FIGURE D.1 Location of tide gages examined with inset showing schematic of a tide gage and benchmark leveling network illustrating fundamental ambiguity that may exist between motion of the tide gage instrument and local land motion.
Tide gages measure the height of the sea surface with respect to their own internal reference level. As the gages require relatively deep water to operate in, away from shore wave breaks and currents, they are often installed on manmade structures such as piers, built out from the coast. Although tide gage locations are chosen carefully in an effort to ensure long-term stability and a minimum of vertical motion, it is impossible to guarantee these qualities in a site. In order to maintain an accessible external reference mark from which the local sea-level datums can be assigned, to allow for tide gage equipment changes, and to confirm the stability of the gage, each tide gage is supported by a network of leveling benchmarks, with one of these designated as the primary benchmark. This is normally chosen to be both as close to the tide gage as possible, and installed in as stable a location as can be found (Hicks et al., 1987). As the local sea-level datums are defined with respect to this primary benchmark, each tide gage has a correction term that is applied to the raw ranges it observes to the sea-surface in order to map them to the vertical datum defined by the primary benchmark. Every time the gage is either changed, upgraded, or experiences some possible vertical offset—due, for example, to being hit by a harbor vehicle—a new correction term is calculated by performing leveling between the primary benchmark and the reference point on the new (or newly offset) tide gage. This term is applied to the data stream so that is it transparent to the end-user of the data, maintaining what is, in theory, a continuous record of sea level with respect to the primary benchmark, rather than the gage itself. Although these constants are recorded as they are recalculated and programmed into the tide gage data logger, they are not all archived digitally, and were not available for this study.
Current recommended operating procedures require a network of at least 10 benchmarks be established and monitored in support of tide gage sea-level observations (Woodworth, 2002). Historically, however, significantly fewer marks have been regularly observed—some of which may no longer exist due to construction (or other reasons) around the typically extremely changeable industrial environment of the harbors, where most tide gages are installed. Leveling of the west coast network is currently performed using 2nd Order Levels standards approximately annually, though historically it was often done much less frequently and to a lower standard. The leveling data are used to determine the general stability of each benchmark as well as the tide gage, and identify any tide gage offsets (any jump greater than 6 mm) necessitating a site correction factor adjustment. The long-term relative vertical velocity of the tide gage with respect to the primary benchmark is not applied as a correction to the sea-level time series.
DATA AND METHODS
The leveling data, corrected for atmospheric refraction, from all occupations of the tide gage benchmark networks for San Diego, Los Angeles, Port San Luis, San Francisco, Point Reyes, and Crescent City, were provided by the National Oceanographic and Atmospheric Administration’s (NOAA’s) Center for Operational Oceanographic Products and Services (CO-OPS). Each benchmark network data set con-