Coral reef habitat mapping in the Red Sea (Hurghada, Egypt) based on remote sensing

Remote sensing can give information about the configuration and composition of coral reefs, about the biophysical parameters of the seas and oceans in which they occur and about the changes over time of these elements. This paper deals with the classification of a Landsat7 ETM+ data set in order to identify the different bottom types (macro-algae, coral, sea grass and sand) occurring on the reefs offshore Hurghada, Egypt. Before classification, the radiance values received at sensor are corrected for atmospheric and water column effects. ‘Depth-invariant bottom indices’ are calculated and form the basis for classification. Besides the bottom type as an ecological classification, also a geomorphological classification is made. After contextual editing of the ecological classification, both results are combined into an open-ended hierarchical classification scheme. An in-depth accuracy assessment still needs to be undertaken but a mean accuracy between 47% and 83% is to be expected

Remote sensing as a tool for bathymetric mapping of coral reefs in the Red Sea (Hurghada-Egypt)

For monitoring coral reefs, a bathymetric map is useful as a base map. Different methods have already been developed to map bathymetry using remote sensing. Two main groups can be distinguished. One group is using active remote sensing data; the other is based on passive sensor generated multi-spectral information. In this article, the focus is on the passive data method. A modified «depth of penetration» mapping method was implemented on a Landsat7 ETM+-image over Hurghada (Egypt). Some 420 depth measurements were used for ground-truthing and accuracy testing. The accuracy test revealed that the resulting bathymetric map is useful for coral reef mapping, but care should be taken when using it. Deviations from reality were caused by assumptions inherent to the theory used, field sampling, satellite image characteristics and errors during implementation of the method

Hyperspectral data for coral reef monitoring. A case study: Fordate, Tanimbar, Indonesia

Coral reefs are endangered world-wide by devastating fishing methods (overfishing, dynamite and cyanide fishery), pollution, tourism, environmental changes and bleaching. The aim of this project is to monitor coral reefs and associated ecosystems (mangroves, sea-grass beds) by integrating different remote sensing data with spectral libraries and field measurements. The study area is Fordate, a small island to the northeast of Tanimbar, Indonesia. The monitoring system under development will enable not only the mapping of the coral reefs but also the localisation of those parts of the reefs that are most affected by degradation. A first test with hyperspectral data from the CHRIS/PROBA sensor shows promising results for the discrimination of different bottom-types on the Pulau Nukaha patch reef east of Fordate. Further field sampling and spectral measurements are needed to validate this preliminary classification

Error determination in the photogrammetric assessment of shoreline changes

The evaluation of error or uncertainty in shoreline change studies is an issue of prime importance for providing an adequate framework for calculated rates of change and to allow the establishment of threshold values above which the rates would be significant. In this note, a practical, easy-to-use method is presented to estimate error involved in the calculation of shoreline changes on aerial photographs, including the three most used types of shoreline indicators: high water line, dune/cliff toe and cliff top. This approach takes into account the specific characteristics of each shoreline proxy, such as relief in the case of the cliff top or tidal oscillations in the case of the high water line. At the same time it includes the error components that are independent from the proxy, basically related to the technical aspects of the process such as photo scanning and georeferencing. A practical example of application of the method is provided for several types of data inputs, based on shoreline changes around the Bay of Cádiz (SW Spain)

Can ASTER-data be used for bathymetric mapping of coral reefs in the Red Sea using digital photogrammetry?

Coral reefs are main contributors to the national income of many developing countries. As they are experiencing more and more stress due to inappropriate human actions and global change, they need to be studied in order to monitor them and preserve them for the future. Bathymetric mapping of coral reefs has always experienced difficulties. Due to their rather shallow depth and/or often remote location in the middle of the ocean, they are not always suitable for standard bathymetric sounding campaigns. Here remote sensing can bring a solution. Different methods have been developed to derive bathymetric information out of remote sensing data. Some methods are based on information gathered by active sensors, e.g. LIDAR; others are based on the different attenuation coefficients of sequencing wavelengths detected by multispectral passive sensors. The aim of this research paper is to examine the possibilities to map bathy-metry of coral reefs using digital photogrammetric processing of stereoscopic ASTER-images. Through-water photogrammetry has already been used for subaqueous mapping, for example by Westaway et al. (2001) and Butler et al. (2002) in close-range river bed surveys using mounted photogrammetric cameras. Due to the refraction of the light on the water surface, calculated depths tend to be underestimated in comparison with the true depth. Westaway et al. (2001) have developed a refraction correction algorithm to deal with this problem. The ASTER stereoscopic data have already proven its usefulness in generating terrestrial DEM’s using the digital photogrammetrical software package VIRTUOZO 3.2. Due to the fact that stereoscopic ASTER - images are only made in the infrared, its application in bathymetric mapping is limited to the upper meters of the reefs. As study area the coral reefs surrounding Big Giftun Island off shore Hurghada (Egypt), are selected. First a DEM is generated in the conventional way. The strict use of absolute ground control points that lay under the water surface, has been proven to be beneficial for the accuracy of the depth estimations. Unfortunately, due to refraction at the air/water interface, an additional shift in X and Y then occurs. Some errors are also occurring after match editing which are distorting the calculated depths. Afterwards the correction algorithm for the two media problem developed by Westaway et al. (2001) has been applied. Although the first, preliminary result presented here, is still subject to some errors, general trends in reef topography can already be noticed. It can be concluded that it is possible to estimate depth of the coral reefs using digital photogrammetry and ASTER data. Further investigation is required for the need of additional adaptation of the refraction correction algorithm to use with ASTER data and the remaining problems with geometric accuracy and the match editing procedure