infrared region of the spectrum, whereas cultural features such as roads, imaged against a great
variety of backgrounds, appear most clearly recorded in visible range of the spectrum. Thus, the
ERTS imagery bands found most useful for providing map revision information have been the red
band 5 (0.6 - 0.7 um) for roads and similar man-made features and the infrared band 6 (0.7 - 0.8 um)
for water features.
Revision Information Contained in ERTS Imagery
The imagery is recorded from an altitude of 900 km and is reproduced at a scale of
1:1 000 000. These facts alone limit the amount of information contained in the imagery of a type
that is normally plotted on a map. However, when one considers the vastness of Canada, and the
fact that 1:250 000 maps are the largest scale available for slightly more than half the country,
there are in fact a number of features which can be usefully detected in the imagery and used for
revision purposes.
Roads - The location of roads in remote areas is difficult to determine accurately from anything
less than vertical photographic coverage that can be linked to surrounding map detail. Only the
most general of plans ever exist for these roads ; they are adapted to the terrain as they are cons-
tructed. On an ERTS band 5 image, a new road through a wilderness area is often highly visible
and tests have shown that it. can be transferred to the 1:250 000 map with adequate accuracy by
local fit to map detail. In fact in one test, the position of a road determined from the ERTS image
was better in one section than that obtained by using large-scale aerial photography. In the latter
case, there was insufficient mapped detail to control the azimuth of the photos and the road became
displaced by 1 500 m. The broader view of the ERTS image uncovered this error.
New access roads are eventually photographed, and map revision proceeds from the lar-
gest scale to the smaller scales. However, often before a road is even in use, let alone photogra-
phed, it has become a landmark feature that should appear on an aeronautical chart. The roads
that have been built to the James Bay power project in Quebec and to the Reindeer Lake project in
Manitoba are examples of new roads extending for hundreds of kilometers through uninhabited ter-
ritory. The ERTS images of these roads have been used to revise the 1:500 000 aeronautical charts
of the areas long in advance of the normal revision methods that will be used to put the roads on
larger scale maps.
Power Transmission Lines - The same hydroelectric power developments that result in roads
being forced into remote areas also result in power transmission lines being erected to feed the
power to the southern urban areas. The swath cleared through the bush for these lines is a linear
feature which, particularly when snow is on the ground, is highly visible on ERTS imagery.
Hydro-electric lines are rarely surveyed in wilderness areas, yet they form a most dis-
tinctive landmark feature, and may be the only man-made feature on some maps. The ERTS image
provides the best means of locating these lines for revision of 1:250 000 topographic maps and
1:500 000 aeronautical charts. This type of revision has been done on 1:250 000 sheets for the power
transmission line between Churchill Falls and Sept-Iles in Quebec.
Reservoirs - Associated with each new power project is a reservoir. These reservoirs change
former river beds into lakes, and often combine scores of small lakes into a single large lake.
The reservoir that formed behind the Kettle Rapids Dam in Manitoba was used to test the
accuracy with which the outline of the reservoir could be obtained from ERTS imagery. Although
revision had been done for the 1:50 000 map sheets using larger scale aerial photography, the
1:250 000 map sheet was not yet revised. Two revision outlines for the reservoir at this scale were
therefore prepared. One was derived from the 1:50 000 maps, anda second was derived from a
band 6 ERTS image of the reservoir obtained a year after the revision photography. The two out-
lines differed very little as is shown in figure 1.
The ERTS image used in this instance was one without any geometric corrections for earth
rotation, so that some of the displacement errors in islands apparent in the test, would probably
be eliminated with current imagery corrections.
Reservoirs, like roads are always photographed after completion to provide the necessary
map revision information. The map-maker wants to wait until the reservoir is completely filled
before he requests photography and starts revision. Canadian weather is not always cooperative
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