How is sea level determined? Is it...

Sea level is determined halfway between high tide and low tide, as measured by a tide gauge. It is a relative, rather than an absolute, measurement and varies from ocean to ocean. It even varies from coast to coast on the same ocean. Sediment deposits at the mouths of rivers, natural and manmade erosion, and engineering projects can affect relative local sea level. Continental drift and global warming also may be factors. Sea level is the standard used to determine local land elevations and geographic features. Mean sea level is the average height of all the oceans and is used to determine the relative height of geographic features worldwide. According to most estimates, mean sea level has risen about four to eight inches over the past hundred years.