USGS TerraWeb: Change Detection, Moloka'i, Hawai'i (USGS-GD-WRCMG)

Geologic Studies of Coral Reef Habitats
Remote Sensing Applications to Coral Reefs: Moloka'i, Hawai'i

Use of Digitized Multitemporal Aerial Photographs to Monitor and
Detect Change in the Clear Shallow Coastal Waters of Moloka'i, Hawai'i

Goal | Change Detection | Results | Moloka'i Title Page

Goal

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.

TOP

Change Detection

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.

TOP

Results

Figure 1: Digital aerial photomosaic of a portion of south Moloka'i (800 x 600, 600 kilobytes)

The digital change image generated using these data shows areas that have under gone change between September 1993 and January 2000. At this stage we can not determine whether the changes are related to long-term impacts or short-term seasonal effects. An important aspect of the results is that they verify that the procedure can detect changes in this environment using these types of data and image manipulation techniques. Areas A and B highlighted in figure 1 (600 kilobytes) were visited in June 2000 to inspect locations that showed a substantial change in brightness/color. On the shallow reef flat (A in figure 1 -- 600 kilobytes), it appears that the majority of the changes detected are related to a change in the amount of algae cover. A small amount of sea grass (Halophila hawaiiana) was present just to the east of area A and close to the fishpond, but not within area A. The dominant algae type in this area is Acanthophora spicifera, with some Schizothrix/cyanobacteria also present. Outside of area A closer to Hotel Moloka'i the dominant algae is Schizothrix/cyanobacteria with several others present (e.g., Ulva fasciata, Padina australis, and Acanthophora spicifera) (personal communication, Lucile Marchese, 2000). The results show that for this area there is less algae cover in January 2000 than September 1993.

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.

TOP

TOPICS: Aerial Imagery | Depth Corrections | Digital Cameras
Moloka'i Title Page | Coral Reefs Title Page | USGS Coral Reefs Page

U.S. Department of the Interior | U.S. Geological Survey | Jobs
Home | Search | Disclaimers & Privacy Web Rings
Send comments to webman@TerraWeb.wr.usgs.gov
URL:
Last Modified: Thu Oct 31 15:12:10 MST 2002