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PREDICTED SEAFLOOR TOPOGRAPHY

NGDC DATA ANNOUNCEMENT NUMBER: 94-MGG-04

A Color Image Available from the
National Geophysical Data Center
and
World Data Service for Geophysics

Report MGG-9

by W.H.F. Smith* and D.T. Sandwell**

view larger gif image of Predicted Seafloor Topography.

*NOAA/National Ocean Service/Office of Ocean & Earth Science/Geoscience Lab

**Scripps Institution of Oceanography

South of 30 degrees S latitude, the depth of the seafloor is not well known because survey lines by ships are hundreds of kilometers apart. There are many areas as large as the state of Oklahoma where no depth measurements are available. Satellites carrying radar altimeters have measured the shape of the ocean along tracks only 3-4 km apart, and from these data we can make very accurate (+/- 3mGal) and high resolution (15km) maps of the marine gravity field. The gravity field mimics the seafloor topography in the 15-160 km wavelength band if sediment cover on the ocean floor is thin. Long-wavelength (greater than 160 km) topography is isostatically compensated and is not correlated with the gravity field. In addition, the satellite gravity field and the available depth measurements were used to determine the correlation between gravity and the seafloor topography. By applying this correlation to the gravity field we predict seafloor topography in the 15-160 km wavelength band. This topography is combined with a long-wavelength component estimated directly from ship depth measurements. The result reveals many new features and is within +/- 100m of actual depths in many cases. The GEOSAT gravity data has been used in combination with this algorithm to produce a 36" x 48" color poster. A color, polar stereographic view of the Predicted Seafloor Topography is now available from the National Geophysical Data Center. The image is also available for viewing on Mosaic (http://www.ngdc.noaa.gov/ngdc.html). The digital data from which this color image was generated is available from the National Geophysical Data Center.

The Predicted Seafloor Topography is an inferred data set and is not true bathymetry. It is, however, the best estimate of seafloor topography available resulting from satellite gravity data and actual ship depth measurements. This data is intended for scientific research and should not be used for navigational purposes. Additional software is required in order to manipulate the data or produce your own graphic images. The coverage for this data base is 30 degrees S - 70 degrees S. The grid spacing is 3 minutes of longitude by 1.5 minutes of latitude (7200 x 1600). The present format is a GMT/netCDF binary grid file. In this format the file is about 47 megabytes.