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beam width used. The results presented were all obtained with a 6° beam, however, the 
effective angle is likely to be significantly smaller. The variations in object height 
within the circle are represented by a single reading which, like an average, lies 
somewhere between the extremes. Second, the radar altimeter pulses out many readings 
per second along the profiles but integrates many of them to produce an additional 
averaging effect. The purpose of the integration is to damp out the random "noise" 
inherent in the system. Thus the "point" sampling of the photo approach is subject to 
greater variation which is levelled out to a considerable extent in the "area" sampling of 
the radar altimeter. 
Anomalies occurred at the 100, 360, 720, 790 and 920 m points along the 
transect. These were cases where the photo forest canopy profile rose substantially 
(sometimes 10 m or more) above the radar profile. Investigation of the large-scale 
photos of these segments revealed that large, loose-crowned trees dominanted the 
transect. The tree at 920 m is illustrated in Figure 8. Whereas the photo profile was 
held to the surface of the tree vegetation, the radar altimeter evidently did not respond 
EEE EEE 
  
FIGURE 8. Tree with very thin crown (centre of centre photograph) which did 
not return the radar signal with sufficient strength. 
to the thinner cover — i.e. insufficient radar energy was reflected from these crowns to 
raise the profile. The radar reading in such cases is based on energy reflected from 
nearby trees and those beneath the large, open crowns. Corresponding anomalies in the 
opposite direction were less evident except at the 690 m point where small openings 
allowed the photo profile to drop to the ground level. The averaging effect of the radar 
prevented the radar readings from reaching the ground level except where the openings 
were sufficiently large such as occurred at the ends of the transect (Figure 2). 
Spot checks of tree height were made on the ground at several points along 
the transect. These are shown as vertical lines in Figure 6. 
The species identification experiment, though limited in scope and incom- 
plete, provided some useful results shown in Table 1 based on many of the trees shown in