Directory Identifier: GRAVCD-haxby
Directory Title: Haxby Worldwide Gravity Grid
Principal Investigator: Dr. William F. Haxby

Contributing Organization: Columbia University Lamont-Doherty
Geological Observatory
Palisades, N.Y. 10964

Summary: In 1985, Dr. William F. Haxby of the Lamont-Doherty Geological Observatory of Columbia University prepared this data base of free-air gravity anomalies, based on the radar altimetry of the SEASAT satellite mission in 1978. SEASAT Image: A full color wall display “Gravity Field of the World's Oceans”, 1:40,000,000 scale, is available from NGDC. What about the data? The utility of SEASAT altimeter data to geophysics is a result of their high sensitivity and uniform global coverage. The precision of SEASAT measurements is about 0.05m, and the distance between adjacent SEASAT tracks is at most 1.5 degrees of longitude. The measurement sensitivity was sufficient to detect features of the marine gravity field with spatial dimensions as small as 20 km. The interpolated gridded data have a latitude and longitude resolution of 5 minutes.

The procedure that was used to derive the model of the marine gravity field included: editing the SEASAT measurements to remove data spikes; adjusting the profiles to minimize discrepancies between measured sea surface heights at intersections between SEASAT tracks; estimating the sea surface height at a grid of equally spaced points by fitting least-squares surfaces to the edited and adjusted measurements; and, finally, computing marine gravity anomalies from the sea surface topography, assuming that the surface conforms to the geoid. In order to preserve linear trends that are defined by similar sea surface signatures on several adjacent SEASAT profiles, lineations that are apparent in the along track data were digitized. The digitized lineations served as input for an anisotropic surface fitting algorithm that was applied in the vicinity of the lineations to revise the gridded sea surface topography.

The accuracy of the gravity model is limited by several factors: by the accuracy and precision of the measurement; by non-geoidal components of the sea surface topography due to currents and tides; by gaps between adjacent tracks that are larger than the potential resolution of the data; and by smoothing as a result of the least-squares gridding procedure. The principal source of error in the SEASAT measurements is uncertainty in the satellite's position. The application of adjustment procedures to minimize crossover differences ensured that these long wavelength errors did not cause short wavelength noise, in the form of artificial along track lineations, in the gridded gravity model. The profile adjustment also reduced some of the inconsistencies introduced by temporal oceanographic variability. However, the large amplitudes of non-geoidal components associated with ocean dynamics result in greater uncertainty in the gravity model in the vicinity of major ocean currents and shallow continental shelves. The precision of the measurements is not a serious limitation, in light of other sources of uncertainty, except at high latitudes where sea ice was present. Sea ice is responsible for significantly higher noise levels in the region surrounding Antarctica and over Arctic continental shelves. The least-squares surface fitting procedure resulted in a significantly reduced noise level, but also reduced the amplitudes of features of the ocean topography with wavelengths less than about 500 km at the equator or 200 km at high latitudes.

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