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’s destructive power and will also enhance understanding of the events.

The data could not be used to provide warning of tsunamis as the measurements take up to five hours to process and it was chance that the two satellites happened to be overhead when the earthquake struck.

Philip Callahan of Nasa’s Jet Propulsion Laboratory in Pasadena, California, said: “There is a very low probability of capturing observations in any given location within two hours of an event like this.

“The fact that Jason captured the tsunami’s signals is serendipitous but is nevertheless a major boon for oceanographers.”

Lee-Lueng Fu, the project’s scientist, said: “The observations made by Jason and Topex/Poseidon are unique and of tremendous value for testing and improving tsunami computer models and developing future tsunami early warning systems. The satellite altimeter data currently take a minimum of five hours to process, so they cannot provide early warning of such events.”

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