The average height of the surface of the sea, for all stages of the tide, over a long (usually 19-year) period, being determined from hourly readings of tidal height. A tidal gauge measures the sea level in relation to the adjacent coast; since the land may also be moving vertically, this does not represent an absolute measure of sea level. Radar altimetry measures the absolute sea level as the distance between the sea surface and the satellite housing the radar transmitter and receiver.