1992 Seagrass and Mangrove Habitats of the Salt River Bay National Historical Park and Ecological Preserve

Habitat maps were created as part of a larger ecological assessment conducted by NOAA's National Ocean Service (NOS), Biogeography Branch, for Salt River Bay National Historic Park and Ecological Preserve (National Park Service).Aerial photographs were obtained for 1992 from the National Geodetic Survey, and were orthorectified by the Biogeography Branch. A classification scheme was set up with 20 benthic habitat types, 19 land cover types, and 13 mangrove habitat types. For this map of seagrass and mangrove habitats during 1992 only the 3 seagrass, and 14 mangrove classification categories were used. These were mapped directly into a GIS system through visual interpretation of orthorectified aerial photographs.

These maps were created to support current and future monitoring and management efforts of the National Park Service (NPS) for this site, and support the programmatic objectives of the NPS service-wide Inventory and Monitoring Program. In particular, maps created from the time series of the photographs cover periods both before and after Hurricane Hugo (1989), providing insight to habitat changes that occurred following the disaster.

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The National Oceanic and Atmospheric Administration (NOAA) National Centers for Coastal Ocean Science (NCCOS) produced this data for Salt River Bay National Historic Park and Ecological Preserve (NPS). NCCOS Biogeography Branch does not guarantee the accuracy of the geographic features or attributes. Please see the metadata records for each data set for complete information on the source, limitations, and proper use. Disclaimer - While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.

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Microsoft Windows 2000 Version 5.0 (Build 2195) Service Pack 4; ESRI ArcCatalog 8.3.0.800

Methods used to create this map were similar to those used to maps of the U.S. Virgin Islands and Puerto Rico by the same office ("Benthic Habitat Maps of Puerto Rico and the U.S. Virgin Islands", 2001). An accuracy assessment conducted for the original maps was designed to determine the accuracy of maps generated from on-screen digitizing using the ArcView Habitat Digitizer Extension. Benthic habitat maps exhibited an overall accuracy of 93.6 percent (Kappa 0.90-0.93) for on-screen digitizing. In comparison, the map described here was created from higher resolution imagery (25 microns), finer minimum mapping unit (MMU) (100 square meters), and habitat delineation occurred at a smaller image scale (1:1500). A greater number of ground truth points over more habitat types were verified for the year 2000 mapping data, in a study site of much smaller area. Ground truth points could not be taken for the 1992 data directly, however, year 2000 points were referenced when appropriate. Mapping methods were otherwise identical to those used previously. Attribute accuracy was therefore presumed to be equivalent to or better than that of the previous map, although a formal assessment was not conducted on the current map directly.

The Minimum Mapping Unit (MMU) restriction was set to 100 square meters and digitizing was conducted at 1:1500 scale. Polygons were digitized directly from georegistered orthophotos with spatial accuracy approximately 1.0 meter. These maps represent conditions at the time the aerial photographs were obtained. When considering horizontal map accuracy, it is important to keep in mind that the ocean floor is a dynamic environment. Physical factors such as currents, erosion, and hurricanes and biological factors such as seasonal growth and die-off, affect the distribution and stability of terrestrial, mangrove and benthic habitats in this environment.

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Delineation of all habitat boundaries was conducted with the image scale at 1:1500. All polygons were digitized at this scale except when subtle habitat boundaries were not easily discernable at 1:1500 and zooming out to a more broad scale was required to place boundaries correctly. Digitizing at a uniform scale ensures that the level of detail produced by the photointerpreter is consistent throughout the project. The Minimum Mapping Unit (MMU) for identifying habitats or features was 100 square meters for visual photointerpretation. The software utilized in this project was designed to alert the photointerpreter each time a polygon was drawn smaller than the MMU. When this occurred, the photointerpreter has the choice of whether to include the polygon in the dataset or not.

Individual color aerial photographs from 1992 were georeferenced and mosaicked. Approximately 10-15 ground control points were used to orthorectify the mosaic. During the digitizing process, image stretches and manipulating image contrast, brightness and color balance were performed in the ArcView Image Analysis Extension to enhance features in the processed imagery. Additional, collateral information including previously completed habitat maps, NOS nautical charts, and other descriptive references dealing with benthic and coastal habitats of St. Croix were used to assist with image interpretation. Over 200 ground truth points were also checked with year 2000 imagery to verify photographic signature with habitat interpretation. Though 1992 habitats could not be directly verified in the same fashion, where appropriate, collateral information from the year 2000 points was considered during 1992 habitat deliniation. GIS topologic quality was established by executing ArcView extension routines that check for: overlapping polygons, multipart polygons, sliver polygons and void polygons. Additionally, checks for adjacent polygons with the same habitat attributes were completed. All errors were identified and corrected. This file is believed to be logically consistent.