Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data
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<p>CASI-2 image data mosaic of Heron Reef, Great Barrier Reef, Australia, displayed in RGB mode using bands with centre wavelengths of 664.5 nm, 564.0 nm and 478.8 nm, respectively. The locations where field data were collected to validate the classified images are shown.</p> ">
<p>Mean spectral reflectance signatures measured <span class="html-italic">in-situ</span> at Heron Reef, and used as the bottom boundary layer in HydroLight modelling.</p> ">
<p>Subsurface reflectance curves for each benthos and substrate type at depths 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 m. The coloured lines represent the full resolution (400–800 nm) spectra, the black lines represent the correspond curves integrated across the CASI-2 spectral response curves. Note that the scale of the y-axis changes between plots, to allow the shape of the spectra be better seen.</p> ">
<p>Classified image showing benthos and substrates on Heron Reef. Below the main figure are insets which show in more detail the mapping of zones dominated by (<b>a</b>) live coral; (<b>b</b>) macro-algae; (<b>c</b>) patch reefs; (<b>d</b>) spur and groove formations and; (<b>e</b>) benthic micro-algae. Inserts are also presented to show more detail of the bathymetry classified image (<a href="#f5-remotesensing-06-06423" class="html-fig">Figure 5</a>), alongside the true-colour and benthos and substrate classified images.</p> ">
<p>Heron Reef bathymetry classified image; showing the internal lagoon with a maximum depth of around 5.0 m, a shallower reef flat, and a harbour which is around 4.0 m on average. Waters beyond the reef slope, and known to be greater than 10 m deep, were unclassified.</p> ">
<p>Standard error matrix. The three divisions of macro-algae (Brown, Red, and Green), and two modes of live coral (blue and brown), were grouped together to form a single macro-algae and live coral group, respectively. Overall accuracy was 65.13%.</p> ">
<p>Examples of field validation transects, with a 5.0 m circular buffer around each point to account for spatial mis-registration. Transects were assessed for a match between the benthos and substrates within the buffer and the classified image. The snorkel transects shown above were considered to display a good match with the classified image, and were representative of the majority of transects.</p> ">
<p>(<b>a</b>) The classified image of Heron Reef bathymetry was validated using boat-based sonar measurements. The estimated water depth from the classified image was plotted against the sonar depth measurements, and showed a high correspondence, with an R<sup>2</sup> value of 0.9285. The maximum depth modelled in the <span class="html-italic">in-situ</span> library, and therefore the maximum estimated depth, was 10.0 m; (<b>b</b>) Water depths along the boat-based sonar track and estimated from the classified image, running from east to west, beginning to the West of Heron Island (shown in <a href="#f1-remotesensing-06-06423" class="html-fig">Figure 1</a>). Values of zero refer to unclassified areas in the classified image, which was only classified to depths of 10 m.</p> ">
<p>The left column shows spectral reflectance signatures measured <span class="html-italic">in-situ</span> with negligible water column attenuation for red macro-algae, crustose coralline algae, and live coral (brown-mode). The coloured lines represent the full resolution (400–800 nm) spectra, and the black lines represent the corresponding curves integrated across spectral response curves of the CASI-2 sensor. The CASI-2 sensor was able to capture the (complex) spectral reflectance features that characterise these benthos and substrates. In comparison, many of these features were not captured when the subsurface reflectance spectra were integrated across the spectral response curves of the CASI-2 sensor (right column, taken from <a href="#f3-remotesensing-06-06423" class="html-fig">Figure 3</a>).</p> ">
Abstract
:1. Introduction
2. Methods
2.1. Study Site
2.2. Acquisition and Pre-Processing of CASI-2 Image Data
2.3. Creating the Reference Spectral Reflectance Library
2.4. Image Classification to Create a Benthos and Substrate Classified Image, and a Bathymetry Classified Image
2.5. Validation of the Benthos and Substrate Classified Image
2.6. Validation of the Bathymetry Classified Image
3. Results
3.1. Reference Spectral Reflectance Library
3.2. Image Classifications
3.2.1. Benthos and Substrate Classified Image
3.2.2. Bathymetry Classified Image
3.3. Validation of the Classified Images
3.3.1. Benthos and Substrate Classified Image Validation
3.3.2. Bathymetry Classified Image Validation
4. Discussion
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Band Number | Centre Wavelength (nm) | Full Width at Half Maximum (nm) |
---|---|---|
1 | 439.3 | 20.6 |
2 | 459.4 | 20.7 |
3 | 478.8 | 18.9 |
4 | 498.4 | 20.8 |
5 | 516.1 | 15.3 |
6 | 528.5 | 11.0 |
7 | 552.5 | 11.0 |
8 | 564.0 | 9.7 |
9 | 574.4 | 11.6 |
10 | 594.2 | 9.8 |
11 | 614.1 | 11.7 |
12 | 624.5 | 9.8 |
13 | 643.5 | 9.8 |
14 | 664.5 | 9.8 |
15 | 674.0 | 9.8 |
16 | 691.2 | 9.8 |
17 | 707.5 | 7.9 |
18 | 732.4 | 8.0 |
19 | 778.5 | 15.7 |
Benthos or Substrate Type | Threshold Angle (Radians) |
---|---|
Sediment | 0.04 |
Benthic micro-algae | 0.07 |
Algal turf | 0.07 |
Crustose coralline algae | 0.05 |
Brown macro-algae | 0.10 |
Green macro-algae | 0.10 |
Red macro-algae | 0.08 |
Live coral (blue-mode) | 0.09 |
Live coral (brown-mode) | 0.10 |
Soft coral | 0.10 |
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Leiper, I.A.; Phinn, S.R.; Roelfsema, C.M.; Joyce, K.E.; Dekker, A.G. Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data. Remote Sens. 2014, 6, 6423-6445. https://doi.org/10.3390/rs6076423
Leiper IA, Phinn SR, Roelfsema CM, Joyce KE, Dekker AG. Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data. Remote Sensing. 2014; 6(7):6423-6445. https://doi.org/10.3390/rs6076423
Chicago/Turabian StyleLeiper, Ian A., Stuart R. Phinn, Chris M. Roelfsema, Karen E. Joyce, and Arnold G. Dekker. 2014. "Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data" Remote Sensing 6, no. 7: 6423-6445. https://doi.org/10.3390/rs6076423