Aggregated Habitat Cover Maps Depicting the Shallow-water Benthic Habitats of the Northwestern Hawaiian Islands Derived from High Resolution IKONOS Satellite Imagery

Shallow-water, aggregated cover maps were produced by combining as many as four or more detailed habitat types into general cover categories. The original detailed habitat maps were produced by rule-based, semi-automated image analysis of high-resolution satellite imagery for nine locations in the Northwestern Hawaiian Islands. This project is a cooperative effort among the National Oceanic and Atmospheric Administration, State of Hawaii Department of Land and Natural Resources, and the U.S. Fish and Wildlife Service to produce benthic habitat maps and georeferenced imagery for the Northwestern Hawaiian Islands. This project was conducted in support of the U.S. Coral Reef Task Force.

The National Ocean Service is conducting research to digitally map biotic resources and coordinate a long-term monitoring program that can detect and predict change in U.S. coral reefs, and their associated habitats and biological communities. This project was conducted in support of the U.S. Coral Reef Task Force (http://www.coris.noaa.gov).

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Not to be used for navigation.

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These maps are NOT to be used for navigation. These aggregated cover maps of the shallow-water benthic habitats of the Northwestern Hawaiian Islands have been reviewed by numerous decision makers and scientists familiar with the Northwestern Hawaiian Islands region.

Statistical analysis to determine accuracy was performed using a subset of the 1,130 site specific benthic characterization data (376 points were used in the accuracy assessment) available for the NWHI. The overall accuracy of the aggregated cover maps was 72 percent (Kappa Statistic 0.59, Tau Coefficient 0.62), and ranged from 94 percent accuracy for unconsolidated sediment with little or no algae or seagrass to 9% for unconsolidated sediment with seagrass present. Correctly classifying benthic habitats that include algae and live coral was challenging. For example, it is difficult to separate Unconsolidated Rubble with dense (>50 percent cover) algae from Pavement with dense (>50 percent cover) algae. Also, it is difficult to separate Unconsolidated Rubble and Sand and Rubble from Pavement habitats. These challenges also were encountered when differentiating between the various habitats with live coral cover.The aggregated cover category Hardbottom with indeterminate cover was not included in the accuracy assessment. The aggregated Hardbottom with indeterminate cover includes those detailed habitat categories where a specific cover was not identified on the map, and no comparison to the field-based benthic characterization information could be performed. An assessment of the overall accuracy of the hardbottom and unconsolidated was performed. That analysis included habitat categories with indeterminate cover and their associated field-based benthic characterization data. That analysis revealed that these two habitats were correctly identified 82% and 90% of the time respectively.

Kure Atoll, Midway Atoll, Pearl and Hermes Atoll, Lisianski Island, Laysan Island, French Frigate Shoals, and Nihoa Island are positioned to within five (5) meters at sea level. Necker Island is positioned to within 15 meters at sea level. Maro Reef is positioned to within eight (8) meters at sea level. Spatial data meets or exceeds national map accuracy standards for fixed features.

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High-resolution satellite imagery was analyzed using rule-based, semi-automated, digital image analysis techniques to produce benthic habitat maps of the Northwestern Hawaiian Island. In total, 1,130 site specific examples of the spectral and spatial characteristics of differing habitats were available and incorporated into the analysis process for the NWHI. The minimum mapping unit (MMU) for identifying habitats or features was 100 sq. m. Generally, features to a depth of approximately 30 m can be analyzed from the satellite imagery. Due to the depth of water surrounding Gardner Pinnacles, very little characterization of the benthic habitats of Gardner Pinnacles was possible. No satellite imagery was available for Gambia shoals.

Multispectral satellite imagery was positioned using National Geodetic Survey ground control data for Kure Atoll, Midway Atoll, Pearl and Hermes Atoll, Lisianski Island, Laysan Island, French Frigate Shoals, and Nihoa Island. Satellite imagery of Necker Island was positioned using satellite ephemeris data. Satellite imagery of Maro Reef was positioned using a combination of satellite ephemeris data and supplemental bathymetry transect data. Once maps were positioned, validation was performed by comparison to repositioned vector shoreline data depicted on recently-revised NOS nautical charts. The draft classified benthic habitat maps were reviewed by scientists in Hawaii with first-hand local knowledge of the Northwestern Hawaiian Islands. Comments were received and incorporated into draft maps. Draft maps also underwent a statistics-based accuracy assessment.