evidence: Scientists now have an unprecedented understanding of changing sea levels, thanks to data collected and distributed by NASA and partner space agencies. For example, international satellites have collected almost 30 years of data on sea surface height from satellite altimeters. [...] The new Sea Level Change Data Pathfinder delves into many of the datasets scientists use to study the status of global and regional sea level change. When scientists refer to changes in sea level, they are referring to changes in the height of the mean sea surface relative to a mean sea level defined by Earth’s geoid. Earth’s geoid is defined as the mean sea surface due to the pull of gravity on the ocean. Satellite altimeters very accurately provide measurements of sea surface height by timing a roundtrip radar signal from the satellite to the ocean surface and back to the satellite. These measurements are used to calculate Global Mean Sea Level.