USGS Geologic Division Coastal and Marine Geology Program GOMAINE

Construction of Digital Bathymetry for the Gulf of Maine

Generating the Grids

This image below shows the combined data coverage from the 7 data sources:

Click image for High-res image


The 15 and 30 second bathymetry grids were constructed using the freely-available Generic Mapping Tools (GMT) software (Wessel and Smith, 1991). The gridding technique consists of (1) pre-filtering the data with a block-median procedure to avoid aliasing small wavelengths, then solving a partial differential equation that combines Laplacian and bi-harmonic terms. See the paper by Smith and Wessel (1990) for details. Here are the GMT scripts we used:

# Generate 15 second grid for the Gulf of Maine using GMT
zcat *.xyz.gz | \
blockmedian -R/-71.5/-63/39.5/46 -I0.25m -V >
surface -R/-71.5/-63/39.5/46/ -Ggom15.grd -C0.1 -A0.73 -I0.25m -T0.35 -V

# Generate 30 second grid for the Gulf of Maine using GMT
zcat *.xyz.gz | \
blockmedian -R/-71.5/-63/39.5/46 -I0.5m -V >
surface -R/-71.5/-63/39.5/46/ -Ggom30.grd -C0.1 -A0.73 -I0.50m -T0.35 -V

Note about NAD83 and WGS84 horizontal datums; The horizontal differences between NAD83 and WGS84 were determined to be insignificant at the resolution of the 15 and 30 arc-second grids. With cell sizes of ~500 and 1000 meters any displacement of data points because of differences in horizontal datum would have no effect on the outcome of the grid cell value after pre-filtering through the GMT block median procedure.

Note about grid cell sizes: we produced the 15 and 30 arc-second grids because of varying source data resolution in the near-shore and offshore areas. The 15 arc-second grid has much higher source data resolution(NOS data) in the nearshore areas and could possibly be recompiled at a higher resolution in certain areas. The 30 arc second grid was produced for a more uniform fit of the data since the DBDB-V, and the USGS DEM contained data spaced at this resolution.

For comments and questions, contact Rich Signell



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