The Indian Ocean tsunami had reached heights of up to 33ft (10m) when it crashed ashore on Boxing Day, satellite images have revealed.
The measurements come from American and French oceanography satellites that passed over the Bay of Bengal two hours after the magnitude 9 earthquake struck southwest of Sumatra and observed a tsunami for the first time.
The Jason and Topex/ Poseidon satellites, which are operated jointly by NASA and the French Space Agency, tracked the tsunami as it crossed the Indian Ocean.
In the open ocean the crest of the tsunami wave stood 20in above the norm and was followed by a trough 16in below the norm.
When the wave reached the coast, however, the shallow water caused it to slow from 500mph to about 20mph, and to rear up into a wall of water up to 33ft high.
Scientists said that the measurements help to explain the tsunami