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Geologic Studies of Coral Reef Habitats
Remote Sensing Applications to Coral Reefs: Moloka'i, Hawai'i |
Goal | Change Detection | Results | Moloka'i Title Page
The main objective of this task was to explore the use of multitemporal aerial photographs to detect, map, and analyze changes in shallow benthic environments over time.
Digitized natural color aerial photographs collected in September 1993 and January 2000 were used to generate digital image maps covering most of the coast and reef tract of southern Moloka'i, Hawai'i. Portions of the georeferenced digital image maps are being used as input to digital change detection and analysis using one-meter pixel resolution. Figure 1 shows a portion of both the September 1993 and January 2000 image maps, and the corresponding digital change image generated using these data. The January 2000 data have a higher spatial resolution and were digitally degraded to approximate the September 1993 one-meter resolution. The near-shore inner reef water depth ranges from approximately 0 to 2 meters and various cover types can be seen in the aerial image maps including fine grain sediments, sand, algae, coral, and coral rubble. One of several fishponds in the area is located towards the top center of the image. On the outer reef crest where water depth ranges from about 1 to 3 meters sand and hard rock are visible in the two images. Information about sand and hard rock cover is likewise visible on the deeper fore reef where water depth ranges from 3 to 10 meters.
![]() Figure 1: Digital aerial photomosaic of a portion of south Moloka'i (800 x 600, 600 kilobytes) |
On the outer reef crest area (B in figure 1 -- 450 kilobytes), it appears that the majority of the changes are related to the amount of sand cover over hard rock. Divers confirmed that the area is a rocky irregular old reef surface having a patchy distribution of sand and algae, with very few coral colonies. Ribbons of sediments and ripples of sand cover the slightly deeper drainages. This area has high wave energy and the amount of sand cover over hard rock can probably change on a daily basis, with substantial change possible during large storms. However, at this stage we can not eliminate the possibility that some of the changes are not related to algae growth.
Further investigations into the use of very high resolution remotely sensed images for detecting change in shallow clear waters is continuing. The change detection work done so far indicates that in certain water depths these types of data could be used to detect and map areas affected by long-term or short-term impacts, such as climate or large coastal storms. We will continue to investigate the applicability of these data to detect and map long-term versus shorter-term seasonal changes in clear coastal shallow waters.
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