Taking measure of the world: radar altimetry in spotlight at Venice event   Global mean sea surface height derived from radar altimeter data. The radar beam from the satellite scans the ocean surface, detecting variations in relief of up to 80 metres in places. Ocean relief, which is stable, is shaped by the geoid - the surface normal to the direction of the Earth's gravity field. The scale here ranges from purple (-80 metres) to red brown (+80 metres).   'Wet' radar altimetry signal returns enable the detection of river and lake levels, as seen here for South America   Radar altimetry-derived slope map of the Greenland ice sheet   Radar altimetry-derived slope map of Antarctica   Altimeter-based chart showing variability of the Gulf Stream, with an inset showing modelled stratification of the temperature field (red is warmest, blue is coldest).   Mean sea level as monitored by altimetry sensors on three separate satellites - Envisat, TOPEX/Poseidon and Jason-1. Release date: 22 February 2008