3. RESULTS 
In addition to the transformation using control 
features, a second order polynomial transformation 
with control points was also done for comparison. 
Fifteen (15) points were used in the point-to- 
point transformation, while 19 features were used 
in point-to-feature transformation. In each case, 
6 independent check points were used. The follow- 
ing results were obtained: 
Mean errors on transformation points: 
(row and column directions respectively) 
Control Points: 1.0 & 0.9 pixels 
Control Features: 0.7 & 0.6 pixels 
Mean errors on check points: 
- Control Points: 0.8 & 1.6 pixels 
S Control Features: 1.0 & 1.2 pixels 
4. DISCUSSION AND CONCLUSION 
The use of straight line features as controls re- 
quires that the image point falls on the straight 
line defined on the map or ground after transfor- 
y mation. This in turn requires that: 
. that the road or track actually be straight 
0 between the two points, otherwise, a more com- 
t plex function would be required to define the 
Y feature. 
. that the two points must be chosen on the 
same part of the road or track, e.g centre line 
or edge, from which the image point has been 
a chosen. For this reason, roads and tracks sym- 
Y bolized with very thin lines are most  approp- 
e: riate on maps. 
. most importantly, that the image point falls 
within the limits of the feature as defined by 
the two points on the map or ground. 
Approximate values of parameters are required for 
the iterative solution discussed above, these may 
jf be obtained by initially assuming a one-to-one 
correspondence between the image point and the 
midpoint of the two points chosen on the feature. 
The rate of convergence depends a lot on how good 
2) the initial approximations are. 
The results show that this concept can be used in 
ye place of the control points method. One of its 
2 disadvantages however is that for the same number 
ge of transformation parameters, the number of fea- 
of tures required is higher than the number of 
2) points. This, in addition to the requirement for 
f- iterative solution, implies an increased compu- 
he tation. 
Re 5. REFERENCES 
al Masry, S.E., 1981. Digital Mapping Using Entities: 
A New Concept. Phot. Eng. and Remote Sensing, 47: 
st 1561-1565 
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or 
the 
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