spot heights in all directions in a model with a lot of water, canals, lakes etc. The tolerance
with this test must be, that we remain within the measuring accuracy of the instrument,
which is in general better than 10 microns in the negative.
Regarding the photographic material used in the plotting instruments in this country
no special methods are applied, to evaluate and test the sharpness and contrast other than
the observation of the images in a normal mirror stereoscope with magnification of 8 times
(binoculars) or with a loup. Referring to what was mentioned in this respect in CHAPTER I
of this report we believe that in future stricter methods must be applied to this important
problem.
Reduction printers or additional devices for improving the photographic quality are
neither used for aerial triangulation, nor for plotting, with the exception of the Kel-O-Watt
and Cintel printers in the Laboratory of the I. T.C. as mentioned in CHAPTER I.
For the large scale mapping projects commissions include the delivery of glass-negatives
or, in the case of film cameras, contact diapositives, both on ultra flat glass-plates. The
flatness of these glass-plates is not tested, except in those cases when doubt arises from
the results of the restitution. The question arises here whether it is not desirable to use in
the Porro-Koppe plotting machines as the Zeiss C 8, the original negatives and to consider
the diapositives as a spare copy. First of all the photographic process of copying always
results in a loss of definition, in some cases quite considerable, and furthermore the errors
in flatness of the negatives will in this case have no influence, on condition that the negatives
are put free of tension in the airsurvey camera and also in the plotting machine.
The stereoscopic plotting instruments are tested on delivery and during the installment
of the instrument by an engineer of the manufacturer. The necessary integrated tests are
made by this engineer, together with a skilled operator of our own organizations. The
measurements are carried out on tested precision grid plates. They are taken with base
inside and base outside and repeated in each of these positions. The accuracy of the
readings on each of these grid points is such that a mean square value of 3 microns reduced
to the scale of the negative can be considered as a maximum. Each new instrument is tested
by means of monocular measurements on at least 25 points. The standard error in the
position of each grid point must be smaller than 10 y and preferably 6-8 u. The geomet-
rical quality is also tested by measuring a grid model with base inside and outside, with
the normal wide angle base-height ratio. With observations on at least 25 points the
standard error in height should for universal and precision machines, be between 0,04
and 0,07°/,, of the flying height.
During normal production, regular tests are carried out on 9 points of a glass grid. In
such tests the standard errors are allowed to reach a maximum of 8-10 y. in planimetry and
0,08-0,10°/,, of the flyingheight in height.
An error in the adjustment of principal distance does little harm in flat areas like this
country, thus it is not necessary to compute the corrections to this principal distance to
within 0,01 mm since the precision of the adjustment is in itself limited to 0,01 mm. The
position of the principal point is subject to the same tolerances as those applying to the
9 point grid measurement, when checking the instrument.
The accuracy of the applied glass grids is tested by the manufacturers who supply a test
protocol. The magnitude of the standard error is on an average, between 1 and 2 microns.
Recent tests have been made to check these data, but results are not yet available.
Our stereoscopic measuring procedure is fairly normal. In numerical orientation the
values h and b and the measured y-parallaxes are recorded. The y, are applied in the
formula, and then the calculated corrections are introduced in the instrument and recorded
with the results of the control measurements (b,).
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