Hi Downwind
I read this article as pasted below that helped explain how and why wave height is measured by different people. It also has a bit of history and culture as to why...
Cheers
Swanie
"Some surfers use 'waist high', 'head high', 'shoulder high' and 'over head' as a wave height indicatior which can be confusing if you're used to measuring wave faces in feet.
Others measure from the back of the wave 'Hawaiian Style'. The most common and simplest method would be measuring the wave face from peak to trough i.e. from the bottom to the top of the breaking wave.
With waves constantly moving, it can be tricky to the uninitiated to judge their actual height. Lying down whilst paddling out can also put a whole new perspective on the height of wave as it smashes onto your head.
Most surfers will call an average height rather than basing a session on rogue set waves/ the biggest of the day.
Rather than calling it 4ft we'd say it was 3-4ft or 5-6ft and so on...
As a general rule, if it's only 1ft, it's pretty difficult to surf on, unless you longboard or are a lightweight grom/ shredding machine!
Here's the best article on the subject from surfing pro Neal Miyake.
The scientific method of measuring wave heights is to calculate the vertical distance from crest to trough of the wave face. With respect to surfing, the measurement should be taken at the highest point just as the wave is about to curl. Sounds simple enough.
However, practical application is actually quite challenging. It is difficult to define the location of the trough, especially from differing vantage points. You actually have to look at shadows and subtle nuances in curvature just to make a decent guesstimation.
And nobody wants to overestimate wave heights. It's like reverse-machismo where downplaying the size establishes a degree of status and legitimacy for the surfer. Having a common wave measuring methodology is also a form of group identification (typically by geographic area), which is actually a part of the "tribalism" of surfing.
In Hawaii, the way most locals call wave heights is using the "Hawaiian scale." It roughly translates to around half of the wave face height. Thus, a three-foot Hawaiian scale wave has about a six-foot face (what some would consider "head-high"), six-foot Hawaiian has about a 12-foot face (around "double-overhead"), and so on. The Hawaiian scale seems to get somewhat more nebulous the higher you get, with a lot more bull$#it and/or bravado coming into the picture. Interestingly, it seems the Hawaiian scale is slightly different between the North Shore and Town, with us "Townies" calling it slightly higher than the Country guys would.
How the "Hawaiian scale" came about is an issue that's shrouded in mystery. Some say the scale measures waves "from the back." When a wave starts to break, it draws water from shore so that the wave trough in the front drops below mean sea level, making the face bigger than the back. Most people scoff at this "from the back" concept, saying that it's impossible to measure the waves from shore, and nobody surfs the back of the wave anyway. Point taken.
Another thought was that the scale was created by California "sandbaggers" who intentionally called the waves smaller than they really were to scare off newcomers. These North Shore pioneers were a fairly tight crew who were always pushing each other to ride bigger and bigger waves, and I could imagine them cutting down each other's calls. More reasonable, but still suspect since wide proliferation of the standard would be difficult.
Recently however, former big wave aficionado Ricky Grigg (who happened to be one of those aforementioned pioneers) wrote a story in The Surfer's Journal (volume 12, number 1) describing his opinion on how the Hawaiian scale got established. He said that back in the day, when the lifeguards made their daily surf height reports they would purposely call it smaller than it really was. Lowering the report typically meant fewer beachgoers for them to worry about and allowed them (most surfers themselves) more time to partake in the waves. As time went by, Grigg surmised that the scale slowly evolved into what it is today. This is the most plausible explanation I've ever heard.
Regardless of how the Hawaiian scale came about, we seem to be pretty well entrenched in it. It has been reinforced and perpetuated by the media, with U.S. surf magazines and local surf reporters aligning with this standard.
The reports are usually given in ranges, which allows the reporter a little bit of extra latitude in the reading. This range also can help surfers determine how consistent a swell is. For instance, a 1-3 foot plus day implicitly says that you may get the occasional overhead wave if you are patient.
In March 2001 however, the National Weather Service sought to standardize reporting, calling the surf by wave face. This was done in the interest of public safety, especially in light of a series of injury lawsuits that claimed that surf reports did not accurately reflect true ocean conditions.
But have the locals converted to the wave face scale? Not really. All the surfers I know pretty much still call it by the old Hawaiian scale. I think the Surf News Network, who reports the lion's share of the daily wave heights on Oahu still does it the old way too, or something close to it (a disclaimer on their website states, "Surf measurements are 2/3 the height of the wave from crest to trough").
Recently, there was a surf/photo contest to determine the biggest wave ridden this winter season. The Billabong XXL Global Big Wave Awards" were announced on April 18 to quite a bit of hoopla and controversy. The winning entrant was a tow-in beast at Jaws, Maui with an estimated wave face height of 66 feet. (Is that 33 feet Hawaiian?) The interesting thing is that photos of competing waves from Belharra, France seems as tall (if not taller), although defining the trough of that wave is very difficult. In the end, the judges deemed that the waves were so close in height that they had to also consider "heaviness." That's where the Jaws wave won hands down.
So height is one way to describe a wave, but to better describe surf conditions, an intensity level might be another neat parameter to use. Of course, this would add yet another level of confusion to the mix, since "heaviness" is an even more subjective attribute.
Due to the constant inquiries to his website on the subject, my surfing friend Buddy has produced a nice graphic that I believe accurately represents and compares wave height scales, including wave face, body reference and Hawaiian scales. Might not be able to use it at Jaws, but this graphic would be perfect for calling those good sessions at Bowls or Rocky Point.
Looking back at the photos from that day on Kauai, I understand why my cousin's friend hassled me about my wave height call. Given my current understanding of the Hawaiian scale, I figure the sets were running around five-feet max - not six. Gotta call ‘um low - no like make shame again."
) that that was the way to measure waves. 
