In order to update the NOAA Biogeography Team's benthic habitat data and enhance the level of detail for select portions of the data set, the most recently (2001 - 2004) available multispectral IKONOS image mosaic and a significantly smaller minimum map unit was used. An average minimum map unit of approximately 1/8 acre was used for the mapping of benthic habitats in four of Guam's marine preserves and other "focus areas"; some polygons with an area less than the minimum mapping unit occur in the data, however. The IKONOS imagery was pan-sharpened using ERDAS Imagine image processing software. The heads-up digitizing functions of ArcGIS were then used to adjust the boundaries of the existing benthic habitat data, create new features, and update feature attribute information. A hierarchical classification scheme similar to that used by the NOAA Biogeography Team was adopted for the data used in the Guam Coastal Atlas.
Four of Guam's marine preserves (Tumon Bay, Piti Bomb Holes, Achang Reef Flat, and Sasa Bay Preserves), as well as Cocos Lagoon, Pago Bay, and East Agana Bay, were targeted for highly detailed benthic habitat interpretation using the smaller minimum map unit; more detailed benthic habitat maps for other coastal areas may become available at a later date. Benthic habitat maps for much of the area beyond the marine preserve boundaries and the focus areas utilized the original NOAA Biogeography Team benthic habitat data, although some alterations (e.g. polygon boundary adjustments, classification corrections/updates) were performed. Ground validation surveys were also performed during the development of the Guam Coastal Atlas with a Trimble GeoXT hand-held GPS receiver that collected GPS-referenced benthic habitat information in situ. Accuracy assessment surveys were not conducted upon completion of the benthic habitat data set, thus the percent accuracy for the data is unknown. An accuracy assessment was performed for data developed by the NOAA Biogeography Team. Because of changes made to the NOAA Biogeography Team's benthic habitat data set for use in the Guam Coastal Atlas, the level of accuracy achieved with the NOAA data does not apply to the data used in the Guam Coastal Atlas. Please refer to the NOAA Biogeography Project Web site (<http://biogeo.nos.noaa.gov>) for more information about their accuracy verification surveys.
Classification Scheme: A hierarchical classification scheme similar to that used by the NOAA team was adopted for the benthic habitat data used in the Guam Coastal Atlas. Using this classification scheme, benthic habitats were described from the broadest level of classification (unconsolidated sediment or coral reef/hard bottom) to a very detailed level of classification (e.g. 50%-<90% macroalgae). The major structure (unconsolidated sediment or coral reef/hard bottom), the detailed structure (e.g. aggregated reef, pavement, patch reef, etc.), the major cover type (e.g. coral, macroalgae, turf algae, uncolonized, etc.), and the percent cover of the major cover type (e.g. 0%, 1%-<10%, 10%<50%, etc.) were also recorded for each habitat delineation.
Although the benthic habitat data developed for the Guam Coastal Atlas used the same classification scheme as that used by the NOAA Biogeography Team, slightly different rules were used to determine the classification an area was to receive. These changes include:
- Defining coral as the majority cover type if the coral percent cover was 10% or greater
- Defining macroalgae as the major cover type if the macroalgae percent cover was 10% or greater and if coral cover was less than 10%.
Additionally, reef zone attribute information provided with the NOAA Biogeography Team benthic habitat data was not preserved during the development of the data for the Guam Coastal Atlas. Instead, adjacent polygons occurring in different reef zones (e.g. reef crest, reef flat, etc.) that otherwise possessed identical habitat classifications were merged in order to simplify the data set and reduce "clutter" on the benthic habitat maps. For more information about the classification scheme produced by the NOAA Biogeography Team and adopted for the Guam Coastal Atlas, including photos of various benthic habitat types, please visit their website at <http://biogeo.nos.noaa.gov/products/us_pac_terr>.
Ground Validation Surveys: A series of ground validation surveys were performed during the development of the benthic habitat data set. A Trimble GeoXT handheld GPS receiver was used to collect benthic habitat information. During these surveys, benthic habitat was described at each of the numerous data points collected throughout the study area. Using the hierarchical classification scheme, the benthic habitat at each point was described from the most basic level of detail (unconsolidated sediment or coral reef/hard bottom) to a high level of detail (e.g. 50% to < 90% macroalgae). The major structure (unconsolidated sediment or coral reef/hard bottom), the detailed structure (e.g. aggregated reef, pavement, patch reef, etc.), the major cover type (e.g. coral, macroalgae, turf algae, uncolonized, etc.), the percent cover of the dominant cover type (e.g. 0%, 1% to < 10%, 10% to < 50%, etc.), as well as the percent cover of coral were recorded for a 1 meter x 1 meter area beneath each point. A large portion of the data points collected at Gab Gab Beach, Dadi Beach, and Tumon Bay also included a digital photograph of a 0.25 sq. meter quadrat in order to assist in the interpretation of benthic habitats using the IKONOS imagery and to compare to future surveys.
A total of 846 ground validation points were collected at various sites around the island (see figures at lower left and lower right). The 203 ground validation points and the 241 accuracy assessment points collected by the NOAA Biogeography team were also used to help interpret the satellite imagery.
Because of environmental conditions, field surveys were limited mostly to the shallow reef flat areas around Guam. Ground validation data was collected with the GPS receiver within the Tumon Bay, Pati Point, and Achang Reef Flat Preserves, as well as at East Agana Bay, Cocos Lagoon, Pago Bay, Ritidian Point, and Gab Gab and Dadi Beaches on U.S. Navy Base property. A much wider cross-section of coastal area, including many fore reef areas, was observed and documented using a digital still camera during snorkeling and scuba diving excursions. These in situ observations allowed for a general assessment of an area's benthic habitat composition and assisted with the interpretation of the satellite imagery, and improved the accuracy of the benthic habitat classification in areas not surveyed using the GPS receiver.
Accuracy Assessment: An accuracy assessment had not been performed prior to the production of the printed and CD-ROM versions of the Guam Coastal Atlas, so the accuracy of any changes made to the original NOAA Biogeography Team's benthic habitat data during the development of the Guam Coastal Atlas have not been assessed. An accuracy assessment will likely be performed at a later date. This information will be available on the Guam Coastal Atlas web site at <http://www.uog.edu/marinelab/coastal.atlas/>.