instruments themselves. Usually this implies large size and weight and often 
means the strengthening of the floor on which the comparator stands. First 
mention of a measuring system which made no use of a pre-defined co-ordinate 
reference frame was probably made by Brown (1967), (2), (3), who recommended 
the use of a trilaterative approach to plate co-ordinates determination. What- 
ever reference system is desired is therefore implicit within the geometry of 
the observational pattern used. The method after Brown while lacking some of 
y possible with direct co-ordinate measuring machines relaxes con- 
siderably the stability demands on the comparator. It also enables the co-ordi- 
nates of each point to be checked, by means other than repetitive measurement, 
before the model formaticn and subsequent computational stages are embarked 
the efficienc 
upon. 
High magnification cannot be applied to any great advantage with stereocompara- 
hic emulsions and low 
tors due to the coarseness of grain of available photograp 
contrast. Eden (1967),(4), mentions that at x40 magnification a general stereo- 
scopic impression of all details on an aerial photograph is not possible because 
of the low contrast of the majority of these. He further states that for magnifi- 
cations beyond x12 the texture of the stereoscopic background starts becoming 
woolly for stereoscopic pointing, the stereoscopic image being lost long before 
x40 magnification is reached. High magnification does however offer certain 
advantages which have been used to some extent in the experiments described in 
this paper. By restricting actual measurement to the order of 50 microns only, 
distances up to 320 mm can be measured to micron accuracy. No special stabili- 
ty requirements appear to be necessary which cannot be found in a well-construc- 
ted table. The purpose of the investigations nas been to show tnat accurate ana- 
lytical aerial triangulation is possible with very simple measuring equipinent 
and that it may be carried out reasonably efficiently in conjunction with an elec- 
tronic computer having remote access and time sharing facilities. Tne I.B.M. 
360/50 and remote terminal 2741 were used in the various tests. 
2. The Trilateration Microscope and Measuring Procedure With The Instrument 
The cost of the measuring components of the apparatus is approximately £100. 00. 
These comprise a glass scale, 5 mm thick, 320 mm long and graduated to 0.1 mm; 
and two microscope readers. Although only one reader is essential two are used 
for the sake of convenience. Accurate graduation of the scale is desirable but as 
long as the graduation lines are of fine gauge the scale is suitable. The scale used 
by the writer has lines approximately 10 microns wide. 
The microscope reader tubes were made 325 mm long and can be fitted with diff- 
erent magnification objectives. A x15 measuring eyepiece completes the reader. 
Each division of the 100 division measuring drum of the eyepiece has the follow- 
ing approximate equivalents when the tube factor is 1 : 
Objective Drum Division (micron) 
x2 2.50 
x3 1.07 
x4 1.25 
x6 0. 82 
Variation of the tube length alters the effective power of the reader and in con- 
     
   
   
  
  
  
  
  
  
   
   
   
   
   
    
    
   
   
  
  
  
  
   
  
  
  
  
   
   
   
   
   
   
   
  
   
   
   
   
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