Hence, by collecting worldwide hydrographic data since 1993, then computing dynamic heights, SLA from ERS-1, ERS-2, T/P, Jason-1, EnviSat or GFO can be associated at the same position and time, using a space-time objective analysis. By subtracting the SLA, we obtain a set of local estimate of the MDT relative to the 1993-1999 period. To reduce noise, these estmates are box averaged (1°x0.5°), assuming that the MDT scales are twice larger:

In a similar way, assuming geostrophy, geostrophic velocity deduced from surface drifters can be used together with geostrophic velocities (anomalies) computed with altimetric slopes. To do so, the Surface Velocity Program drifters from 1993 to 2000 are processed and corrected from the wind driven effects [2]. The local estimates of mean geostrophic velocities are also box averaged (1°x0.5°):

    

All these height and velocity estimates are combined in a multivariate objective analysis [4], [5] in order to improve locally the guess computed with the direct method. The figure below illustrates where local estimates are improving the guess, along western boundary currents and the circumpolar current: