Wave Height Estimation -- Hawaii
The link you just followed used to point to this page: surfing.about.com/blwaveht.htm ("Estimating Wave Height"), but that URL is no longer valid. It was a brief article on how surfers decide wave height and offered a good description of the Hawaii-surfer estimation standard by a well-known Australian (I think) surfer (I can't recall his name).
If I'm remembering correctly, the gist of it was: (1) take the wave face and divide by two; (2) examine the result, the quotient, for the number of multiples of five; and, (3) for each multiple subtract one foot. That's it.
How big is a wave with a 20-foot face?
(1) 20 / 2 = 10
(2) 10 / 5 = 2
(3) 10 - 2 = 8
Answer: 8 feet.
Thusly, a wave with a 30-foot face is a 12-foot Hawaii-style wave. This particular calculus gives the most accurate result based on my experience. That is to say, it is the best mathematical expression of how most people use the Hawaii-surfer method of underestimating wave height.
Some comments. First, you just have to guess as to the base wave-face value. This definitely will vary with the observer. One way would be to use a surfer on the wave as a measuring stick. Again, this is necessarily a subjective exercise.
Second, for wave-face heights of less than ten feet you could use the alternative measure of body-reference points, e.g., knee-high, waist-high, chest-high, head-high, slightly overhead, etc. Basically, I would call a slightly-overhead wave a 3-footer; a head-high wave, a 2-footer.
Lastly, it often helps to verify or arrive at an estimation by asking other people for their opinion. This is determination through consensus. If I really care about figuring a height, then I might ask some of the other surfers in the water or even someone watching on the beach. It's surprising how the responses of several people will usually converge toward a narrow range, that is, they will be in general agreement.
Here are some articles on the topic:
"Isle converts to global wave measures."
"Willard Bascom: Estimating Breaking Wave Height."