The information will be used by the military to rehearse and plan missions. Special public data will be used to study flooding, soil erosion, landslides, earthquakes, weather and climate. SRTM imagery of Pasadena, Calif., with the San Gabriel Mountains to the north. The data collected during SRTM represents mountains, valleys and flat land. Mountains and valleys are measured from sea level.
Sea level is the average height of the ocean.
So, if someone tells you Mount Everest, the tallest mountain on Earth, is 29,028 feet, he or she means if you took the mountain and put it on the ocean and measured straight up through its middle it would be that tall.