International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
Table 1. Primary Purposes of Two Types of Lidar Systems.
Not all lidars in each category share all purposes for that category.
Small-footprint
Large-footprint
Common Goals | — ground surface elevation, DTMs
— canopy top elevation
— ground surface elevation, DTMs
— canopy top elevation
Different Goals | — .5-3m horizontal resolution
— 10-170m horizontal resolution
2 PURPOSES OF TWO TYPES OF LIDARS
The biggest differences in purposes of small- and large-
footprint lidars are in scale, types of terrain surveyed and
vegetation characterization. Small-footprint lidars are required
to be able to sense relatively small features, and large-
footprint lidars are required to characterize complete vertical
canopy structure or are used for measuring ice sheet
elevations (Table 1).
Both types of lidars evolved from early profiling lidars that
returned a single first reflection (Aldred & Bonnor, 1985; Arp
et al., 1982; Schreier et al., 1985) and bathymetric lidars
typically operated in profiling mode (Jensen et al., 1987;
MacLean & Krabill, 1985; Nelson et al., 1984). From these
beginnings small-footprint lidars developed to meet needs of
geotechnical applications such as road and pipeline right of
ways, large-scale DTMs, surveys of high-power electric
transmission lines and to generate 3-D urban models
— vertical distribution of canopy surfaces
(Axelsson, 1999; Flood & Gutelius, 1997; Wagner, 1995).
Their use for these purposes has expanded greatly in the last
three-four years. Several large footprint lidars have been
developed by NASA in the last six years towards the ultimate
purpose of a satellite system to survey earth topography and
vegetation height and cover. They are designed with
footprints large enough to usually capture the top of one tree
canopy and the ground in one pulse and can be used to map
topography, vegetation height and canopy height profile
(Blair & Coyle, 1996; Dubayah et al., 1999; Harding &
Roark, 1999). Other scanning lidars are used to accurately
measure elevations of ice sheets and glaciers (Krabill et al.
1999; Echelmeyer et al., 1996).
3 DESIGN
The different mix of purposes (Table 1) has led to different
designs and functioning (Table 2). In large-footprint lidars
these purposes lead to greater horizontal spacing, larger
footprint size and digitization of full waveform reflections.
More widely spaced footprints allow covering the desired area
Table 2. Differences in Design and Function of Two Types of Lidar Systems, Typical Values.
Small-footprint, discrete return
V | Large-footprint, waveform return
Horizontal point spacing, 2-3
m
Footprint diameter, meter 0.2-0.9
Reflection collection
1-5 discrete reflections,
separated by minimuml.5-2m
Energy, mJoule/pulse .0125-.2
Pulse rate, kHz 5-15
Flying height, km 0.2-1.0
Swath width, m 70-1200
Platform helicopter, fixed wing
10-25
10-70
full waveform digitization
(.012-.02) 5-75
0.04-0.5
4-400
800-8000(discontinuous)
fixed wing, earth orbit
1/(Baltsavias, 1999a) and e-mail communication with several lidar firms.
2/(Blair et al., 1999; Dubayah et al., 1999; GLAS Team, 1999; Krabill et al., 1999a; SOAR Staff, 1999).
International,
at a lower pulse rate
lower pulse rate fa
energy per pulse.
operating height an
reasonable choice fc
height can be a prim
greater swath width.
The different functi
capabilities (Table :
footprint lidars allov
and create finer D’
generally narrower -
for covering very la
to find a canopy op:
do not. Large-foo
spacing and greate
characterizing topo;
reflected waveform
vertical distribution
small-footprint lidar
reflection, though
intermediate reflecti
The problem with :
Dense vegetation n
small-footprint lidai
between successive
m above the grou
between returns is 2
these cases the top
— Accurate ele
— Closer shot
— Resolve fine
— Create DTN
— Probably les
areas, e.g., 10
— Fewer shots
depending on
— Difficulty m
— To get vege
— Data on veg
from many sh
