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the v-parallaxes. Then the operator could make properadjustments when residual 
pzrailaxes are observed at points not used by the automatic stereoplotting device. 
Absoiute Orientation 
  
The automatic stereoplotting device has no special arrangement for the 
absolute orientation, and consequently this process is practically identical to that 
normally employed. The only difference is that the elevations of orientation points 
may be read by the automatic stereoplotting device. The advantages here are that 
elevation readings are very consistent and very rapid. Actually, the elevation 
reading is available as soon as the scanning head is brought to a point. On the 
other hand, as will be shown later, the readings are somewhat affected by terrain 
details that surround the point whose measurement is being made. Proper allow- 
ance should be made for this fact when the automatic stereoplotting device is used 
for the absolute orientation. 
The automatic stereoplotting device will speedup the process of absolute 
orientation when its capabilities are fully exploited. One important capability is 
that the device can follow the elevation of a point continuously, even if the eleva- 
tion is changing rapidly. Therefore, the operator can watch the height readingata 
point while he makes corrections for absolute orientation. In this way the required 
result may be obtained more rapidly. 
Accuracy in Determination of Spot Elevations 
  
The accuracy of height measurement by the automatic stereoplotting 
device was investigated on the Renfrew stereogram that was used in the interna- 
tional test on small-scale mapping. The photography was at a scale of 1:50, 000 
and the resulting scale of the model in the Nistri Photornapper was 1:10, 000. The 
height counter of the device records to 0.1 mm, which means that the elevations 
could be read to only 1 meter. 
The device was used to measure automatically the elevations of 63 
control points, which were well-distributed over the test model. The same points 
were also measured visually by an experienced operator. Thus, the results of the 
automatic stereoplotting device could be compared with elevations determined in 
the field and elevations measured by a human operator on the same model. 
It was found that the automatic stereoplotting device required very little 
time for elevation determination. At the moment the scanning head reached the 
point the reading was already available. The readings were most consistent, no 
drift was observed, and when brought to the same point several times the readings 
were always identical within the reading accuracy. 
In about half of the measured points some amount of ''jitter'" was 
observed in the readings. The reading dial oscillated rapidly over a range that 
varied from a fraction of an interval (1 m) in the dial to a maximum of about 
2 intervals (2 m). This jitter originates partly in the servo system that drives the 
reading dial and partly in the character of the point being measured. . More jitter 
was observed if considerable elevation differences were present near the point. 
Incidentally, excessive jitter was observed when the relative orientation was not 
perfect. This may turn out to be a valuable warning in practical operation.