Light Detection And Ranging (Lidar) use laser ranging to measure the distance
between a sensor and target based on half the elapsed time between the emission of a
pulse and the detection of a reflected return. Lidar is an active optical remote sensing
technology and has been used to measure three dimension vegetation structure.
Lidar systems are classified as small footprint lidar (laser footprint less than
1 m scale) and large footprint lidar (laser footprint 10 m or greater) based on the
size of laser footprint or profiling lidar (recording only along a narrow ling at nadir
along the flight path) versus scanning lidar (recording across a wide swath along
the flight path) systems or discrete return and full waveform recording (Dubayah
and Drake 2000; Lefsky et al. 2002; Lim et al. 2003). Discrete return systems
record single or multiple returns from a given laser pulse. As the laser signal is
reflected back to the sensor, large peaks (i.e., bright returns) represent discrete
objects in the path of the laser beam and are recorded as discrete points. Most
small footprint lidar systems record discrete energy returns. In contrast, full
waveform recording lidar systems digitize the entire reflected energy from a
return, resulting in complete sub-meter vertical vegetation profiles. The waveform
is a function of canopy height and vertical distribution of foliage, as it is made up
of the reflected energy from the surface area of canopy components such as
foliage, trunks, twigs, and branches, at varying heights within the large footprint.
The total waveform is therefore a measure of both the vertical distribution of
vegetation surface area and the distribution of the underlying ground height.
Waveform recording instruments are mainly large footprint lidar systems; however, recent advances have seen full waveform instruments with increasingly
smaller footprint sizes (Wagner et al. 2006, 2008).
Lidar measures the three-dimensional distribution of plant canopies. The
measured canopy structure parameters include vegetation height, cover, and
canopy structure. Canopy height is calculated as the distance between the first
significant return above threshold and the ground. Canopy cover, the fraction of
background obscured by vertically projected foliage and woody area above a
certain height, is calculated using the cumulative laser returns from the canopy to
that height divided by the total returns from the canopy and the ground. Canopy
height profile (CHP) quantitatively represents the relative vertical distribution of
canopy surface area. All the lidar measured vegetation structure parameters are
closely related to field measurements and are then used to derive AGB estimates.
3.3 Approaches
3.3.1 Field Samples
In situ data of forest biomass are inevitable in biomass estimates. These data serve
as a tool to establish biomass predictive models from remote sensing, to evaluate
the developed models, and to validate the accuracy of biomass calculations. In situ
data can be obtained using a destructive method for a single tree or on an area
66
X. Zhang and W. Ni-meister
between a sensor and target based on half the elapsed time between the emission of a
pulse and the detection of a reflected return. Lidar is an active optical remote sensing
technology and has been used to measure three dimension vegetation structure.
Lidar systems are classified as small footprint lidar (laser footprint less than
1 m scale) and large footprint lidar (laser footprint 10 m or greater) based on the
size of laser footprint or profiling lidar (recording only along a narrow ling at nadir
along the flight path) versus scanning lidar (recording across a wide swath along
the flight path) systems or discrete return and full waveform recording (Dubayah
and Drake 2000; Lefsky et al. 2002; Lim et al. 2003). Discrete return systems
record single or multiple returns from a given laser pulse. As the laser signal is
reflected back to the sensor, large peaks (i.e., bright returns) represent discrete
objects in the path of the laser beam and are recorded as discrete points. Most
small footprint lidar systems record discrete energy returns. In contrast, full
waveform recording lidar systems digitize the entire reflected energy from a
return, resulting in complete sub-meter vertical vegetation profiles. The waveform
is a function of canopy height and vertical distribution of foliage, as it is made up
of the reflected energy from the surface area of canopy components such as
foliage, trunks, twigs, and branches, at varying heights within the large footprint.
The total waveform is therefore a measure of both the vertical distribution of
vegetation surface area and the distribution of the underlying ground height.
Waveform recording instruments are mainly large footprint lidar systems; however, recent advances have seen full waveform instruments with increasingly
smaller footprint sizes (Wagner et al. 2006, 2008).
Lidar measures the three-dimensional distribution of plant canopies. The
measured canopy structure parameters include vegetation height, cover, and
canopy structure. Canopy height is calculated as the distance between the first
significant return above threshold and the ground. Canopy cover, the fraction of
background obscured by vertically projected foliage and woody area above a
certain height, is calculated using the cumulative laser returns from the canopy to
that height divided by the total returns from the canopy and the ground. Canopy
height profile (CHP) quantitatively represents the relative vertical distribution of
canopy surface area. All the lidar measured vegetation structure parameters are
closely related to field measurements and are then used to derive AGB estimates.
3.3 Approaches
3.3.1 Field Samples
In situ data of forest biomass are inevitable in biomass estimates. These data serve
as a tool to establish biomass predictive models from remote sensing, to evaluate
the developed models, and to validate the accuracy of biomass calculations. In situ
data can be obtained using a destructive method for a single tree or on an area
66
X. Zhang and W. Ni-meister