orthophoto map postes (Ref.: Lester and Malan
1971) "the contours are drawn using a stereoplotting machine as the
dropped line method is not considered dre able."
Apart from the low accuracy, dropped lines or dropped line segments
present another problem: "much time is, consumed in extracting the ap-
proximate contours from the data. The approximate contours must then
be cartographically finalized by the compilation technician to best
portray the terrain features" (Ref.: Powell 1971). "This work requires
a well trained technician and the time requirement is nearly the same
as for stereoplotting the contours in the conventional way" (Ref.:
Krauss 1971).
À practical example may serve to illustrate the economics of using con-
ventional stereoplotting instead of the rapped line system. In this
example it is assumed that grthophoros at 1:5,000 scale are to be pro-
duced on the Jena Topocart, coupled directly to the Orthophot and Oro-
graph, and that contours at 2 meter intervals are to be overprinted on
the photomap.
Using the dropped lines, the C-factor of the system would he 650, the
flying height thus 1300 meters and the negative scale 1:8,500. Plotting
contours in the conventional way, the C-factor of the system would be
1300, the flying height thus 2600 meters and the negative scale 1:17,000.
Production estimates are:
e
orthophoto instrument time* 100 x 5 hours. -.500 hrs
conversion of droplines into contours: ‚100 x 4 hours. = 400 hrs
: a 900 hrs
for 25 models 1:17,000:
orthophoto instrument time: 25 x 10 hours = 250 hrs
conventional stereo contouring: 25 x 15 hours += 375 hrs
625 hrs
From this we may conclude that apart from the much higher cost of the
aerial photography in the first case the map production time is also
significantly higher.
In this context, it is understandable that the most recently developed
new orthophoto equipment, such as the Wild Orthophoto-attachment to the
A8, the new U.S.G.S. system of Profiler-Autoline-Orthophotomat, and the
Hobrough Gestalt Photomapper, do not incorpxate devices for the simul-
taneous production of dropped line data.
Several instrument manufacturers are seeking improvement of the equip-
ment and of the systems to overcome the difficulties of lower accuracy
and longer contour compilation time of the dropped line systems. Zeiss-
rm