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Figure 1 - Revision accuracy for water areas.
Two outlines of the reservoir were obtained. One based on 2000! /in photography, Oct. 1971 and
one based on ERTS imagery taken August 1972.
The discrepancies between the two outlines are shown. The solid black shows where the ERTS
“outline gives a "higher' water-level, the double lines show where the ERTS outline was "lower".
In some places the discrepancy is an obvious shift of detail, rather than a water-level difference.
for aerial photography, so several years may easily elapse between the formation of large lake and
its appearance on a map. Long before this, the lake has become a landmark feature for pilots.
ERTS images provide a means of getting these features on aeronautical charts, where at a scale
of 1:500.000 the difference in outline of a partially filled or totally filled reservoir is academic,
and not nearly so confusing to the pilot as finding himself over a huge lake when his map tells him
the nearest lake that size is many kilometers off his course.
Coastline and Islands - The recent dramatic increase in interest and activity in the Arctic Islands
due to oil and gas developments imposes its demands for mapping. Efficient and safe air and sea
navigation require the accurate depiction and positioning of the myriad of small off-shore islands.
In the vicinity of large sediment carrying rivers such as the MacKenzie shorelines change in subtle
manners. This area is not easy to map because the photographic season is short and cost of esta-
blishing ground control is high. ERTS imagery can show shoreline changes, pin-point islands (or
lack of them) that have been erroneously charted, and perhaps even aid in the positioning of islands
separated from the nearest ground control by large water gaps.
Relief shading - Portrayal of the character of the ground by means of relief shading is an integral
part of the World Aeronautical Chart Series. At a scale of 1:1 000 000 contours are not effective
for this purpose, although they are the basis from which the person doing relief shading works. He
must supplement his information of the topography with aerial photography. When working at a scale
of 1:1 000 000 this involves looking at hundreds of photos. It has been found that ERTS images, par-
ticularly winter ones with snow on the ground can be very helpful in picking out the minor topogra-
phic features which give character to the ground - the fault lines, the folds, the ridges - which con-
tours fail to portray. It is additionally helpful that the scale of the ERTS image matches that of the
maps being compiled.
Conclusions
For a large country with many small scale maps to keep revised, ERTS imagery can be-
come a useful source of revision information. Features such as roads, reservoirs and transmis-
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