It appears that RMSE not only explains amplitude discrepancies
between tidal and altimetry observations, but is also a measure of
quality of the subsequently derived vertical land movement of the
tide gauge. However, RMSE is only available after intensive altimetry
and tidal data analysis, so some pre-analysis indicator of RMSE
would be useful in practice. Fig. 6 illustrates the strong relationship
between RMSE and distance between TOPEX ground tracks and the
tide gauge. At almost all stations, smaller ground track/tide gauge
distance results in low RMSE, which in turn results in low standard
deviation of the vertical landmotion. Our altimetry solution is driven
by TOPEX/Jason altimetry because its shorter revisit period contributes
more measurements. As such, TOPEX ground track/tide
gauge distance can be used to predict RMSE. A simple rule can then
be derived from our results, when TOPEX-class altimetry measurements
within 100 kmof a tide gauge are available, a good estimate of
vertical land motion can be expected. With this rule of thumb and a
regional or global plot showing the TOPEX-class altimetry ground
tracks with the locations of tidal stations having no nearby GPS, tide gauges whose vertical motions can potentially be derived, can be
identified