Check the video: http://www.guardian.co.uk/sport/video/2013/jan/29/garrett-mcnamara-100ft-wave-video"There are a quite a few prerequisites to surf a wave of the magnitude of those in Nazaré, Portugal. For a start, you can't exactly paddle out through these waves, as they are just too powerful. In this case, you can see on the video how the jet-ski approaches the wave from deep water and then pulls Garrett McNamara into the wave before exiting safely to the side of the wave.Also you need a wave that focuses and breaks in one place and then "peels". That is, the breaking point will move along the wave crest at a rate that will allow the surfer to maintain a position just ahead of it.In order for the wave to peel, you need the seabed to have a particular structure – a shallower area, where the wave focuses its energy and breaks, with deeper areas to the sides, leaving this section of the wave unbroken.The wave at Nazaré is quite unusual. A deep underwater canyon is instrumental to its formation. The waves move faster in the deep canyon than in the adjacent shallow areas. This causes the wave crests to bend (refract), focusing the wave energy and creating a large peak that breaks in a very regular and controlled fashion. Perfect for surfing.It's very unlikely there were any measurements of the wave taken directly through scientific means when McNamara was surfing at Nazaré. Sometimes you do get measurements from wave buoys and there are estimates from computer models, but neither of these methods can fully reflect the breaking wave that is actually surfed. This is because the wave is modified a lot from the offshore area to the point of breaking.As it isn't really possible to use these measurements or estimates, it is probable that the estimates of wave height were derived from photographic and video data. This can provide reasonable approximations of the actual face of the wave surfed. If the height of the surfer is known, then this can be used to scale the wave face observed in the image. Providing the images are collected from a suitable vantage, the error in this method could be as little as plus or minus one foot.Mark Davidson is associate professor of coastal processes at Plymouth University"