'OMAPS 
yancil of Canada 
vey, USA 
jible to most users 
: information from 
211 as those in- 
[cally through the 
icies of height 
t the larger map 
/ed systems of ob- 
nage, combined 
information on any 
sreoscopic ortho- 
juncil of Canada. 
oreoscopic ortho- 
duced on equipment 
peed of production 
ined from large 
e 
reatments for 
of resolution in 
phasis on repro- 
all numbers of 
se used in pro- 
uch as urban maps, 
charts are dis- 
state or national 
naps 
s ~ either large 
lopment (Ref.: 
otated orthophoto 
rabia (Ref.: 
ensable. 
on (i.e., of the 
a relative sense, 
ements for semantic 
information (i.e.,.for small details to be extracted from the aerial 
photography for annótation onto the photomaps) that govern the choice 
of the aerial negative scale and thus, with a certain camera, the 
flying height. For a map 1:1,000 the semantic requirements may often 
lead to the choice of‘ a negative scale of 1:5,000 while the contour 
interval (C.I.) required is often 0.5 meter. Using photography taken 
vith a wide-angle 15 cm f.1. camera the C.I. is then 1/1500 of the 
flying height. For a map 1:50,000, however, the negative scale may be, 
perhaps, 1:80,000, and the C.I. 20 meters. Using wide-angle photog- 
raphy, the C.Í. is then as large as 1/600 of the flying height and, 
with superwide-angle photography, 1/350 of the flying height.  Further- 
more, the height accuracy required increases when the terrain becomes 
flatter i.e., the required C.I. for a map 1:1,000 of rather flat 
terrain is often 0.25 meter and for a map 1:50,000 often 10 meters. 
Generally we have that, the larger the map scale and the flatter the 
terrain, the higher are the relative height accuracy requirements. 
For extremely flat terrain, orthophoto maps may, of course, be pro- 
duced through conventional rectification, using classical optical- 
mechanical rectifiers. Spotheights and (interpolated) contours are 
then obtained either by Stereo-plotting from the photography also used 
for the rectification (Ref.: Lambert 1971), or from additional photog- 
raphy of larger scale and preferably with a larger B/H-ratio, or by 
field survey methods, (Lambert. 1971). 
Àn interesting example of using two photographic coverages, of which 
one is used for conventional rectification and the other for the ex- 
traction of hypsographic information, has been reported on in the East 
German review "Vermessungstechnik" (1967, p. 238). The application 
concerned the production of photo maps 172,000 scale, with a contour 
interval of only 25 centimeters, of some 25,000 sq.km. of rather flat 
Semi-desert areas for the design of an irrigation system, Photography 
taken with a narrowsangle 50 cm £.1. camera, 30x30 cm format, at 
negative scale 1:5,500 was rectified at 1:2,000, and a photo map as- 
sembled. Next, stereomodels of photography at negative scale 1:3,000, 
taken with a superwide-angle camera, / cm f.1. 18x18 cm format, were 
orientated absolutely to the photomap and to nine field-surveyed height 
control points per model. A dense grid of photogrammetric spot heights 
was plotted and used for interpolation of 25 cm contour lines. . 
Ine accuracy obtained was: 6 (standard error in the heights) » 5 cm; 
the cost was: 480 Rubel per Sq. km. Field compilation methods would 
have cost 750 Rubel per sq. km., would have resulted in the same ac- 
curacy, but would have taken much more time. 
In order to produce photomaps in cases where the terrain is not flat, 
or where conventional rectification cannot be applied, we have the pos- 
sibility to use commercially available "orthophoto"-praducing equipment, 
sometimes called "differential rectifiers." 
This equipment has generally been developed with the thought of the 
production of the orthophotos only, and that, as a by-product, height 
information could be extracted simultaneously as well in the. form of 
dropped lines, or dropped line segments, etc. This applies to equip- 
ment in which the profiling is done manually (such as the Zeiss Planimat- 
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