9 
measures 
sxpansion 
j that the 
ourse, on 
Dry work, 
e a priori 
■ficient of 
Ly at our 
ble. 
it not for 
mgulation 
ice of the 
nal block 
be found 
the more 
tie smaller 
very best 
t the best 
measures; 
determine 
towns and 
a «better 
jr answer, 
ise of the 
work, it 
is a better 
ions: what 
is to stop 
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speaking, 
eatures of 
and not a 
s of maps 
5 with the 
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o walk up 
¡round, its 
te valuable 
this shape 
[ is surely 
ted result, 
¡eometrical 
alone but 
rdinates in 
and polar 
coordinates (heights). The idea that photogrammetric triangulation has errors 
due to earth curvature comes simply from the failure to accept the obvious fact 
that a height is not a rectangular coordinate and should not be treated as if it were. 
3. THE RÉSEAU 
A word or two must be said about the use of the réseau in analytical photogram- 
metry. It is unfortunate that while this device has been in use for many years 
in Great Britain, it is only relatively recently that it has been incorporated in 
cameras having modern high performance lenses.^ It has therefore been hitherto 
impossible to carry out realistic tests to decide whether and by what amount the 
use of the réseau improves aerial triangulation with good photography. The 
opportunity occurred to do such a test in Southern Rhodesia. The Surveyor 
General was anxious to see whether analytical techniques could be used for 
providing coordinated property boundaries; and he initiated an experiment with 
the cooperation of University College London and Messrs. Fairey Surveys who 
undertook the photography and provided an observer for making the observations 
on the Hilger and Watts stereo-comparator at University College. The camera 
used was the Hilger and Watts 105 fitted with the Aviogon lens and réseau; and 
the computations were carried out by Mr. P. Boniface on the University of London 
Ferranti-Mercury Computer. 
For a number of reasons it has been impossible to present a complete analysis of 
the results for this conference, but one set of results is available and is instructive. 
The photographs for this set were taken at a height of 7500 feet giving a scale of 
1/15,000 with the six inch lens. There were 12 photographs in two strips of six 
pictures forming a roughly rectangular area with a perimeter control of six points. 
All control and check points were pre-marked. The adjustment was carried out 
by the Amer method. 
Each photograph to be observed was placed on a stageplate carrying an accurate 
centimetre grid so that the lines of this grid were displaced by a small amount 
from the réseau crosses appearing on the pictures. For every photographic point 
observed, the operator measured the difference of coordinates to the nearest réseau 
intersection and also to the nearest grid intersection. Pointing errors to photo 
graphic points were thus common to the réseau and non-réseau observations and 
differences in the results could thus be ascribed absolutely to the use or non-use 
of the réseau. 
The results, on an adequately large number of check points, were very good 
indeed, the root mean square vector errors at plate scale being + 13 ( u with the 
réseau and + 12/x without. The difference may be statistically significant but it 
is clearly unimportant; but what is interesting and requires explanation is that 
the réseau gives no improvement. The same diapositives used in the Thompson- 
- Watts plotter gave a vector error of + 18 u which, while larger, is still very good 
for an analogue instrument. Unfortunately, as we have said, a full analysis has 
not been done, and explanation of these results must be to some extent conjecture.