12
to monitor stand structural features, such as height
and cover (e.g., Wimbush et al 1967). As mentioned above, upward-looking hemispherical photography is a long-standing method of obtaining
information on canopy gap fraction and derived
variables, such as LAlor foliar distribution (reviewed by Rich 1990). Furthermore, photographs
can be readily scanned to generate digital images
for quantitative analysis.
Video and digital cameras lack the spatial resolution of film photography but are a convenient
means of producing panchromatic or color imagery
for digital image analysis of vegetation. Most systems are based on solid-state, charge coupled device (CCD) array sensors. Video images generally
must be converted from analog (videotape) to digital form using a frame grabber. Digital cameras,
on the other hand, acquire and store digital images
directly. The maximum spatial resolution (measured as resolution elements per square millimeter
of surface) of current video and low-cost digital
cameras is roughly 5 to 10 times coarser than that
of standard film products (King 1995), although
cost-performance is improving rapidly. Moreexpensive professional systems offer 1600 X 1200
or greater resolution and are beginning to approach
the resolution of film while offering the advantage
of real-time image editing, enhancement, and
production.
Aerial Remote Sensing
Laser Altimetry
Aircraft- and satellite-borne laser systems transmit
and analyze monochromatic light that interacts with
and is partially reflected by the illuminated surface.
LIDAR (light detection and ranging) laser systems
measure the travel time of short pulses of light to
estimate the sensor-surface distance, while more
sophisticated systems measure changes in light
quality to measure absorptive properties of the atmosphere (differential absorption LIDAR) or the
velocity of a moving target (Doppler LIDAR).
Beam divergence from airborne LIDAR systems
can be widened to increase the illuminated ground
area or "footprint," which typically ranges from 0.5
to 10 m. Time-varying return signals from targets
at different distances from the sensor are sampled
at high frequency to construct a "waveform" of disFrank W. Davis and Dar Roberts
tances to targets along the flightline. The waveform
of heights can be analyzed to estimate the vertical
distribution of vegetation and ground surface
within the footprint.
Although low penetration oflasers through dense
vegetation can lead to underestimation of vegetation height, LIDAR profiles obtained from aircraft
show real promise for estimating vegetation height,
stratification, and canopy closure over large regions
(Nelson et al. 1988; Weltz et al. 1994; Nilsson
1996). In the United States, the National Aeronautics and Space Administration (NASA) has deployed several aircraft-borne laser altimeters, the
most recent known as the laser vegetation imaging
sensor (LVIS) (Blair et al. 1994; Blair and Coyle
1996). Nilsson (1996) describes an airborne Scandinavian system that uses a near-infrared (NIR)
beam to measure distance to the surface or canopy,
and a green beam to measure vegetation height profiles based on analysis of the waveform. In 2000,
NASA is scheduled to launch the vegetation canopy LIDAR (VCL), a five-laser system designed to
obtain global measurements of surface and vegetation height at 25-m horizontal resolution and I-m
vertical accuracy along sampling tracks separated
by 2 km (http://essp.gsfc.nasa.gov/vcl).
Aerial Photography and Videography
Aerial photography has been in widespread use
since the first quarter of the century and it continues
to be the primary means for mapping and inventorying vegetation structural features, such as timber volume, shrub or tree crown diameter, crown
closure, and vegetation height (reviewed in Philipson 1997). Large-scale «1:100) photography can
be obtained from balloons, cherrypickers, towers,
and other low platforms. Medium- to small-scale
photography is obtained from aircraft or satellite.
Standard large-format film (23 X 23 cm) still provides the highest resolution, readily available, remotely sensed imagery.
Stand-level analyses generally employ true color
and color-infrared film and acquire photos at
1: 12,000 to 1 :24,000 scale. Scale or "representative
fraction" is used to measure distance in rectified
photography. Shadow length in a single image or
image parallax in stereo photo pairs can be used to
measure object heights. Dot grids and other photogrammetric tools are typically used to estimate
crown closure and composition.
to monitor stand structural features, such as height
and cover (e.g., Wimbush et al 1967). As mentioned above, upward-looking hemispherical photography is a long-standing method of obtaining
information on canopy gap fraction and derived
variables, such as LAlor foliar distribution (reviewed by Rich 1990). Furthermore, photographs
can be readily scanned to generate digital images
for quantitative analysis.
Video and digital cameras lack the spatial resolution of film photography but are a convenient
means of producing panchromatic or color imagery
for digital image analysis of vegetation. Most systems are based on solid-state, charge coupled device (CCD) array sensors. Video images generally
must be converted from analog (videotape) to digital form using a frame grabber. Digital cameras,
on the other hand, acquire and store digital images
directly. The maximum spatial resolution (measured as resolution elements per square millimeter
of surface) of current video and low-cost digital
cameras is roughly 5 to 10 times coarser than that
of standard film products (King 1995), although
cost-performance is improving rapidly. Moreexpensive professional systems offer 1600 X 1200
or greater resolution and are beginning to approach
the resolution of film while offering the advantage
of real-time image editing, enhancement, and
production.
Aerial Remote Sensing
Laser Altimetry
Aircraft- and satellite-borne laser systems transmit
and analyze monochromatic light that interacts with
and is partially reflected by the illuminated surface.
LIDAR (light detection and ranging) laser systems
measure the travel time of short pulses of light to
estimate the sensor-surface distance, while more
sophisticated systems measure changes in light
quality to measure absorptive properties of the atmosphere (differential absorption LIDAR) or the
velocity of a moving target (Doppler LIDAR).
Beam divergence from airborne LIDAR systems
can be widened to increase the illuminated ground
area or "footprint," which typically ranges from 0.5
to 10 m. Time-varying return signals from targets
at different distances from the sensor are sampled
at high frequency to construct a "waveform" of disFrank W. Davis and Dar Roberts
tances to targets along the flightline. The waveform
of heights can be analyzed to estimate the vertical
distribution of vegetation and ground surface
within the footprint.
Although low penetration oflasers through dense
vegetation can lead to underestimation of vegetation height, LIDAR profiles obtained from aircraft
show real promise for estimating vegetation height,
stratification, and canopy closure over large regions
(Nelson et al. 1988; Weltz et al. 1994; Nilsson
1996). In the United States, the National Aeronautics and Space Administration (NASA) has deployed several aircraft-borne laser altimeters, the
most recent known as the laser vegetation imaging
sensor (LVIS) (Blair et al. 1994; Blair and Coyle
1996). Nilsson (1996) describes an airborne Scandinavian system that uses a near-infrared (NIR)
beam to measure distance to the surface or canopy,
and a green beam to measure vegetation height profiles based on analysis of the waveform. In 2000,
NASA is scheduled to launch the vegetation canopy LIDAR (VCL), a five-laser system designed to
obtain global measurements of surface and vegetation height at 25-m horizontal resolution and I-m
vertical accuracy along sampling tracks separated
by 2 km (http://essp.gsfc.nasa.gov/vcl).
Aerial Photography and Videography
Aerial photography has been in widespread use
since the first quarter of the century and it continues
to be the primary means for mapping and inventorying vegetation structural features, such as timber volume, shrub or tree crown diameter, crown
closure, and vegetation height (reviewed in Philipson 1997). Large-scale «1:100) photography can
be obtained from balloons, cherrypickers, towers,
and other low platforms. Medium- to small-scale
photography is obtained from aircraft or satellite.
Standard large-format film (23 X 23 cm) still provides the highest resolution, readily available, remotely sensed imagery.
Stand-level analyses generally employ true color
and color-infrared film and acquire photos at
1: 12,000 to 1 :24,000 scale. Scale or "representative
fraction" is used to measure distance in rectified
photography. Shadow length in a single image or
image parallax in stereo photo pairs can be used to
measure object heights. Dot grids and other photogrammetric tools are typically used to estimate
crown closure and composition.