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National Centers for Environmental Information (NCEI)


Offshore Washington/Oregon Coast

Data Announcement 87-MGG-19

The mean sea surface height (SSH) derived from satellite altimeter data is a good approximation to the marine geoid, a surface of gravitational equipotential. The short wavelength components of the SSH are most influenced by submarine topography as well as shallow crustal density.

The July-October, 1978, flight of the NASA "SEASAT" satellite produced detailed measurements of the height of the sea surface by 13.5 GHz microwave altimetry. The basic measurement consists of the two-way travel time of a short radar pulse (3.1 ns) originating from the satellite and reflecting from the ocean surface. When corrected for the satellite position and variable delays due to the atmospheric index of refraction, this measurement provides an estimate of the instantaneous height of the ocean surface. If the temporal variations due to tides, storms, or atmospheric disturbances are removed or minimized, the result is an estimate of the mean ocean height.

The geoid anomaly caused by a feature such as a seamount is the sum of two opposing terms: the excess mass of the feature itself, and the mass deficiency caused by its isostatic compensation. Although the masses are equivalent, a positive anomaly is still observed at the sea surface because the mass excess is closer to the observation plane (at the sea surface itself).

The National Geophysical Data Center (NGDC) (now the National Centers for Environmental Information (NCEI)) has available three versions of height and gravity-anomoly data derived from the SEASAT data, which cover all ocean areas from 72 deg. North to 72 deg. South:

1. SEA SURFACE HEIGHT CONTOUR OVERLAYS: The NASA Jet Propulsion Laboratory together with the Scripps Institution of Oceanography have produced a contour overlay for the east and west half of each of the 16 GEBCO (GEneral Bathymetric Chart of the Oceans).

2. 5-MINUTE GRIDDED GRAVITY-ANOMALY DATA DERIVED FROM SEASAT ALTIMETRY: Free-air gravity anomalies based on filtered SEASAT altimetry data have been calculated for a 5-minute latitude- longitude grid by Dr. William F. Haxby of the Lamont-Doherty Geological Observatory. Data are given to 0.1 milligal precision. Coverage extends 72 degrees either side of the Equator. Data are on 1 magnetic tape.

3. 15-MINUTE GRIDDED GRAVITY ANOMALY DATA DERIVED FROM SEASAT ALTIMETRY: Free air gravity anomalies based on filtered SEASAT altimetry data have been calculated for a 15-minute latitude- longitude grid by the International Gravimetric Bureau (BGI) of France. Gridding computation was by the inverse Stokes operator method combined with a high-degree and order spherical harmonic model of the gravity field based on the SEASAT and GEOS3 sea- surface altimetry. Data are on 1 magnetic tape.


1. Sea Surface Contour Overlays
       (plastic sepia) Product #978H200001 (paper sepia) Product #978H190001 (blackline) Product #978H180001

2. 5-min Gridded Gravity-Anomaly Data:
       1 Magnetic tape (ASCII, 1600 bpi) Product #978H07001

3. 15-min Gridded Gravity-Anomaly Data
       1 Magnetic tape (ASCII, 1600 bpi) Product#978H07002