Lots of different measurements. If you measure the back of the wave, it's generally thought to be more accurate for surfing conditions because you'd be measuring the wave, not the swell as it grows in shallower water. Interestingly, this method is somewhat larger than 1/2 of the face height (Crest to Trough).
Most of the world seems to use 2/3rds the height. Different for inland waterways than the ocean as well. Generally speaking, if the wave face on the lake is 4', then there are 4 footers. I think the NWS takes the face height average of two thirds of the highest waves.
Being out in the ocean on three to fives can be significantly different than Champlain, or the Great Lakes. The period is much shorter (usually) on inland lakes, and while your boat in in the trough, the crest of the next wave can be headed for the top of your bow. If they're close enough together, larger boats can effectively ride the top for a ways if consistent enough.
We've had some very windy days on Champlain now since mid-August, so I got to see a lot of how NOAA measures them. 2-3s are pretty irritating and painful for my little 22'. 3-4's mean I shouldn't have gone out, pay attention. One day I was doing the cross lake journey, it started out as 2-3's, built up to 4-5's. That meant I could be SOL. Thankfully, they were close enough that I didn't have to ride them down. But they were large enough that I couldn't go into the straight following sea, the bow would have stuffed on many of the larger ones.
My guess from the picture was 1-2. But pictures rarely show the proper perspective
Then you can see whacked out waves like these