Satellite altimetry and tide gauges provide observations of the instantaneous sea surface. As a further source an oceanographic model of the Baltic Sea also provides sea-level heights. The different sea-surface heights are of comparable precision and complement each other well in terms of their spatial and temporal resolution and their information content.

When the different sea-level information are combined, variations in the time series can be reduced, the quality of the observations can be checked, and phenomena of special interest can be separated.

One application for the combination of observed and modelled heights is the estimation of secular sea-level changes. By reducing a large part of the high frequency sea-level variations in the observations, the trend estimation from short time series can be improved. This is of interest especially for satellite altimeter observations, which, though having a high spatial resolution, cover a rather short time span.