USGS Data Series 835: Data Processing - Archive of Single-Beam Bathymetry Data Collected from Select Areas in Weeks Bay and Weeks Bayou, Southwest Louisiana, January 2013

DATA PROCESSING

Single-beam BATHYMETRY

Navigation

The coordinate values of the GPS base station are the time-weighted average of values obtained from the National Geodetic Survey's On-Line Positioning User Service (OPUS). Each GPS session is submitted to OPUS and a solution is returned to the user. All the solutions are entered into a spreadsheet and the time-weighted average of the ellipsoid value is calculated for the entire occupation. Any of the individual ellipsoid solutions that differed from the average ellipsoid by more than 3 standard deviations (σ) were excluded and the final coordinate value determined.

The final coordinate value (World Geodetic System 1984 (WGS84) (G1150)) was used in GrafNav version 8.3 (Waypoint Product Group) as the point-base location to reference all processed x,y,z values (table 5). Using GrafNav, the base station GPS session files were processed to the respective GPS session files of the roving vessel. During this process, steps were taken to ensure that the trajectory produced from the base to the rover was clean and produced fixed positions. The quality assurance and quality control (QA/QC) was accomplished through the interpretation of graphs, trajectory maps, and processing logs that GrafNav produces for each processed GPS session. The processes for eliminating poor GPS data, at this point included, but were not limited to, excluding data from a satellite flagged by the program as having poor health, eliminating satellite time segments that have had cycle slips, and adjusting the satellite elevation mask angle. From these processes, differentially corrected GPS (DGPS), precise position files at 1-s intervals were created for every roving GPS session. These files were exported in text format and serve as the post-processed, differentially corrected navigation for the single-beam bathymetry.

Site Name	Latitude, WGS84 (G1150)	Longitude,, WGS84 (G1150)	Weighted Ellipsoid Height, WGS84 (G1150)
Table 5. Final coordinate value derived from the National Geodetic Survey's On-Line Positioning User Service (OPUS) and used to post-process the navigation data. Latitude and longitude values are in degrees, minutes, decimal seconds (DMS.SS), and ellipsoid height is in meters.
MORT	29 48 28.26610	91 48 25.47357	7.790

Soundings

The raw HYPACK version 10.0.5.31 data files were imported into CARIS® HIPS (Hydrographic Information Processing System) and SIPS (Sonar Information Processing System) version 8.0. Next, the corrected DGPS positions exported from GrafNav were imported into CARIS using the generic data parser tool within CARIS. The SVP profile casts were entered and edited using the SVP editor within CARIS. Once all components of the raw bathymetric files were loaded, and correctly modified for use in the CARIS program, they were then merged and geometrically corrected. Then the soundings, attitude data, and navigation data were edited for any additional outliers. Bathymetric data were further reviewed in the Subset Editor utility to review the crossings and identify any remaining outliers or any trouble areas within the dataset. The geometrically corrected x,y,z point data were then exported as an ASCII text file in the ellipsoid datum of WGS84 (G1150), equivalent to ITRF00.

DIGITAL ELEVATION MODEL (DEM)

single-beam elevations were imported into Esri ArcMap version 10.1, where a shapefile of the individual data points (x,y,z) was created and plotted in 1-m color coded intervals. An initial 10-m raster grid and a tinned model surface were created using the raster interpolation "natural neighbors" tool and the data management "TIN" tool, respectively; both are part of the ArcGIS 3D Analyst tool suite. The grid, tin surface, and x,y,z point-data shapefile were used in conjunction to visually scan for any remaining discrepancies. For example, irregular looking or converging tin lines indicated problem areas, deep holes, or abnormal areas. All were indicators of potentially false or incorrect x,y,z data values. Any areas identified as questionable were reviewed, and if found to be unsatisfactory the values were either deleted or statically adjusted.

Once all data were reviewed, a final 10-m grid was generated. A polygon was created using the extent of the survey tracklines and then converted into a raster mask using the ArcGIS "polygon to raster" conversion tool. The final grid was clipped to the raster mask using the ArcGIS Spatial Analyst "extract by raster mask" tool. The final product was a digital elevation model.

This analysis method was performed individually on each survey area. This resulted in an accumulative amount of 470 and 708 x,y,z data points having an ellipsoid height range of -27.83 m to -35.85 m in WGS84(G1150). Table 6 lists simple statistics of the final ellipsoid heights indicative of each survey area including point count, minimum, maximum, mean, and standard deviation. The DEMs are made available on the Data Download page of this report.

These data were then converted horizontally and vertically into NAD83 (CORS96) UTM Zone 15 N and NAVD88 (orthometric height) using GEOID12A. The conversion was completed using the National Oceanic and Atmospheric Association (NOAA) VDatum software conversion tool version 3.2 (reported vertical transformation error is 5.4 cm). From these values DEMs were created in ArcMap as described above and also made available on the Data Download page of this report. This process resulted in an orthometric height range of -0.53 m to -8.37 m. Table 7 lists simple statistics of the final orthometric heights indicative of each survey area including count, minimum, maximum, mean, and standard deviation.

Statistic	Port Channel North	Port Channel South	Weeks Bayou	Shark Island Bayou
Table 6. Statistics of the final ellipsoid heights of Port Channel North, Port Channel South, Weeks Bayou, and Shark Island Bayou, Louisiana.
Ellipsoid Height (meters) - WGS84 (G1150) UTM Zone 15 North
Point count:	66,409	119,344	216,324	68,631
Minimum:	-35.49	-34.03	-35.85	-33.80
Maximum:	-30.92	-28.22	-28.11	-27.83
Mean:	-33.32	-29.97	-30.23	-29.00
Standard deviation:	1.09	1.48	1.47	1.01

Statistic	Port Channel North	Port Channel South	Weeks Bayou	Shark Island Bayou
Table 7. Statistics of the final orthometric heights of Port Channel North, Port Channel South, Weeks Bayou, and Shark Island Bayou.
Orthometric Height (meters) - NAVD88 UTM Zone 15 North GEOID12A
Count:	66,409	119,344	216,324	68,631
Minimum:	-7.93	-6.51	-8.37	-6.49
Maximum:	-3.36	-0.72	-0.63	-0.53
Mean:	-5.77	-2.47	-2.75	-1.69
Standard deviation:	1.09	1.48	1.47	1.02

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