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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-9by W.H.F. Smith* and D.T. Sandwell**
*NOAA/National Ocean Service/Office of Ocean &       Earth Science/Geoscience Lab 

**Scripps Institution of Oceanography 
Free .gif             Poster PreviewFree             Data Download
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. 
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