3 arc second digital elevation model of the Gulf of Maine

Data Compilation: Bathymetry data were sought in an effort to compile as many quality bathymetry soundings as possible for the Gulf of Maine region. Digital bathymetry were acquired from: United States Geological Survey(USGS), National Oceanic and Atmospheric Association (NOAA) National Aeronautics and Space Administration (NASA), Gulf of Maine Mapping Initiative (GOMMI), Naval Oceanographic Office, University of New Hampshire, and Canadian Hydrographic Service . In order to fill gaps and supplement these data, images of fishing and nautical charts acquired from National Oceanic and Atmospheric Association (NOAA) were georeferenced and contours were digitized in ArcMap 9.2. In the case of digitizing contours, the contours were subsequently converted to points in ArcGIS 9.2 using XToolPro (v. 4.1.0 (Build 375))) by generating a points randomly along the line. The Generic Mapping Tool command 'blockmedian' was used to resample the data at intervals of six (6) arc-seconds. In addition to bathymetry data sources, land elevation data were acquired from United States Geological Survey(USGS), NASA (National Aeronautics and Space Administration), NGA (National Geospatial-Intelligence Agency, and NOAA's Ocean Service (NOS), Coastal Services Center (CSC).

Data Formatting: All data were converted to ASCII XYZ format including decimal degrees, positive up. The USGS Massachusetts Bay Bathymetry data, the National Elevation Data (NED), and the Shuttle Radar Mission Topography (SRTM version 4.1) were all in ESRI binary grid format. The Geospatial Data Abstraction Library (GDAL) tool gdal_translate was used to convert these data to the gridded file format. The conversion from Gridded Data Format to ASCII XYZ was done using the GMT command 'grd2xyz'. The Northwest Atlantic Dataset in the Self Describing Structure (SDS) architecture utilized an SDS toolset to convert the data to ASCII XYZ format. Although in ASCII format, the NOAA Coastal Services Center Lidar had to be reformatted from a comma-delimited text file to ASCII XYZ. The following data sets underwent a horizontal datum transformation before being formatted to ASCII XYZ: USGS Massachusetts Bay, USGS Middle Ground Shoals, NED, and SRTM.

Manual Buffering and Clipping: NGDC/NOAA Hydrographic Soundings with depths greater than 400 meters were eliminated to avoid producing artifacts caused by data overlapping with deeper water, higher resolution data sets such as Smith and Sandwell (version 12.1) and the University of New Hampshire. This was done by mapping the surveys in ArcMap 9.2 to compare the spatial extent with the deeper water data. A buffer of 1000 meters was applied to the deep water data. Any NOAA survey point lying within the buffer was clipped from the data. The Gulf of Maine Mapping Initiative (GOMMI) data had to be processed to a coarser resolution of 30 meters due to the large size of the 5 meter resolution files. This procedure was done to increase the proficiency and speed of the gridding algorithm. The large data sets cause the algorithm to bog down and stop all together during initial processing. Generic Mapping Tools (GMT) blockmedian was used to decrease the resolution from 5 meters to 30 meters using a grid spacing of 30 meters for the region of interest (71.5 - 63.0 W and 39.5 - 46.0 N). A buffer of 1000 meters was applied to any higher resolution data overlapping the Smith and Sandwell Global Topography data. If any of the Smith and Sandwell data fell within the buffer, it was clipped (removed) to avoid producing artifacts during the surface interpolation of the final digital bathymetry grid. The Olex data were clipped to remove any points that fell within a 1000 meter buffer around the NWATL data to avoid producing artifacts. The clipping was done by using ArcMap 9.2 "near" tool from the ArcMap ArcToolbox Analysis tools. Any National Elevation Data (NED) points falling within the buffer were removed from the final data set to ensure there was no overlap between the data sets. This same method was used with the NOAA CSC Coastal Lidar data. Any National Elevation Data (NED) points falling within the buffer were removed from the final input data-set.

Horiztonal Datum Transformation: The final horizontal coordinate system chosen for this work is geographic, NAD83. The USGS Massachusetts Bay bathymetry data used the GDAL tool gdalwarp to reproject the data from the original Mercator, NAD83 projection to geographic, NAD83. After the reprojection, the Generic Mapping Tool (GMT) grd2xyz was used to convert the gridded data format to ASCII XYZ. The Gulf of Maine Mapping Initiative Data (GOMMI) used the PROJ 4 Cartographic Projection library to reproject the UTM, Zone 19, NAD83 data to geographic, NAD83. The USGS Middle Ground Shoal bathymetry data used GDAL's gdalwarp tool to reproject the data from UTM, Zone 19, WGS84 to Geographic, NAD83.

Vertical Datum Transformation: All depth values were converted to NAVD88 using information from the NOAA VDatum Project (http://vdatum.noaa.gov/). The VDatum software does not extend into Canadian waters, but the underlying ADCIRC tidal model does. The MLLW and MLW surfaces from the model were used to convert depths from the MLW or MLLW datum into the tidal model datum. The depths were further transformed into the NAVD88 datum by subtracting 0.09 m, the median value of NAVD88 values above the tidal model datum obtained at five long-term tide gauges in the Gulf of Maine. The maximum cumulative uncertainty from these transformations is estimated to be 0.13 m (VDatum Team, 2011).

Grid Input: Combine all the data into a single XYZ file using AWK.

Sub-region Processing: Clip ASCII XYZ data into 1.2 by 1.2 degree squares, which split total coverage area into 7 rows of latitude and 9 columns of longitude. This was done using MATLAB and AWK. Each sub-region was run through pre-filtering process using the GMT software tool blockmedian. Blockmedian reads in the ASCII XYZ values: longitude, latitude, and depth, along with the user specified region of interest and desired grid spacing. For the Gulf of Maine region, the region of interest was 71.5 to 63.0 W and 39.5 to 46.0 N and the grid spacing was 3-arcseconds. Each working sub-region was then run through GMT surface algorithm to create an adjustable tension continuous curvature surface. The construction of the Gulf of Maine grid used a tension factor of 0.35. The aspect ratio was set to 0.75, the convergence limit was set to 0.1, and the grid spacing was chosen as 3-arcseconds for the region of interest: 71.5 to 63.0 W and 39.5 to 46.0 N.

Sub-region Cropping: Crop the sub-regions from 1.2 by 1.2 to 1 by 1 degree. The continuous surface grid of each subregion was then clipped to 1 degree by 1 degree squares using the Generic Mapping Tool (GMT) grdcut. Grdcut works by extracting the user specified region from an existing grid file. For example, the sub-region 69.5 to 68.5 W and 41.0 to 42.0 N was extracted from working sub-region 69.6 to 68.4 W and 40.9 to 42.1 N.

Grid Cell Conversion: Convert grid cells from grid-line registered to pixel-registered. The Generic Mapping Tool command grdedit was used to modify the content of the final sub-regions of the digital bathymetry grid. This was done by specifying the "-T" option, which makes the necessary changes in the header to convert from grid-line registered to a pixel-registered grid, or vice-versa. This processing step was necessary to assure no visible seams once the final product was pasted together. The grids had to be assembled exactly on the edges in order to provide a seamless image. Content of the final sub-regions of the digital bathymetry grid. This was done by specifying the "-T" option, which makes the necessary changes in the header to convert from grid-line registered to a pixel-registered grid, or vice-versa. This processing step was necessary to assure no visible seams once the final product was pasted together. The grids had to be assembled exactly on the edges in order to provide a seamless image.

Grid Assembly: The final digital bathymetry grid was assembled by pasting the sub-regions together on a common edge. This was done using Generic Mapping Tool's grdpaste. The user specifies two input grids and a single output grid file name. Grdpaste searches for the common edge and pastes the two grids together to form the final grid. Since there were many sub-regions of the Gulf of Maine, a pasting method had to be followed. Sub regions 1 and 2 of a specific row were pasted together as the base grid. Grdpaste was then done iteratively along each row, pasting the western-most sub-region adjacent to the adjacent sub-region to the east until each row was complete. Grdpaste was then done to each full row using the same method working from the southern-most row adding adjacent rows to the north.

The final grid grid file was uploaded to the THREDDS server where additional information was added to the file by virtue of NcML in the THREDDS catalog. This NetCDF file, downloaded from the THREDDS server is what is available.

GDAL version 1.9.2 (part of OSgGeo4W) on a Windows 7 machine was used to convert the NetCDF file to a GeoTIFF image using the following command: gdal_translate -a_srs EPSG:4269 gom03_v1_0.nc gom02_v1_0.tif. The additional metadata information in the header of the NetCDF file is carried over to the GeoTIFF. The -a_srs flag specifies that the CRS is NAD83 (EPSG:4269) and that information is added to the GeoTIFF header. Additionally, an xml file (gom03_v1_0.tif.aux.xml) was created also containing the NcML metadata from the NetCDF file.

GDAL version 1.9.2 (part of OSGeo4W) on a Windows 7 machine was used to convert the NetCDF file to an Arc/Info ASCII grid formation using the command: gdal_translate -a_srs EPSG:4269 gom03_v1_0.nc -ot Float32 -of AAIGrid gom03_v1_0.asc. This process also creates gom03_v1_0.prj and gom03_v1_0.asc.aux.xml. The prj file contains the projection information, and the xml file contains the additional NcML metadata from the NetCDF file.

Each data format file(s), combined with the FGDC compliant metadata were bundled into individual compressed files for download using WinZip version 14.

Edits to the metadata were made to fix any errors that MP v 2.9.30 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

The larger work citation had one originator field, but with two authors. This was adjusted so that there are two originators, each in its own field.

Added keywords section with USGS persistent identifier as theme keyword.