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Analyzing Benthic Data: Examples

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Summary Puerto Rico Coral Mapping

Narragansett Bay Eelgrass

Puerto Rico Coral Mapping

Aerial photo showing delineated sand substrate. Click image for a more detailed view and substrate comparison.

The National Oceanic and Atmospheric Administration (NOAA) designed a project to create consistent and comprehensive maps of benthic habitats throughout the U.S. Virgin Islands and Puerto Rico. Visual interpretation of aerial photographs was used to classify and delineate various habitats including coral reefs, seagrass beds, and mangrove forests. These maps provide useful information for fisheries management, coastal zone management, disaster mitigation, and coral reef research programs.

Prior to mapping, a habitat classification scheme was devised to provide a standardized methodology for photo interpretation. This classification scheme was based on several factors, including existing classification schemes for benthic habitats of coral reef systems, limitations of the data (minimum mapping unit of one acre), quantitative data for habitats in the region, and management needs. Twenty-six distinct habitats were classified by location ("zone") and substrate or cover type ("habitat").

There were several sources of error that had to be removed from aerial photographs prior to visual interpretation. Spatial distortions due to factors such as camera angle and specific lens characteristics can cause distance and scale inaccuracies. These errors were eliminated through a process called orthorectification. During this process, aerial photographs were digitally scanned and algorithms were applied to remove each source of spatial distortion. The resulting digital mosaic contained several photographs with uniform scale. Habitat boundaries could then be more accurately and efficiently delineated from the orthorectified images.

Example images illustrate orthorectification. Landsat TM image (left) in original orientation with locations of ground control points highlighted in black circles; full image rectified and reprojected (right). Note the shift in image orientation and location of ground control points. Courtesy: U.S. Geological Survey

Using geographic information system (GIS) software, digital images were analyzed by tracing a polygon around each distinct habitat. Differentiation of habitat type depended on visual cues such as perception of color, tone, texture, and geographic context.

Aerial photography was used to generate detailed benthic habitat maps over a large and diverse nearshore area. Courtesy: NOAA Biogeography Program

Field surveys were conducted to validate the visual photo interpretations. These validation surveys included direct observations from snorkeling, free diving, and from boats in clear, shallow water. During this phase of the project, transect surveys across many representative habitat types encompassing various depths and water conditions were performed. This validation process confirmed preliminary habitat delineations and assisted in classifying sites that were difficult to interpret from aerial photographs alone.

A significant goal of the Benthic Habitats of Puerto Rico and the U. S. Virgin Islands project was the development of new technology to improve the efficiency and accuracy of habitat delineation from aerial photography.

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