For the purposes of this data set, a "marsh island" was defined as an island located between the mainland and coastal barrier islands that had an area greater than one-eighth of an acre (500 square meters). Furthermore, an island was only considered a marsh island if it possessed terrestrial vegetation - sand spits and other ephemeral forms were not included as islands. The boundaries established in this data set are not intended to define legal boundaries. The extent of the terrestrial vegetation was used to determine each island's boundary, approximating SC OCRM's critical line; marsh grass and sand were not included as part of an island's boundary. Boundary determination for the spoil island, paved/concrete bridge, earthen bridge/causeway, and mainland polygons followed these same conditions. Islands located within managed areas (water bodies entirely isolated from tidal flow by impoundments) were not considered marsh islands. Also, many small islands contiguous with a highway, whether existing prior to highway construction or created by dredge spoil during highway construction, were not considered marsh islands. In such cases, the area of the island and the highway were enveloped by a single polygon. Not all islands contiguous with a highway were considered part of the highway, however. Smaller islands were generally included as part of a highway, but the decision to include the island as part of a highway was ultimately made by SC OCRM staff with expert local knowledge of the area in question. "Barrier islands" were defined as islands directly bordering the open ocean and separated from the mainland by open water (lagoon, bay, estuary) or salt marsh. "Spoil Islands" were defined as islands formed by dredge material. These islands exist as "rings" of dredge material which may completely isolate the island's interior from natural tidal flow; the interior of a spoil island is generally composed of shallow water or inundated soils/vegetation. Two types of bridges were also captured for this data set: earthen causeways and paved/concrete bridges. Earthen causeways, were defined as anthropogenically modified earthen structures connecting the mainland or barrier island to a marsh island or connecting two or more marsh islands. Structures connecting two parts of the mainland or connecting two barrier islands were not included in this data set. Earthen causeways inhibit tidal flow entirely, with the possible exception of small culverts constructed through the structures. Paved/concreted bridges were defined as structures connecting the mainland or barrier islands to one or more marsh islands or connecting two or more marsh islands that have been surfaced with asphalt or other type of pavement material or are composed of concrete building material. These bridges may be earthen and may not allow tidal flow, but were considered distinct because of the impervious surface applied to the structure. More often, however, these polygons represent highways or other roads that are raised above the water and do not restrict natural tidal flow.
The polygons in the South Carolina Marsh Islands data set were created using two methods. The first method involved converting Digital Ortho Quarter Quads (DOQQs) to vectors using the object-oriented image analysis software eCognition. The vectors, or island polygons, were then edited by hand if necessary. The second method used was on screen hand digitizing from the DOQQs. The automated method was used to create island polygons for approximately 67% of the DOQQs in areas where a large number of marsh islands occur. In areas where very few marsh islands exist, hand digitizing was used. The following description lists the steps used in the automated vector generation with eCognition:
One-meter resolution DOQQs created from 1999 color infrared (IR) National Aerial Photography Program (NAPP) photography were downloaded from the South Carolina Department of Natural Resources (SCDNR) web site (<http://www.dnr.state.sc.us/water/nrima/gisdata/>) in MrSID format. There were 187 DOQQs downloaded for this project. Using ERDAS Imagine 8.5, the DOQQs were exported from MrSID format to a Geotiff format, then imported back into ERDAS and saved as an .img format. The datum and projection information was input for each image and the images were resampled from one-meter to three-meter resolution with a nearest neighbor convolution. This was done to reduce the amount of image processing time and keep the vector file sizes small. The ERDAS images were then imported into the eCognition software for further processing. Each DOQQ was processed individually within eCognition. The images were segmented in eCognition using the following criteria: scale = 25, color = .8, shape = .2, smoothness = .8, compactness = .2, mode = normal, and weight = 1 for the red, green, and IR bands. The image segments were classified into land and no land classes using the ratio of the IR band combined with the membership function's increasing curve. The range of values for the minimum and maximum number varied in each DOQQ, but generally fell in the range of .34 to .38 for the minimum value and .35 to .39 for the maximum. The classified segments were exported as a classified Geotiff image for ArcGIS. In ArcGIS, the Spatial Analyst extension was used to convert the Geotiff image to a polygon shapefile using the Raster to Features command with the Generalize Line box unchecked. The shapefile was edited to remove large water and marsh polygons and to merge the remaining polygons into a single polygon with the merge command. ArcToolbox was used to convert the island polygon shapefile to a coverage. A Build and Clean were performed on the coverage and it was converted back to a shapefile with ArcToolbox. ArcToolbox was also used to define a projection for each shapefile. Island polygons were inspected and their boundaries were edited by hand when necessary. The minimum size island polygon is 1/8 acre or 500 square meters. Islands smaller than the minimum size were deleted.