9-11 Nov. 1999 
S 
  
, 1997; Wagner, 1995). 
anded greatly in the last 
tprint lidars have been 
'ars towards the ultimate 
sy earth topography and 
ey are designed with 
pture the top of one tree 
and can be used to map 
canopy height profile 
al, 1999; Harding & 
s are used to accurately 
] glaciers (Krabill et al. 
1) has led to different 
In large-footprint lidars 
izontal spacing, larger 
] waveform reflections. 
'overing the desired area 
CS. 
m return 
n 
  
R Staff, 1999). 
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
at a lower pulse rate reducing sizes of large data files. The 
lower pulse rate facilitates building systems with greater 
energy per pulse. Greater pulse energy allows greater 
operating height and so large-footprint systems are the 
reasonable choice for orbital platforms. Greater platform 
height can be a primary design goal. This in turn allows a 
greater swath width. 
4 CAPABILITIES 
The different functioning of these systems leads to different 
capabilities (Table 3). The closer shot spacings of small- 
footprint lidars allow them to resolve finer topographic details 
and create finer DTMs. Their lower flying altitudes and 
generally narrower swaths may make them less economical 
for covering very large areas. Small footprints are less likely 
to find a canopy opening and in dense vegetation most shots 
do not. Large-footprint lidars with their wider footprint 
spacing and greater platform heights may be better for 
characterizing topography of larger areas. Also, the full 
reflected waveform they capture, provides a measure of the 
vertical distribution of reflecting surfaces. In contrast most 
small-footprint lidars capture only the first or first and last 
reflection, though several are capable of capturing 
intermediate reflections. 
The problem with dense vegetation 
Dense vegetation near the ground may be impenetrable to 
small-footprint lidars that have a minimum trigger distance 
between successive returns. If the lidar detects a return at 1.5 
m above the ground and the minimum trigger distance 
between returns is 2 m then it will not detect the ground. In 
these cases the top surface of a dense low-lying vegetation 
layer may be interpreted as the ground. This problem is 
probably worse when there is a dense fores canopy and a 
dense understory. Such vegetation strata occur in the moist 
climate of the Pacific Northwest of USA where conifer- 
dominated stands can have thick shrub understories (Franklin 
& Dyrness, 1973). In one portion of a study area in 
Washington, USA, with dense overstory, preliminary analyses 
indicate only 1-5% of lidar shots reached the ground (J.E. 
Means, unpublished data). In this case the true resolution of a 
derived DTM will be much reduced. One approach to 
compensate for this is to use a very small footprint size, e.g., 
10cm, and a high shot density so that hopefully an acceptable 
number of shots will not encounter reflecting surfaces within 
their minimum trigger distance above the ground. 
Large-footprint lidars work around this problem of dense 
vegetation because somewhere in their large footprint are one 
or more small openings to the ground that taken together 
allow enough photons to be reflected from the ground to 
provide a discernible peak in the return waveform. The large 
footprint and full waveform work together to this effect as 
both are needed. This effect can be seen in the success of 
Figure 1. 
  
Figure 1. 
Table 3. Capabilities of Two Types of Lidar Systems. 
  
Small-footprint, discrete return 
Large-footprint, waveform return 
  
— Accurate elevations, 15-30cm with care 
— Closer shot spacing, .2-10m 
— Resolve finer topographic details 
— Create DTMs 5x5 to 2x2m and finer 
— Probably less economical creating DTMs of very large 
areas, e.g., 10's-100's km? 
— Fewer shots penetrate dense vegetation, 25-596 or less 
depending on cover percent 
— To get vegetation heights must first build DTM 
— Difficulty making accurate DTMs under dense canopies 
— Accurate elevations, 15-30cm with care 
— Farther shot spacing, 2-170m 
— Not resolve fine topographic details 
— Create DTMs 10x10 to 25x25m 
— Probably more economical creating DTMs of very large 
areas, e.g., 10's- 100's km 
— Distinguish ground in many or most pulses through dense 
vegetation 
— Relatively effective creating DTMs under dense canopies 
— Ground elevation and vegetation height measured in one 
shot 
  
— Data on vegetation height profile must be synthesized 
from many shots or returns 
  
— Data on height profile of canopies obtained directly