Provides billions of topographic ground control points at 1 m accuracy.
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Land cover characterization for terrestrial
ecosystem modeling and prediction
-Vegetation height
In addition to providing a unique metric, i.e., the vertical dimension,
to classify vegetative cover at global scales, height is highly correlated
with aboveground biomass (Oliver and Larson 1990, Avery and Burkhart 1994,
Nilsson 1996). Biomass in forests represents the major reservoir of carbon
in terrestrial ecosystems that can be quickly mobilized by disturbance
or land use change (e.g., Houghton et al. 1987, Dixon et al. 1994).
-Vertical distribution of above ground surfaces
By recording the complete time-varying amplitude of the return signal of
the laser pulse between the first and last returns (representing the canopy
top and the ground), VCL captures a waveform that is related to canopy
architecture, specifically the nadir-projected vertical distribution of
the surface area of canopy components (foliage, trunk, twigs, and branches).
Like the simple height estimate, the vertical distribution of laser return
provides a new means to classify vegetation and functions as a predictor
of the successional state of a forest. Trees in younger stands tend to
exhibit a more leptokurtic vertical distribution of phytomass concentrated
near the ground. As a stand ages and grows, the vertical distribution of
canopy components becomes more platykurtic. Bimodal distributions are associated
with the presence of an understory which may occur in more mature stands.
Older stands characterized by canopy gaps and trees of multiple ages and
sizes exhibit a more even distribution of canopy components (e.g., Aber
1979, Brown and Parker 1994).
When combined with greenness measures from other sensors, such as TM,
MODIS or AVHRR, VCL observations may be used to determine whether the greenness
signal is the result solely of low-lying vegetation (via the height distribution).
Many areas of the world have ground covers with greenness indices comparable
to those of forests, making landcover discrimination based on greenness
measures alone difficult. Measures of the vertical organization of canopy
components are also critical for modeling factors that relate to biophysical
and micrometerological processes at the atmospheric-vegetation boundary
layer such as radiative transfer, evapotranspiration, and trace gas flux
(Gro 1993, Fournier et al. 1995).
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Land cover characterization for climate
modeling
Direct measurements of canopy height, canopy vertical and spatial structure,
and ground topography would allow determination of landcover properties
critical to GCMs, SVATs and mesoscale models, whether used for climate
modeling or numerical weather forecasting. There have been efforts to include
complex land surface parameterization schemes both on-line and off-line
in these models, as well as to understand the effects of various schemes
on model outputs (e.g. see the Project for Intercomparison of Landsurface
Parameterization Schemes - PILPS - activities). The determination of bulk
aerodynamic parameters that control the transfer of energy, mass and momentum
between the atmosphere and the surface, roughness length, zero-plane displacement,
and canopy and ground resistances, is generally regarded as a major source
of uncertainty. For example, in SiB2 (Sellers et al. 1996) 1° X 1°
maps of the bulk aerodynamic parameters are found by first using a global
landcover classification. Mean values of eight canopy and ground morphological
and physical parameters (canopy top height, canopy base height, ground
roughness length, leaf-area density inflection height, leaf width and length,
leaf-angle distribution factor and leaf area index) are assigned from the
literature based on landcover type. These variables are then used with
estimates of LAI from satellite data to derive the bulk parameters at 1° X 1°
spatial resolution. This procedure can only be seen as inadequate
given the sparsity of literature values for canopy variables and their
gross spatial and categorical generalization.
In contrast VCL would provide direct measurements of canopy top height,
canopy base height, sub-canopy ground roughness length, and vertical density
of interceptors (woody and non-woody). Maps of these variables would then
be available globally, and gridded to a resolution as fine as 4 km X
4 km. These can then be used to derive the bulk aerodynamic parameters
at accuracies and spatial extents never before possible, with a resultant
improvement in our ability to model momentum, water vapor and sensible
heat fluxes. In addition, a global data base of canopy structure, especially
when combined with other remote sensing data, such as greenness indices
from MODIS, will enhance our ability to model the interaction of energy
with the surface via better estimates of LAI and FPAR (fractional absorbed
photosynthetically active radiation), as well as the interaction of precipitation
with the surface through interception and retention, with resulting improvements
in model photosynthesis, net primary production, trace gas and hydrologic
fluxes.
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Globally distributed topographic control
points
The strong scientifc need for accurate, global topographic data bases
has led to recent progress in limited release of portions of the Defense
Mapping Agency Digital Terrain Elevation Data (DTED) Level 1 data that
has 90 spatial (horizontal) pixel resolution and 16 m vertical accuracy.
Despite this progress the scientific benefit for global MTPE studies is
not nearly realized. The existing DTED Level 1 data will not be released
for the entire Earth and space-based imaging sensors now in orbit and planned
in the EOS-AM1 era (1998-2003) require global topography at DTED Level
2 (30 m spatial, 16 m vertical) for full realization of their science potential
in land cover/global productivity, short-term climate modeling, and natural
hazard studies. The Shuttle Radar Topography Mission (SRTM) has been announced
to address these needs. Since IFSAR data, or the conventional photogrammetric
data sets, are only relative in their measurement of surface elevation,
direct measurements are needed to "control" the vertical dimension of the
topographic image. Estimates of TCPs needed for a global DTED Level 2 are
well in excess of 100,000,000. Only a limited number of these TCPs can
be provided by ground-based radar targets and GPS receivers, the remainder
will have to be estimated from existing maps and digital elevation models
that do not routinely achieve the 1 meter level of vertical accuracy and
do not have a common reference frame. The VCL surface elevation measurements
will have a common, global reference frame and are "direct" rather than
inferred. Furthermore, only VCL will address the vegetation cover issues
that limit all present mapping techniques to tens-of-meters rms in forested
areas. The VCL Mission, by virtue of its primary vegetation canopy measurements,
will provide billions of sub-canopy surface elevation points.
Vegetation
Canopy Lidar (VCL)