Division of the Canadian Department of Mines and Technical 
Surveys. The procedure followed may be outlined as follows; 
The region to be mapped is generally covered with Tri-Metrogon 
photographs, from which the planimetry is charted, The mapping 
scale is 8 miles to the inch or about 1 : 500,000, and the 
contour interval is 500 feet or about 150 meters. In order to 
complete the topographical detail, the entire area is covered 
by parallel radar profiles at 16-mile (26-kilometer) intervals- 
Vertical photographs are made at the same time as the radar 
profiles so that the position of the profile on the ground might 
be identified. 
After making the correction in the profile for the 
gradient of the barometric pressure and any further corrections 
which can be made with the aid of existing fixed points, well- 
defined and reliable points characteristic of the terrain are 
chosen and recorded in the appropriate position. The contours 
are plotted by interpolation between the given points; at this 
stage, of course, use is made of the preliminary aerial photographs 
and the stereoscope. If significant elevations appear between the 
individual profiles, secondary spot heights can be reconstructed 
with the aid of the Tri-Metrogon photographs. 
This method has proved to be very rapid and convenient 
for the purpose in question and in the past three years it has 
been possible to map vast, almost inaccessible areas of Canada. 
At the same time as this work was being done on the 
production of aeronautical maps, the Aeronautical Charts Division 
in collaboration with the National Research Council, on the one 
hand, and the Photographic Survey Corporation, on the other; were 
carrying out a number of experiments in an effort to solve various 
problems encountered in the instruments used and in the general 
application of the method. Several thousand miles of profiles were 
made and analyzed. Of these preliminary tests, we would like to 
cite here the results obtained from a profile of about 500 km 
(306 miles) in length. This test is of particular interest because 
the only elevation known in this profile was that of the initial 
point; moreover, the course of the aircraft was changed several 
times. The corrections in elevation were therefore calculated 
only on the basis of the formula mentioned above and the absolute 
elevations of various water surfaces, as deduced from the profile, 
were compared with the actual elevations. The results are shown 
in Table I. 
The méan square elevation error, which amounts to only 
+ 3.7 meters (+ 12 feet), and which appears to bear no relation 
to the distance between the point measured and the initial point, 
speaks for itself; at the same time it shows clearly that an 
isobaric surface at a certain elevation above ground-level is 
incomparably better as a reference surface than is the case in the 
classical barometric measurements in the field. 
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