It’s been almost a year now since footage of 45-year-old Hawaiian Garrett McNamara surfing a mountain of water in Nazaré, Portugal stormed the web at the start of the year. Along with it had been huge speculation and excitement that this wave may be a fabled ‘100 footer’! The funny thing is that McNamara wasn’t even claiming it at the time!

Everything was cleared up at the Billabong XXL Global Big Wave Awards in May, so maybe we shouldn’t jump to conclusions next time a monster wave is tamed. When all the speculation was being thrown around at the start of the year, there had been no mention or reference to a credible source. Garrett himself was actually keen to point this out in an interview with Outside, when asked how big he thought the wave was. “Surf Europe was the first one we’ve found to put it up…It’s all from a picture, they don’t even know if it’s Photoshopped or not. My mother-in-law told me, ‘Believe none of what you hear and half of what you see.’ That’s a good way to think about it.”

So how was this monster wave actually measured before the XXL Big Wave Awards? Contrary to popular believe, the Guinness World Records does not deal with wave height measurement. This role falls in the hands of a carefully selected group of surf meteorologists, surfers and surf journalists, forming the Billabong XXL Global Big Wave Awards judging panel. They have been scrutinizing photographs, footage and other data in order to verify wave heights since 1997. Back then the XXL was not sponsored by Billabong but was known as the K2 Big Wave Challenge (a year-long competition launched by former Surfing Magazine editor Bill Sharp, where the surfer to paddle into the biggest wave of the year would earn payout equal to $1,000 for every foot of wave height). The quest for bigger waves and thus more speed since then has required the use of jet skis to tow surfers into waves. This has resulted in the XXL being split into two categories, paddle (where a surfer has to rely on their own paddle power to catch the wave) and tow.

The first step in measuring a wave height is to determine the surfer’s height on the board, i.e. in a crouched surf position. This is the most important and fundamental measurement since it gives the judges a frame of reference to the wave. Therefore efforts have been taken to increase the measurement precision to the inch, through new techniques. Since the perspective height of a surfer varies with angle of photograph, the respective surfer’s shinbone is now measured, which is multiplied by a ‘shinbone multiplier’ to gain a more accurate crouched height measurement. The next stage involves identifying where the trough (flat water in front of the wave) begins and where the wave starts to curve upward in height. This can be a difficult task and varies with location. At some spots, like Jaws, Hawaii, the wave’s transition to vertical is normally well-defined and thus a judgement is quite straightforward. However at other spots, this is not always the case and judges have to interpolate from the available information where the trough actually was. At Nazaré, Portugal, where the waves can be much more ‘slopey’ and often obscured by spray, the determination of the trough’s position proves even more difficult. Therefore judges need to gauge dimensions by observing footage of the wave taken at different angles at the same time. Once the trough’s location has been pinpointed, wave height can be calculated quite simply by taking a multiplier of the surfer’s height from the trough to the crest, either via digital means or by using a pair of calipers.

Photo’s of McNamara’s wave on 28th January 2013 are mostly slightly hazy and from a straightforward position. Since the wave is partly obscured by the headland in much of the shots its very difficult to identify the position of the trough, if at all. From a straightforward point of view, McNamara appears to be half way up the wave face, however in other shots taken at different angles he appears to be nearer the trough at the same moment. This suggests that the wave height could be substantially smaller that 100 feet (perhaps nearer 60 feet (18 m) – which is still a massive wave in my book! Anyway you get the picture, judging the size of monster waves is a tricky job.