Benthic habitat maps of the nearshore marine environment of St. John, U.S. Virgin Islands were created by visual interpretation of remotely sensed imagery. The objective of this effort, conducted by NOAA's Center for Coastal Monitoring and Assessment - Biogeography Branch in partnership with the U.S. National Park Service (NPS), was to provide spatially-explicit information on the habitat types, biological cover and live coral cover of St. John's coral reef ecosystem. The data described herein represent accuracy assessment (AA) sites explored by NOAA field scientists to evaluate the thematic accuracy of the benthic habitat maps of St. John. Considering vessel drift during habitat characterization, the entire evaluation period was most accurately represented by a series of GPS points logged at five-second intervals.
The purpose of this survey data was to collect information on current habitat conditions at random locations throughout the mapping area. Locations were determined by an iterative, GIS-based, stratified random sampling technique to ensure that all bottom classifications would be assessed. This information was used to evaluate the thematic accuracy of the St. John benthic habitat map.
Data were collected on 486 ground validation sites over a two-week field mission from February 9-20, 2009 aboard U.S. National Park Service small research vessels. As anchoring at each site was not feasible for rapid assessment, wind and wave effects caused the survey vessel to drift along a transect. Post-processing of the GPS data allowed for a series of points to represent the actual survey location.
ground condition
Not for navigation.
1305 East West Highway
The points in this data layer were acquired by GPS data using a Trimble GeoXT receiver. All data point characteristics were recorded in the GPS data logger automatically and seamlessly transferred to ArcGIS shapefiles using Trimble Pathfinder Office software. Attributes are all expected to be correct.
Data points were collected with mapping-grade GPS receivers. The data were post processed for differential correction to the Continually Operating Reference System (CORS) station at St. Thomas, U.S. Virgin Islands (VITH). All attribute information was recorded by the GPS receiver and stored directly in the raw GPS files. This automated process was assumed to be error free. This file is believed to be logically consistent.
486 field positions were occupied and habitat characterizations were conducted for accuracy assessment during this work. A small subset of the accuracy assessment sites did not have transect information in this dataset, depending on the evaluation technique. Snorkel and visual inspection from distances did not allow for a GPS receiver to occupy the exact location, as a result, approximated GPS locations were used instead without the opportunity for transects with the underwater camera.
The true positional accuracy of individual epics was determined to be within 1 m for 96% of the logged points. However, it is estimated that horizontal accuracies may be 0 - 20m from the true position of the underwater camera assessment due to line "lay-back" during drift.
GPS data, which were originally recorded as code-phase signals in Trimble SSF format, were differentially post-processed to the Continually Operating Reference System (CORS) station at St. Thomas, U.S. Virgin Islands (VITH).
A benthic habitat map was generated from delineating features by visual inspection of remotely sensed imagery and incorporation of field data acquired during a ground validation mission. Target locations for the accuracy assessment of this habitat map were determined by an iterative, GIS-based, stratified random sampling technique to ensure that all bottom classifications would be assessed. Based on guidelines from other recent accuracy assessment analyses (Battista et al. 2007), a minimum of 25 points were assigned to each of the 13 detailed structure classes within the draft habitat map. An additional 175 points were distributed based on the proportion of area of each structure class in the map. Points were randomly placed within each class using Hawth's Analysis Tools (Beyer 2004) in ArcGIS at a minimum distance of 50 m apart. Next, the number of points that fell within each detailed primary cover class was calculated. Where necessary, additional points were randomly added and re-distributed from classes with many points to ensure that there was a minimum of 25 points within each detailed cover class. Process Dates: 2008-2009
NOAA field scientists explored the accuracy assessment locations with a suite of assessment techniques depending on the conditions at each site. A combination of underwater video (primary method), free diving, snorkeling and surface observations were used to survey the ecological characteristics at each location. At the start of every morning, the boat captain selected a general region to begin the day's work. Navigating to field locations was accomplished using a Garmin GPS 76 device with the uploaded GV site coordinates. The boat captain maneuvered the vessel to within 5 m of the target location and made every effort to maintain that location without jeopardizing crew and equipment safety. Once on site, NOAA scientists would simultaneously deploy a SeaViewer Sea-Drop 950 camera and begin logging a waypoint on a Trimble GeoXT GPS receiver.While the video camera was capturing bottom imagery, an observer viewed the video real-time on a Panasonic Toughbook aboard the survey vessel. They categorized each site according to the levels of the habitat classification scheme: major and detailed geomorphological structure, major biological cover, percent major biological cover and percent coral cover. Data was entered into a custom data dictionary generated in Trimble Pathfinder Office software and loaded onto the Trimble data logger.
Trimble Pathfinder Office software was used to post process and differentially correct the raw GPS data to the Continually Operating Reference System (CORS) station at St. Thomas, U.S. Virgin Islands (VITH). All positions acquired for a single site were plotted to represent the entire duration of bottom evaluation as the boat drifted.
Ultimately, precisely positioned GPS positions, the evaluation transect and the associated classification data were viewed in a GIS to assess the accuracy of the draft benthic habitat map.
Internal feature number.
ESRI
Feature geometry.
Maximum Positional Dilution of Precision
Automatically generated by Trimble
Maximum Horizontal Dilution of Precision
Date of logged position
Time of logged position
Vertical Precision
Hortizontal Precision
The National Oceanic and Atmospheric Administration (NOAA) National Centers for Coastal Ocean Science (NCCOS) is publishing this data on their website. NCCOS Biogeography Branch does not guarantee the accuracy of the geographic features or attributes. Please see the written report and metadata records for each data set for complete information on the source, limitations, and proper use.
Contact NOAA for distribution options (see Distributor).
