3 Chapter 6: The use of laboratory spectroscopy to establish Pteronia incana spectral trends and separation from bare surfaces and green
7.5 Application of pixel and sub-pixel based classifications to separate P. incana
The output in pixel-based methods is often a composition of materials within a pixel (Adam and Gillespie, 2006). In scenarios that may not require local detail, reliable P.
incana invasion mapping can be achieved using aggregation of pixel components in HRI. Results in this study show that consistent separability can be achieved when the pixel based PVI is applied to HRI. The biggest advantage of PVI application in P.
incana separation is its ability to minimise the effect of background soil reflectance in P. incana invaded environments. This is particularly important in P. incana invaded environments often characterised by inter-patch bare surfaces.
Whereas pixel based techniques like the PVI may be an option in land cover mapping, such techniques may not provide accurate mapping of P. incana invaded surfaces depending on, the spatial resolution of the imagery used. However, this study showed that sub-pixel techniques that de-convolve surface types within a pixel based on selected end-members can be used to account for major cover types within P. incana invaded environments.
106 In keeping with other SMA applications, the reliability of P. incana fractions is dependent on the quality of endmembers selected. Due to the spatial coverage and limited cover types that characterise P. incana invaded surfaces, image based endmember selection is a more suited technique for extracting P. incana fractions.
Depending on the number of unique spectra in an image, these characteristics enable fraction extraction from both high and low resolution imagery. In this study, the identification of green vegetation endmembers in P. incana invaded environments was relatively straightforward. However, care should be taken when identifying P.
incana and bare surfaces endmembers as their spectral differences were generally small.
Whereas the SMA has commonly been used in low spatial resolution imagery, (see;
Souza and Barreto, 2000; Sobal et al., 2002; Uenishi et al., 2005), it has also been successfully used in medium (see Robichaud et al., 2007) and low (see Zhu, 2005;
Miao et al., 2006) spatial resolution situations. This study further confirms that an application of spectral mixture models should not be limited to medium and coarse spatial resolution imagery. In a similar study using a 1x1m spatial resolution Compact Airbone Spectrographic Imager (CASI), Miao et al. (2006) showed reliable mapping of Centaurea solstitialis (Yellow starthistle) invasion in California’s Central Valley grassland using spectral un-mixing.
A combination of pixel based techniques like PVI and sub-pixel techniques like SMA in P. incana mapping can be used to enhance the reliability of invasion interpretation.
Whereas it is acknowledged SMA applications may not produce reliable results with a large number of components within a pixel (Adam and Gillespie, 2006), its application within P. incana invasion environments which are often characterised by two other major constituents (green vegetation and bare surfaces) increases its potential as a tool to P. incana mapping.
In summary, this study managed to identify relationship between P. incana invasion and a range of variables. The importance of isohyetic gradients as determinants of invasion boundaries was identified. The study also demonstrated the implications of P. incana invasion for surface moisture flux, particularly the potential of conversion
107 of invaded areas to dysfunctional landscapes. Spectral analyses confirmed that P.
incana has unique spectral characteristics from other vegetation types and showed the potential of complimenting pixel and sub-pixel based analyses in P. incana mapping.
P. incana spectral investigation was limited to its difference from green vegetation and bare areas. Consequently, to provide further understanding of remote sensing applications in P. incana invasion and its interaction with invaded environments, the following directions for future research are recommended:
i) A comparison between P. incana and typical green vegetation internal leaf structures as potential causes of spectral differences.
ii) Collection of spectra for P incana and other invader vegetation types, some of which have similar characteristics, with a view to assembling a spectral library for delineating invaded environments using imagery.
The main research questions raised in this study namely:
• What is the pattern of P. incana occurrence across a range of gradients?
• What is the hydrological response of P. incana invaded surfaces as compared to grass and bare surfaces?
• What is the ideal wavelength for separating P. incana from bare surfaces and green vegetation cover?
• Can consistency be achieved in separating P. incana invaded areas using multi-temporal HRI? Are sub-pixel techniques more effective than pixel ones in P. incana separation using HRI?
have all been addressed.
The study has inter alia confirmed the reliability and consistency of HRI in the delineation of P. incana using both pixel and sub-pixel techniques. The imagery is therefore a useful tool in the rehabilitation of areas invaded by undesirable vegetation species.
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