192 CARTES A PETITES ECHELLES, DISCUSSION 
Je donne la parole au Colonel Thompson pour 
la méthode utilisée au Canada. 
Mr J. I. THomPson: This problem is not only 
a fascinating one but it is one in which we in 
Canada are directly engaged. My comments are 
based upon our experience over a number of 
years combined with my wishful thinking of 
what we would like to do if we were start- 
ing again. This type of reconnaissance map 
should be considered strictly from the user's 
point of view, and not from what we as photo- 
grammetrists would like to produce. Our users 
have told us that they wish to be able to use this 
map on the ground in exploration and in the air 
from light aircraft. They have no desire for high, 
absolute accuracy, either in the vertical or the 
horizontal. They wish for accurate, relative 
shapes again on both horizontal and vertical 
features. They have agreed that their need is 
met by our choice of a publication scale of 
1 : 250,000, with form lining at 100' intervals 
in areas of low relief and 500' in mountainous 
areas. Except in exceptional cases, they will not 
require any further information until a larger 
scale of the order of 1 : 50,000, or larger, is 
requested. However, they do stipulate that they 
want these maps now and not in generations to 
come. A conservative estimate of our produc- 
tion in Canada this year, in this particular field 
of mapping, is approximately 300,000 square 
miles. That is approximately one million square 
kilometres, and we anticipate having to meet this 
yearly target for at least five years to come. 
In reality, one of the basic factors in our con- 
sideration of our procedure has been that our 
methods in all phases will lend themselves to a 
simple and efficient procedure which will allow 
us to use our limited resources of trained per- 
sonnel to the greatest advantage. In actual fact, 
our tests have indicated that the average mean 
error in absolute horizontal position does not 
exceed 150 metres, approximately six-tenths of 
a millimetre at publication scale; and the error in 
the vertical values on which our form lining is 
based does not exceed 50' to 60', approximately 
20 metres. 
Were we to re-fly our basic cover — un- 
fortunately it is now done or virtually complete 
— we would call for a minimum of photography 
taken with a superwide angle lens in areas of 
low relief, and the wide angle lens in areas of 
high relief, with a flight altitude of approximate- 
ly 30,000 feet. 
To effect economy, we would require that 
radar altimetry would be carried out simulta- 
neously with the vertical photography and would 
be keyed to it. In Canada, because of the nature 
of the land, we would use the helicopter and 
light aircraft to support Tellurometer operations 
in the field to establish horizontal control. It 
would be run in a grid pattern in which the grid 
dimensions would be approximately 150 miles. 
Vertical control, to which the radar altimetry 
would be tied, would be run by instrumental 
levels using water transfer procedures on the 
vast number of bodies of water that we have. 
These level lines would be transferred to our 
photographic flights at minimum intervals of 
150 miles and maxima extending to 250 miles. 
Form lining would be carried out on the pho- 
tographs by skilled field topographers, based 
on the height deduced from the radar altimetry 
coupled with the basic natural drainage pattern. 
Their instruments would be simple conventional 
universal stereoscope, and possibly the parallax 
bar on occasions. In the areas of difficult relief, 
the Multiplex and lower orders of plotters would 
be used. The information on the photographs 
would be transferred to the compilation man 
equipped with such simple transfer devices as 
the Sketchmaster or the Stereoscope. 
In the past, in Canada, we have used shoran 
to a tremendous amount, but economically we 
have now dropped this method. However, in 
areas where aerial navigation is extremely diffi- 
cult, if not impossible, shoran could be used to 
its best advantage to provide a most effective 
means of navigation, as well as to provide 
horizontal control. This, coupled with simulta- 
neous radar altimetry, keyed to photography, 
would provide you with all your supplementary 
horizontal and vertical control. 
Monsieur M. BAUSSART: Je vais vous rappe- 
ler briévement la méthode qui a été adoptée par 
l'Institut Géographique National pour la solu- 
tion des mémes problémes. Cette méthode per- 
met de déterminer directement les altitudes des 
quatre points nécessaires pour l'orientation de 
chacun des modéles. Je ne l'exposerai pas car 
elle a fait l'objet d'une communication à la 
Commission IV. Je ferai à son propos unique- 
ment quelques remarques. 
Son principe consiste, une fois la couverture 
normale exécutée, à utiliser un émetteur-récep- 
teur aéroporté et à faire des enregistrements de 
profil le long des zones communes aux bandes 
longitudinales, ainsi que le long de certains 
axes transversaux. L'originalité de cette méthode 
consiste en deux choses: d'abord l'enregistre- 
ment des profils ne se fait plus en méme temps 
que la prise de vue, mais tout à fait séparément 
et à une altitude plus basse — d'autre part et 
  
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