Geoscience Reference
In-Depth Information
tic confusion (Comber et al., 2005). Whenever land cover information of different
dates derived from different methodologies and interpretation systems is used
together, the inconsistent class defi nitions can cause problems in understanding the
resulting land cover change maps. However, formal methods exist for dealing with
inconsistent classifi cation systems (Comber et al., 2004).
Remotely sensed data have also been applied in studies modelling the processes
of land-use and land-cover change (De Almeida et al., 2005). Since they provide a
spatial and temporal data source, they enable new insights into space-time dynamics
of the land surface and its driving factors.
Forest biomass
Forest biomass is a parameter of great economic importance to forest enterprises,
natural resource managers and climate scientists. It can be indirectly estimated from
stand parameters and canopy height using sets of published species-specifi c allome-
tric equations. Forest canopy height can be estimated from stereophotogrammetry,
LiDAR, and Synthetic Aperture Radar (SAR). On a landscape-scale forest parame-
ters can be mapped at high accuracy and a spatial resolution of 0.5-2 m using
Imaging LiDAR. The basic principle is that a model of the underlying unvegetated
terrain is subtracted from the signals received from the canopy top to derive a veg-
etation canopy height model. The terrain model can be generated from the last
return data of the LiDAR, but it generally needs to undergo several fi ltering and
interpolation steps.
Detailed maps from airborne remote sensing have a high spatial resolution and
are used in land and habitat management, forestry and agriculture. In this way,
remote sensing can help update forest inventory geographic information systems
like those used by the British or the Russian Forestry Commissions. Forest biomass
is also directly correlated to carbon content of the vegetation. This information is
useful for informing government agencies about aboveground carbon stocks, which
are needed for national reports to the Secretariat of the United Nations Frame-
work Convention on Climate Change under the Kyoto Protocol to combat climate
change.
Researchers interested in global change often require large-scale coverage that
can only be obtained from a satellite-borne instrument. The fi rst spaceborne profi l-
ing LiDAR instrument, the Geoscience Laser Altimeter System (GLAS), was launched
on-board ICESAT on 12 January 2003. While the mission focus is to measure ice
sheets, clouds and aerosols, the instrument can also be exploited for land and veg-
etation applications. GLAS fi res 40 pulses per second and records the intensity of
the refl ected radiation as a function of travelling time, resulting in approximately
70-m footprints, which are spaced about 170 m apart along the track. Between
orbital paths, footprints are between 2.5 km (near the poles) and 15 km (near the
equator) apart (University of Texas, 2003). Each pulse results in a full waveform
measurement, which provides a profi le of the illuminated footprint in the third
dimension (fi gure 19.3). Biomass indicators that can be derived from the full-
intensity waveform are terrain elevation, canopy height and crown depth (Harding
and Carabajal, 2005).
Whereas LiDAR technology essentially measures the time that the radiation takes
to reach the target and back at a specifi ed point or footprint location, Synthetic
Aperture Radar (SAR) is based on active microwave radiation. Since microwaves
