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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
similarity between species, mixed stands, complex patterns of 
crown illumination and shadow and texture caused by gaps and 
variable canopy density. 
5. CONCLUSIONS AND DISCUSSION 
By integrating small footprints over an appropriately sized area 
it is possible to generate detailed descriptions of vertical 
structure in even the most complex, semi-natural vegetation 
canopies. Using simple graphical techniques, accurately 
calibrated LiDAR data from an ALTM 3033 sensor has been 
used to demonstrate the basic principle. First pulse interacts 
with the outer surface of the canopy and the frequency 
distribution of heights characterises the canopy surface 
morphology. First pulses which reach the under-storey layer of 
the canopy can be used to map the location of gaps in the 
canopy. 
By combining both first and last pulse data it is possible to 
examine vertical structure wifhin the canopy by virtue of the 
fact that last pulses associated with first pulses near the canopy 
top reflect the degree of laser penetration. In the Birch canopies 
studied here it was found that a significant number of both first 
and last pulses reached the under-storey underlining the 
openness of the canopy. By contrast, in Hawthorn and Sallow, 
which have very dense canopies, very few first returns penetrate 
the upper canopy layer. Nevertheless, last pulse data is still 
returned in sufficient quantities to characterise the under-storey 
layers. Even the vertical structure of the complex, multi- 
layered, mixed woodland category was evident in the combined 
first and last pulse data. 
For all four of the canopy structures, boundary layers separating 
under-storey, stems and base of crowns could be identified. In 
all cases these were distinct, near horizontal features except for 
the case of Birch which showed considerable, vertical 
variability in the height of the layer. Ultimately, these layers 
may hold the potential for thresholding laser returns. First 
pulses reaching the under-storey provide a basis for identifying 
gaps and modelling the external canopy morphology (height 
and shape) Last pulses reaching the under-storey with 
associated first pulses above the base of the canopy could be 
used to provide a measure of canopy openness and dense 
canopy can be represented where both first and last pulse are 
returned from above the under-storey layer. 
This ability to identify gaps, areas of open and areas of dense 
canopy is clearly of considerable significance for understanding 
light penetration to the under-storey and the impact it has on 
composition and vigour. The next stage of this work will 
involve implementation of this approach for wide area canopy 
mapping. 
In addition to characterising vertical structure, the cumulative 
height distributions from the histograms also provided very 
clear discrimination between the vegetation classes involved. 
This suggests that their incorporation with spectral data in 
classification schemes will result in significant progress towards 
detailed mapping of semi-natural vegetation communities. As 
the introduction to this paper suggests, this progress is urgently 
needed in many parts of the world. 
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7. ACKNOWLEDGEMENTS 
The authors would like to acknowledge the support of Lin Kay 
and Peter Purcell at the NERC Airborne Remote Sensing 
Facility for their input to and support of this research. They are 
also grateful to Jim Gammie of English Nature for supporting 
acquisition of survey data as part of the Great Fen Project and 
also for providing access for field data collection. The 
Woodwalton Fen LiDAR research has been made possible by 
the generous support of the Sir Isaac Newton trust. 
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