Light Detection And Ranging (Lidar) systems
are a key complement to optical scanner technology for future development
of forest inventory, biomass measurements, gap modelling and carbon
accounting applications. Lidar is a technique used to measure distance
by precisely timing the return time of a laser pulse reflected (backscattered)
from a target. With a solid object, the return from a laser pulse
will be sharp and distance can be calculated precisely. With spatially
distributed objects, such as the foliage elements of a forest canopy,
the return is more complex and contains information about the spatial
arrangement and density of the target components. The objective
of vegetation canopy Lidar studies is the extraction of this information
from the return pulse. The CSIRO Canopy Lidar Initiative (CLI) has
been working for a number of years to develop algorithms to extract
this forestry related information from current and future airborne
Lidar systems.
The main use of commercial airborne Lidar systems (and the major
driver for their design specifications) is to derive digital elevation
models. Thus the instrument specifications and data processing methods
designed around such instruments aim to eliminate the effect of
above-ground features (such as trees and other vegetation) and retrieve
information relating only to the underlying terrain. When information
relating to the forest structure is the primary objective, alternative
instrument specifications may be used, including an increase in
the beam divergence, resulting in larger footprints, and digitisation
of the return intensity to provide laser ‘waveforms’ – or the complete
time trace of return power to the airborne platform. A Lidar with
such specifications is commonly referred to as a ‘canopy Lidar’.
As one of its strategies, the CLI is investigating the capacity
of current airborne
terrain Lidar systems to provide data from which useful forest
inventory parameters can be derived. Although the inherent limitations
of these instruments place constraints on the accuracy and cost-effectiveness
of derived forestry information products, the CLI is developing
processing methods that minimise bias in products derived from this
currently available and very useful source of data. A more detailed
discussion of the issues involved can be found in Lovell et al.
(2003). In the future, when dedicated canopy Lidar systems become
available to the forestry industry, a range of more sophisticated
products and more cost-effective outcomes are anticipated.
The CLI has a complementary focus
on the development of an advanced ground based scanning Lidar system
known as ECHIDNA™. This system
can be used, in combination with airborne Lidar, to scale up detailed
forest variables to large areas or as a stand-alone plot-based measurement
tool, providing a complete site-based forest inventory with both
primary productivity and environmental data outputs. At this time,
a prototype is being used for preliminary field testing and evaluation
of theoretical concepts on which the final ECHIDNA™ instrument
will be based. A fully functional research instrument is also being
constructed and is expected to be ready for field deployment in
early 2004.