Just as the surface of the Earth is not flat, the surface of the ocean is not flat. For instance, the absolute water level height is higher along the West Coast of the United States than the East Coast. The surface of the sea changes at different rates around the globe.
The term “global sea level,” refers to the average height of all of the Earth’s ocean basins. “Global sea level rise” refers to the increase in the average global sea level trend. “Local sea level” refers to the height of the water measured along the coast relative to a specific point on land. Tide stations measure local sea level. “Relative sea level trends” reflect changes in local sea level over time. This relative change is the one most critical for many coastal applications, including coastal mapping, marine boundary delineation, coastal zone management, coastal engineering, sustainable habitat restoration design, and the general public enjoying their favorite beach.
Sea levels change in response to astronomical forces (sun and moon) and to meteorological forces (winds, pressure changes, etc). Over time these changes can be observed, averaged, and modeled into the future to predict what the water surface will look like at some point in the future. This is done on various time scales. Sea level rise predictions usually go out to a century, based on global climate models. Tidal predictions are on a monthly basis, and storm surge and tsunami forecasting is done at the event scale (minutes to hours). In order to accurately map any predicted ocean surface, that surface has to be either be modeled, interpolated from tidal or water level gages or high water marks, or estimated as a single value of the study region. Ocean surfaces are usually referenced to local tidal datums and have to be converted to terrestrial or orthometric datums like NAVD88.
The map shown here illustrates the variation of the mean higher high water (MHHW) surface in the NY Harbor/ Long Island Sound region, as modeled by the NOAA VDATUM model. This averaged surface shows up to a 1m (3 feet) difference in elevation. This will have a major influence on mapping inundation imacts of sea level rise. Note the effects of embayments and areas of restriced tidal flow (inlets) on water surface elevations.
For more details on sea level change, visit these links.