XXXIX-B8, 2012
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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
serving as the limits of the integrals in (1). Details of the
derivation of ground phase and tree heights for that particular
study could be found in (Mercer et al., 2011). The PCT profiles
were calculated for slant range coherence images as transects in
range and azimuth direction. The averaging window size for
coherence calculation was 11 by 11 pixels in slant range
coordinates.
2.3 Biomass extraction
A ‘standard’ operational method for direct biomass estimation
is through the use of ground sampling sites as shown in
equation (7).
Biomass, = p C DBH?). f AN, (7)
where stem Diameter at Breast Height (DBH), tree height (h),
stem density (N) and wood density p[ g / cm’ ] are determined
on site. The tree is modelled as a tapered cylinder (constant
factor f), leading to the stem biomass per unit area B[tons/ha].
Remote sensing methods often use an allometric relationship
where some observable such as backscatter in the case of radar
(Le Toan et al., 1992) is regressed against biomass estimates
obtained from ground sample sites. Generally such methods
become insensitive at biomass levels of about 150 tonnes/ha or
greater. A more robust relationship has been shown to exist
when tree height (as obtained from POLInSAR or lidar for
instance) is the direct observable (Mette ef al., 2004A; Mercer
et al, 2009B). However the form of the allometric relationship
seems to have species dependencies, and stimulates the interest
in tomography which offers the potential to address the mixed
species problem. The key idea here is to differentiate species by
classification according to structure function. An allotropic
approach with specific parameters would then be derived. In
this paper we restrict ourselves to the first part of the problem —
derivation of the tomograms.
3. DESCRIPTION OF THE DATASET
3.1 Radar System
The SAR data used in our study was collected during an
airborne campaign (Mercer et al., 2009A, 2009B) carried out in
winter 2008 in Alberta, Canada, near the town of Edson. A
proof of concept design was built to accommodate and
demonstrate the feasibility of an airborne PolInSAR system at
L-Band by Intermap Technologies to recover tree height and
ground surface elevation. The system was carried onboard an
Aerocommander aircraft. Some of the system parameters are
presented in Table 1:
Central wavelength 0.226 m
PRF/channel 0.4 kw
No. of channels 12
Polarisation full quad
Horizontal baseline 3.5 m
Flying altitude over ground 1000 m
Azimuth resolution 1.0 m
Slant range resolution 1-1 m
Table 1: Design parameters of Intermap technologies' L-Band
fully polarimetric InSAR radar system.
A quad-pol antenna pair supported by a rigid beam was
mounted across track and provided a horizontal baseline of 3.5
meters. The flying height was around 1000 meters above
ground level and ground resolution was close to 1.25 meters.
The test altitude was chosen to obtain a satisfactorily large
signal to noise ratio while also optimizing conditions for
POLINSAR inversion (Cloude, 2006B).
3.2 The Test Areas
The particular test site (Mercer et al., 2009B) for generating the
tomograms is a forested area near Edson, Alberta, Canada. It
consists of densely forested areas interspersed with clearcut
areas at various early stages of regrowth and with moderate
variations of topography.. This test site was also chosen for
availability of ancillary data such as airborne LiDAR and X-
Band generated digital terrain models, and is an effective
candidate for biomass estimation of pine-lodged trees. Stand
heights ranged from 10 to 30 meters. The forest stem density
was estimated between 100 to 300 stems/ha, based on high
resolution photography analysis.
4. RESULTS
4.1 Tomograms
A composite polarimetric image (Pauli) of one of the passes is
shown in Figure 1 with overlaid red lines indicating the location
of particular tomographic profiles in the azimuth (along-track)
and slant range (cross-track) directions. The green-colored
texture is forest and the darker patches are the clearcut/regrowth
areas. Figures 2 and 3 show the PCT profiles (tomograms) for
these lines as indicated. The tomograms show clearly the
distinction between clear cuts with low tree regrowth patterns,
and older, 15-30 m canopy patches. The color scale represents
the normalized structure function and should be a proxy for the
internal backscattering intensity as a function of vertical
location within the canopy. As explained in the above section,
the upper and lower limits (tree tops and ground phase) are
given from the RVoG inversion estimates which have
accuracies of about 1-4 m (RMS) (Mercer et al, 2009B).
Range (pixels)
Figure 1: Pauli image and transects
Singe FCT L-Bsnd for HY
Figure 2a: Tomogram along Azimuth (1)
PO L-Bami for Hv
Ray
Figure 2b: Tomogram along Azimuth (2)
