Medium and large scale production mapping processes frequently utilise combinations of 
"high flown/low flown" aerial photographs in which the smaller scale models are triangulated 
to provide planimetric control for the larger scale stereomodels from which the machine 
plotting is performed. 
The high flown photographs are generally taken from twice the flying height of the low flown 
sortie, resulting in a 4:1 area coverage ratio. Scale control for the low flown photographs 
are subsequently obtained from single model triangulations, executed by observing the 
planimetric photogrammetric coordinates of all terrain established control data in the high 
flown overlap, together with a number of suitably positioned (on the large scale photographs) 
supplementary points. The terrain control serve as pivotal elements in deriving parameters 
for a linear conformal (Helmert) transformation of all supplementary points from the 
photogrammetric system to the terrain framework. In this respect the conventional 
expressions for conformal transformation from the (x,y) system to the (X,Y) system, namely 
X J a b Cx x 
Y |» a. |Qvy y 
© extended to allow for an adjustment procedure for deriving the unknowns a,b,Cx and Cy 
m more than two control observations. This results (after a change of origin to the 
centre of gravity of the control points) in the expression 
Cx 1/0 : | [x] 7 
Cy : 1/n i . LY'J 
a | = . . 1/[x'? 4 y'] . []. [vv :vf:1 
b ; : 1/ [x2 + y'2) [y'X' - xr 
where n is the number of control, and in matrix notation 
-1 -17. : : ui. 
X=Q TF where Q is the variance-covariance matrix, and 
qox = qe : 1/n 
q^ i oo 1/ [x + y'à] the cofactors of the adjusted unknowns. 
©... consequently possible to derive the expected theoretical precision of various 
transformed points, based on certain control distributions and weight considerations. 
This paper presents the results of a number of practical tests of single model triangulation 
in which various model scales and control combinations have been used. Whilst absolute 
orientation of the stereomodel prior to triangulation is clearly not essential, limits must 
nonetheless be imposed on arbitrary @ and W settings before commencement of the relative 
orientation since the above expressions do not correct for false tilts. Of the four tests 
described below, three involved levelled models and one dealt with a relatively orientated 
model with non excessive tilts. In each case, different photography was used. 
The test area (known locally as the A.O.C. Test Area) consists of an overlap in which 28 
pre-marked control points form a near penfect rectangular grid as shown below. 
28 T 
  
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