Knowledge Bank

Gravity anomalies and sea surface heights have been computed on a 0°.125 grid in the ocean areas from a combined Geos-3/Seasat altimeter data set. The basic procedure used least squares collocation estimation where model covariance models were tailored to individual areas through altimeter residual variance scaling, Preliminary tests led to production prediction procedures using a reference model defined by a set of potential coefficients complete to degree 180, Comparisons of the predicted anomalies with ship derived values showed agreements varying from ± 9 to ± 30 mgals. No corrections to the altimeter implied sea surface heights were made for sea surface topography effects. The maximum anomaly predicted was 396 mgals near Hawaii and the most negative anomaly was -361 mgals over the Puerto Rican Trench. The 0°.125 data set has been used to display the results using color plots, contour maps, and perspective views. The gridded data was used to compute 1° x 1° and 0°.5 x 0°.5 mean values. 1° mean anomalies were compared to terrestrial data where a difference of ± 7 mgals was found in comparing 10139 values. The actual resolution of the data was estimated to be about 0°.19 based on a power spectrum analysis. The resolution is limited by our data selection process which used 300 data points, a thinned altimeter data set for the prediction of the 0°.125 in a 3° x 3° block with one data selection, and matrix inversion. The predicted data were used to compute 104 potential coefficient spectra using flat earth approximations developed by Forsberg. The spectra were classified in terms of smooth, mild, or rough areas. The great majority (78) of the spectra were in the smooth classification. Good agreement was found from spectra computed from topographically reduced land data and the mean ocean data, but the spectra from two models were substantially higher than found from the ocean data. The decay of the system could be represented in the form of ℓ-3.6 which follows from the Forsberg data and the altimeter data considered to degree 800. The geoid undulation information between various spherical harmonic degrees was computed using the spectra and compared with other results. The geoid information was quite sensitive to the type of region from which the spectra was computed. [Some mathematical expressions are not fully represented in the metadata. Full text of abstract available in document.]