NOAA's NOS/NCCOS/CCMA Biogeography Branch, in collaboration with NOAA vessel Nancy Foster and territory, federal, and private sector partners, acquired multibeam bathymetry data south of St. Thomas and St. John, USVI, from 3/18/10 to 4/6/09. Data was acquired with a hull-mounted Kongsberg Simrad EM 1002 multibeam echosounder (95 kHz) for water depths greater than 75 meters, and with a moon pool flange-mounted Reson 7125 multibeam echosounder (dual frequecy, 200/400 kHz) for water depths of up to 75 meters. It was processed by a NOAA contractor using CARIS HIPS software. Data has all correctors applied (attitude, sound velocity) and has been reduced to mean lower low water (MLLW) using final approved tides and zoning from NOAA COOPS. Data is in UTM zone 20 north, datum NAD83. The processed CARIS data was used to generate a CARIS BASE surface based on CUBE. A BAG file was exported from the BASE surface, and converted to a GeoTIFF in ESRI ArcMap 10. Rugosity was derived from this surface using NOAA's Benthic Terrain Modeler extension.
The project was conducted to meet IHO Order 1 and 2 accuracy standards, dependant on the project area and depth. All users should individually evaluate the suitability of this data according to their own needs and standards.
NOAA - NMAO personnel and R/V NANCY FOSTER
"...rugosity derivation relies, in part, on a neighborhood analysis using a 3 grid cell by 3 grid cell neighborhood. An algorithm is passed through the Raster Map Algebra Operation object within Spatial Analyst that calculates the planar distance between the center point of the center cell and of each of the eight surrounding cells in the neighborhood. Next, using the Pythagorean Theorem, the surface distance is calculated for each planar distance using the difference in elevation between the cells. The result of this function is sixteen separate grid data sets with each cell value equal to this surface distance. The next step in the process is to calculate the area formed by three adjacent sides. The result is eight triangular surface area grids. These grid datasets are combined to obtain a surface area data set for the input bathymetric data set. The final step in the process is to create a data set that represents the ratio of surface area to planar area. This final data set represents rugosity for the study area."
More information on the specific algorithms used, and contact points for questions, can be found in the BTM's documentation.
Rugosity values near 1 represent flat smooth terrain, while higher values reflect increasing rugosity or terrain roughness. For rugosity grid interpretation, is recommended that the grid be reclassified according to standard deviation divisions.