springs has become uncertain as demands on this iconic
groundwater resource increase. The impacts of existing water
extractions for mining and pastoral activities are unknown. This
situation is compounded by the likelihood of future increasing
demand for extractions.
Despite their importance, GAB springs have received scant
inventory and documentation of their vegetation extent,
distribution and composition. Hyperspectral remote sensing
provides the necessary spectral and spatial detail to discriminate
wetland vegetation communities and has shown great potential
in previous studies for mapping wetlands vegetation (Torbick
and Becker, 2009; Hestir et al., 2008; Zomer et al., 2009). This
technology will enable accurate and repeatable baseline and
longer-term mapping of these remote and spatially disparate
groundwater dependent ecosystems (GDEs) to be achieved.
The objectives of this paper are to discriminate the spatial
extent and distribution of key spring wetland vegetation
communities associated with the GAB springs evaluating
Spectral Angle Mapper (SAM), Mixture Tuned Matched
Filtering (MTMF) and Spectrally Segmented Principal
Component Analysis (PCA) hyperspectral techniques. In
addition, to determine if the hyperspectral techniques developed
can be applied at a number of sites representative of the range
of spring formations, their floristic and geomorphic diversity
and at two temporal intervals (March 2009 and April 2011).
1.2 Study area
The GAB springs found in South Australia (SA) are located
along the western margin of the GAB and are formed from the
natural outflow of groundwater. The GAB springs surface
expression is composed of wetland vegetation, saline flats and
carbonaceous substrate. The wetland plant communities of GAB
springs vary between spring groups (cluster of springs sharing
the same water source) and complexes (clusters of spring
groups which share the same geomorphological settings), some
of which are rare, relic and endemic to this region. This paper
focuses on three sites Hermit Hill Springs complex (latitude
29.60° S and longitude 137.41° E), Freeling Springs, Mount
Denison complex (latitude 28.06° S and longitude 135.91° E),
and Dalhousie Springs complex (latitude 26.45° S and
longitude 135.51° E). The three sites are representative of GAB
springs within SA. The sites also differ in their geomorphic
setting, vegetation community composition, spatial extent and
distribution of springs.
2. MATERIALS AND METHODS
2.1 Image Data
Two epochs of HyMap airborne hyperspectral mosaicked image
cubes were captured for this research in March 2009 and April
2011. The two epochs of HyMap imagery were captured under
dry antecedent conditions (March 2009) and wet antecedent
conditions (April 2011). The HyMap imagery is composed of
126 wavebands with a bandwidth of ~15 nm and wavelength
range of 450 — 2,500nm, a ~3 m ground sampling distance
(GSD), and swath width of 1.5 km. The raw HyMap imagery
was radiometrically corrected along with geometric correction
and colour balancing of swaths to form a seamless mosaic
(Kruse et al., 2009; Cocks et al., 1998). Colour digital aerial
photography at 30 cm GSD was acquired concurrently with the
HyMap imagery.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
Results discussed in this paper focus on the March 2009
baseline image capture for all three sites. More detailed
comparison of changes between March 2009 and April 2011 is
focused on the Hermit Hill springs complex.
2.2 Field data
The image acquisition coincided with a field campaign of
spectroradiometry measurements and a botanical survey, which
included recording vegetation cover and composition within 9 x
9 m sample plots representative of the range of spring
vegetation types and cover. The spatial locations of field
measurements were recorded with GPS.
2.3 Image processing
To identify key wavebands which have the greatest capability to
discriminate vegetation communities of the GAB springs and
surrounding area three hyperspectral data reduction techniques
were employed: (i) the Minimum Noise Transform (MNF); (ii)
the Pixel Purity Index (PPI); and (iii) Spectrally Segmented
PCA (SSPCA). SSPCA was applied to NDVI-masked
vegetation portions of the HyMap imagery with wavelength
regions spectrally segmented for the VIS-NIR (450-1,350 nm),
SWIR 1 (1,400-1,800 nm) and SWIR 2 (1,950-2,480 nm). The
pure pixels identified from the SAM and MTMF analyses were
interactively verified as specific spring vegetation communities
using spatial location and attribute data collected from the
botanical survey plots overlayed onto colour composites of the
HyMap imagery, MNF and PPI output images. The resulting
pure endmember image pixels of the vegetation communities
identified were used as target spectra for input into the SAM
and MTMF algorithms which identify pixels within the HyMap
image which are spectrally similar to the target spectra.
3. RESULTS AND DISCUSSION
The resulting SAM and MTMF output images of the identified
target vegetation communities are currently being validated
through formal accuracy assessment using field observations
and digital colour aerial photography. Spring wetland
vegetation communities successfully discriminated include low
lying reeds and sedges along spring tails (Baumea spp., Cyperus
spp., Fimbristylis sp., Gahnia trifida sp.), dense homogenous
stands of Phragmites australis reeds and White Tea Tree
(Melaleuca glomerata sp.). In addition sporadic patches of salt
couch grass (Sparabolus spp.), Samphire shrublands
(Haloscarcia sp. and Sarcocrina sp.) and Sea Heath
(Frankenia sp.) which fringe the springs were also
discriminated.
VIS-NIR PCs 2, 3 and 9 identified key vegetation
discrimination wavelength features, in particular, the green peak
at 555 nm, chlorophyll absorption feature at 685 nm, VIS-NIR
and red-edge contrasts. The SWIR 1 PC 3 mapped Phragmites
australis, with notable loadings at wavelengths 1,450, 1,645-
1,715, 1815, and 1,825 nm associated with water, lignin, and
cellulose absorptions, respectively. The SWIR 2 region revealed
little spectral variation (White and Lewis, 2010).
Differences between March 2009 and April 2011 HyMap image
captures focused on the Hermit Hill springs complex. The
extent and distribution of spring wetland vegetation
communities varied between the two image dates. Most notable
were decreases in Phragmites australis reeds in contrast with
