along a test line 9 kilometres in length and at an average altitude
of 5,700 feet above mean sea level. The mean latitude and longitude
of the points were 2Ö°io' South and 28°oo' East, respectively.
Both terminals (University Pillar and Waterval) were points co
ordinated on the South African National Survey System, and con
sisted of stable cylindrical concrete pillars.
In planning the research programme it was decided to spend
several days using the well-known tracking or reversal point method
before carrying out comparative measurements using the transit
method of Schwendener [4].
The tracking method involves the following-up of the oscillating
gyroscope, observing the "reversal point" at which it is momentarily
stationary, and repeating the procedure for as many extremal
point as is desired. Three oscillation extremities will provide the
the azimuth, 0O, of gyro-indicated north, from
I [|(£i L3) Li\, the Schuler mean.
Any additional observations may be reduced in a similar way or
by one of the methods described in references [2] or [3].
The transit method, for use with the Wild GAKi gyro-attach
ment, or instruments of similar type, consists of timing the passage
of the gyroscope fiducial mark across the index line of the auxiliary
scale of the autocollimator while the GAKi gyro-attachment
remains clamped in an approximately north orientated position.
The difference in time, dt, between successive transits, when the
gyroscope is moving to the east and to the west, is generally non
zero. Proportionality exists between dt and dA, the correction to
the gyro-orientation, along the near-linear range of the oscillation
curve where this crosses the axis of symmetry. The correction to
approximate north is obtainable from
dA c.a' .dt
where c (A's AT)/(«Tdfc a'adfc)
a' the amplitude read on the auxiliary scale
dt tetw, the difference in transit times
for the gyroscope moving from left and right,
respectively
A'1, A'2 the approximate north settings used in the deter
mination of c.
Thereafter, A A\ -J- c.a'i.dti A'2 c.a'i.dti, where A is
the direction of gyro-indicated north and dA takes the sign of dt
vhen te and tw are assumed to be positive and negative, respectively.
It should be noted, of course, that the transit method can only be
applied with instruments fitted with autocollimator auxiliary scales
on which the oscillation amplitude can be measured. Little informa
tion is available of practical results using this observing procedure,
as yet. However, tests indicate that accuracies comparable with the
better known tracking procedure can be achieved. A particular
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