I 
I 
62 
Then the law of error propagation gives 
Qdx dx = Qxo*o+ x 2 Qss + y 2 Qaa 
QdY dY — Q r 0 r 0 + y 2 Qss + * 2 Qaa 
QdX dY — x y Qss — x y Qaa 
We obtain 
QdX dX = QdY dY — 1 / 6 + (x 2 + y 2 ) / 513 ; QdXdY ~ 0 
To get an average value of the standard error of the correction we integrate 
this expression over the image area and divide by the area 
dx dy 
—10 —6 
Qcorr; corr 0.255 
The image co-ordinates are assumed to have the standard error o as estimated 
in formula 8.3.6. The standard error of unit weight of the transformation is 
o ]/1.5 as in formula 8.3.5, and the transformed image co-ordinates will then 
have a standard error 
Strans j/O 2 + Qcorr; corr O' 2 1.5 
Strans 1.1 75 O 
Assuming only the errors treated above, a single point resection in space should 
give a standard error of unit weight corresponding to that estimated by this 
formula. For o = 13.23 jam we get Strans 15.55 (288 df). 
8.3.2. Test object 
In this case the test object is a piece of plexiglass with 18 steel balls (0.2 mm 
diameter) fixed in drilled holes. The holes have diameters somewhat smaller 
than the balls, and depths somewhat larger. The steel balls are thus fixed by 
friction. There are 9 balls on each side of the plexiglass plate. Fig. 17. 
Fig. 17. 
Test object used to calibrate the iodine 125 system in Fig. 16. The plexiglass plate is 10 x 30 x 
30 mm. The balls on the upper surface use an area 12 x 20 mm. and on the lower surface 9 x 
14 mm. The needlepoints are used to connect the measurements of the two surfaces to each 
other.