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REVISION DES CARTES
THE UTILIZATION OF SATELLITE IMAGERY FOR MAP REVISION
E.A. FLEMING
Surveys and Mapping Branch
Ottawa, Canada
Introduction
Canada's mapping organization faces a tremendous task compared to similar organizations
in other countries. The area to be covered is vast and the funds available for mapping are compa-
ratively small. Full coverage of the country is at present available only at scales of 1:250 000 and
smaller. Complete coverage at a scale of 1:50 000 cannot be available before about 1995 if revision
of existing maps is to be maintained at an acceptable standard, for, implicit in the production of
new topographic maps, is the commitment to maintain the currency of the information depicted on
existing maps.
Revision cycles have been set for Canadian maps which reflect the varied nature of the
country and the fact that urban areas would require revision more frequently than maps in the Arctic.
About 8 % of the country can be classified as urban-suburban areas requiring an 8 year map revi-
sion cycle ; rural and recreational areas make up 32 % of the area for which a 15 year revision
cycle is appropriate and the remaining 60 % of the country can be classified as a wilderness area
that has been assigned a 25 year revision cycle. But wilderness areas remain wilderness areas
only so long as oil, mineral or hydroelectric power developments do not occur in the area. Al-
though exploration in Canada depends largely on airborne and seaborne support in the northern
areas, when a development occurs, overland routes must be established. These routes traverse
hundreds of kilometers of often uninhabited territory in order to link the new development with the
communication routes of the south and in so doing they create an immediate need for map revision
regardless of the theoretical revision cycle of a particular area. Such a road, railroad, power line
or pipe line may be the only man-made feature on a map sheet, and any map user rightly expects
tofindit portrayed there. .
The revision information for these routes comes from many sources, but obtaining re-
liable information in a form that can be related to an existing map is not always easy. Provincial
governments at times forget to report the building of development roads ; or roads are reported
complete with construction plans, but it turns out that the construction crews had to leave the pro-
posed route to avoid muskeg, or to find road building material. Delays are encountered in obtaining
new photography of the feature, or only partial information is obtained.
A new source for this revision information has been imagery from the Earth Resources
Technology Satellite (ERTS-1).
Satellite Imagery
The imagery from the Earth Resources Technology Satellite, launched by the National
Aeronautics and Space Administration of the United States in 1972 has been providing cyclic cove-
rage of Canada since launch.
A receiving antenna at Prince Albert in central Canada records the satellite transmissions
which are then sent to Ottawa for processing at the Canada Centre for Remote Sensing. The imagery
produced at the Centre is at a scale of 1:1 000 000 in each of the four bands of the multispectral
sensor. Colour composites can be made for any three of the bands together.
Four ERTS-1 orbits a day cross Canada ; 75 orbital paths are required to cover the land
mass of Canada and each path in repeated at 18-day intervals. The amount of ground imaged on any
pass is, of course, highly dependent on the cloud cover at the time. Even after 2 years of operation
not all possible image centers have been acquired with cloud-free, snow-free imagery.
As with aerial photography, the spectral sensitivity of the sensor affects the manner in
which the imaged details are separated by tone. "Water-land interfaces are best delineated in the
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