Declaration 2- Publication and manuscripts
2.8 Overall challenges and future research
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and water, and atmospheric conditions. Further research is needed to develop indices that can reduce the effects of background and atmospheric quality.
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From a research perspective, however, there are several major challenges in the application of remote sensing in wetland species that need to be addressed.
First, the most current remote sensing techniques in mapping vegetation have been undertaken in arid and semi-arid regions with low vegetation cover and less complexity within the vegetation unit. These techniques are, therefore, of little use for narrow vegetation units that characterize wetland ecosystems. Additional research effort is needed to adopt more classification techniques to improve the accuracy of the spatial resolution of the current sensors which varies from 20 m to 30 m. Hyperspectral radiometers are considered to be the sensors of choice in the future for mapping and monitoring wetland species. This has increased the need to build comprehensive spectral libraries for different wetland plant species under different plant conditions and environmental factors. Additionally, the fundamental understanding of the relationship between the reflectance measurements, wetland species’ canopy density, and bottom reflectance parameters should be studied further. The spectral libraries of wetland species will help in discriminating not only between wetland species, but also between wetland species and upland species as there has been no specific research dealing with the difference in spectral response of canopies of wetland species and upland species.
Second, in the southern African region, more research is needed to enhance ability in discriminating wetland vegetation and estimating its biophysical and biochemical properties which have been overlooked in the scientific research. For example, papyrus swamps (Cyperus papyrus L.) (which characterize most of the tropical African wetlands, with a high rate of biomass production, a tremendous amount of combined nitrogen, that play vital roles in hosting habitats for wildlife and birds) are omitted in the application of remote sensing in discriminating wetland vegetation.
Third, although some studies have been undertaken on estimating the vegetation biophysical and biochemical parameters (e.g. LAI, water content, biomass, pigment concentration, and nitrogen) in different ecosystems, there is paucity of research on wetland species. After the progress in the field of spectrometry, researchers began to measure vegetation properties in complex ecosystems using new narrow band indices (Mutanga and Skidmore, 2004a) and red-edge position (Mutanga and Skidmore, 2007). These efforts should be further extended and developed so as to cope with wetland species environments where the saturation and the atmospheric vapour affect the near-infrared region. A fourth research prospect is the
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availability of hyperspectral sensors which could allow mapping both of species and their quality in wetland ecosystems. This will enhance a fundamental understanding of the spatial distribution of the quality and quantity of wetland species, which could lead to the development of early warning systems to detect any subtle changes in wetland systems such as signs of stress and lead to the development of techniques to classify wetland area conditions (e.g. healthy or disturbed) based on their species quality and quantity.
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CHAPTER THREE
Spectral discrimination of papyrus (Cyperus papyrus L.) using a hand-held spectrometer under field conditions
This chapter is based on
:
Adam, E., and Mutanga, O., 2009. Spectral discrimination of papyrus vegetation (Cyperus papyrus L.) in swamp wetland using field spectrometry. ISPRS Journal of Photogrammetry and Remote Sensing, 64, 612-620.
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Abstract
Techniques for mapping and monitoring wetland species are critical for their sustainable management. Papyrus (Cyperus papyrus L.) swamps are among the most important species rich habitats that characterize the Greater St Lucia Wetland Park (GSWP) in South Africa. This paper investigates whether papyrus can be discriminated from its co-existing species using ASD field spectrometer data ranging from 300 nm to 2500 nm, yielding a total of 2151 bands. Canopy spectral measurements from papyrus and other three species were collected in situ in the Greater St. Lucia Wetland Park, South Africa. A new hierarchical method based on three integrated analysis levels was proposed and implemented to spectrally discriminate papyrus from other species as well as to reduce and subsequently select optimal bands for the potential discrimination of papyrus. In the first level of the analysis using ANOVA, we found that there were statistically significant differences in spectral reflectance between papyrus and other species on 412 wavelengths located in different portions of the electromagnetic spectrum. Using the selected 412 bands, we further investigated the use of classification and regression trees (CART) in the second level of analysis to identify the most sensitive bands for spectral discrimination. This analysis yielded eight bands which are considered to be practical for upscaling to airborne or spaceborne sensors for mapping papyrus vegetation. The final sensitivity analysis level involved the application of the Jeffries–Matusita (JM) distance to assess the relative importance of the selected 8 bands in discriminating papyrus from other species. The results indicate that the best discrimination of papyrus from its co-existing species is possible with six bands located in the red-edge and near-infrared regions of the electromagnetic spectrum.
Overall, the study concluded that spectral reflectance of papyrus and its co-existing species is statistically different, a promising result for the use of airborne and satellite sensors for mapping papyrus. The three step hierarchical approach employed in this study could systematically reduce the dimensionality of bands to manageable levels, a move towards operational implementation with band specific sensors.
Keywords: Papyrus. Greater St Lucia Wetland Park. Field spectrometer measurements. CART.
Jeffries–Matusita.
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