Ben Hamlington (Old Dominion University, USA); Dallas Masters (Old Dominion University, USA); Mark Merrifield (University of Hawaii, USA); Steve Nerem (University of Colorado, USA); Philip Thompson (University of Hawaii, USA)

It was shown very early in the altimeter era of sea surface height measurement that vertical land motion (VLM) errors at in situ tide gauges were the limiting factor in estimating possible altimeter drift errors. These errors were in turn recognized as the limiting errors on altimeter estimates of sea level change. This situation has not changed in the nearly 20 years since. In those early days there were few estimates of VLM, but now there are multiple global estimates from multiple sources that can be used in altimeter, tide gauge comparisons. Two questions naturally arise, which our group has been working to address. First, we ask whether inter-comparisons of the different VLM estimates can be exploited to make estimates of the bias and random errors for the various VLM estimates, and to describe the covariance in the errors of the different VLM estimates. If successful, such an analysis could be used to define an optimal VLM estimate at each tide gauge. Second, we ask about the sensitivity of the tide gauge based drift estimate for the climate data record from altimetry to the different VLM measurements. It would be good if the altimeter drift estimate was insensitive to the specific VLM estimates used, but to our knowledge this sensitivity has not been sufficiently quantified. In our talk we will present results that should aid in answering both of these questions.