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. Methods were otherwise the identical to those used previously. Attribute accuracy was therefore presumed to be equivalent to or better than that of the previous map, although an assessment was not conducted on the current map directly.
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.
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 weather to include the polygon in the dataset or not.
Twenty-one distinct benthic habitat types within eight zones were mapped directly into a GIS system using visual interpretation of orthorectified aerial photographs. Benthic features were mapped that covered an area of 1600 km^2. In all, 49 km^2 of unconsolidated sediment, 721 km^2 of submerged vegetation, 73 km^2 of mangroves, and 756 km^2 of coral reef and colonized hardbottom were mapped.
During the digitizing process, image stretches and manipulation of 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 Salt River National Historic Park and Ecological Preserve was used to assist with image interpretation.
A first draft map was completed and features in the imagery where uncertainties existed, due to confusing or difficult to interpret signatures, were identified for future ground validation effort. An ArcView GIS point theme was generated with points positioned on the features of uncertain habitat type or along transects through gradients between habitat types. The GIS points were converted to GPS waypoints, and were visited for on the ground habitat validation.
Benthic habitat characterization was conducted at each site by snorkeling, free diving, or via observations from the surface where water depth and clarity permitted. Mangrove habitat characterization was conducted by driving, kayaking or hiking to points and visually identifying mangrove type and cover. GPS data was collected at each location, and site ID, depth, habitat type, and the method used to make the assessment were recorded. The ground validation data was incorporated into the second draft of each map.