ABSTRACT
This chapter describes the in situ measurement systems including a brief history of their implementation and examples of their uses together with satellite altimetry and explores auxiliary measurement systems. It shows how satellite and in situ observations combine to form an integrated ocean observing system and illustrates the limitations of the observing system. The chapter discusses sea surface height changes in response to changes in the mass of water per unit area and to changes in density. It is important to avoid the impression that tide gauges and altimetry provide alternative sea level monitoring systems that are somehow in competition. The chapter describes the considerations that enter into selection of tide gauges to use for altimeter calibration. Thermal expansion is a large component in the global and regional sea level budgets, and observations of steric variability and water column heat content are prominent among the objectives for in situ observations in relation to satellite altimetry.
