"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 
nterval should 
een used when 
way. 
  
  
form of dropped 
3 no height 
by distances 
ie dropped 
ap of t 
cal consic 
rors-are negli- 
the horizontal 
equal to the 
rain slope). 
from dropped 
ying height) 
stereoplotted 
actors’. of 
<r 
/ 
- t. 
1i 
3. 
0 
i 
4 
T) 
2 
a 
lC 
de 
Jjora- 
too serious 
re relatively 
e following 
a larger con- 
aps), since the 
om the photo- 
photographic 
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