Others have touched on these same issues so I apologize if it repeats or gets redundant.
An apex is nothing more than the highest point. In this case the highest point, in the rocker curve, on the bottom of the board. Depending on how you rotate the board and then fix it in space the Apex could be anyplace on the curve all the way from the nose to the tail. Of course, this pretty much makes the term Apex irrelevant as the term cannot apply to a single location along the rocker curve unless we first establish an absolute way that the rocker curve, is going to be located in space. That is way I have been stressing the need for a cooperative measuring system.
In my view there are only 2 logical ways to “hang” the rocker curve in space.
1 is to use the center of the curve, and from there establish the tangent line and then level that line to a level horizon. In this method the center will become the Apex, not only on this board but also on any board that is measured using this method.
2 is to scribe a level horizon line and then place the bottom rocker curve on it with the ends of the curve just resting on the horizon line. Airfoils of all sorts are often viewed this way. In this method the Apex will float all over the place depending on 3 factors.
A. How much nose rocker or how much tail rocker the board has.
B. How thick the nose beak is or the tail tip is.
C. How that rocker curve is proportioned.
It quickly becomes clear that between these 2 methods, # 1 is far superior because it creates an absolute system of measurement that will consistently reproduce the same results regardless of the actual character of the curve being measured or who, when or where it is being measured. Including what the rest of the boards design features are.
Method #2 cannot do this. Especially since the bottom curve in a board is also attached to the deck and the thickness of the nose and tail. If you try to lay the board on its deck and then align a LEVEL rocker stick on the bottom, to find the Apex, that Apex and where it lands along the curve will be dictated by the nose and tail rockers and thicknesses.
The whole point of measuring rockers is so that we can know what we are riding. If we know what it is, we can then alter what it is, and then learn what the effects of those alterations are, both visually and functionally. But without being able to establish a common standard of measurement for determining what it is, we can’t share the data we collect and we can’t know if we have actually altered it. Nor can we know if the differences we feel are related to the changes we have made or if they are due to some other changes that we don’t even realize we have made because we don’t have a common method of measuring them and accounting for them.
Having a reasonably accurate measuring system that is commonly shared and agreed to is the only way to sensibly discuss board design and be able to acquire some kind of peer review of the theories and results. Even if we could all ride each other’s boards in the same surf, we still need a common language and measurement system, to be able to discuss them, suggest changes, make those changes and actually know that we have made the intended changes.
Some have suggested that it is not reasonable to discuss or measure rocker without also considering the deck. I disagree. The rocker curve is simply what it is. It is not dependent upon the deck for its existence. It is a line scribed in space and it can be accurately measured and defined without ever knowing what the deck is going to look like.
Now before everyone freaks out about the statement above, let me assure you that I fully realize that, in action, the deck and bottom are intrinsically connected and the total function of a board must also take into consideration not only the bottom but also the deck and all other features of the board. But at the risk of sounding rude, we all know and recognize this don’t we? So lets not waste time going there. Once we agree to a common measurement system, we can get into the performance effects of altering those measurements and how they interact with other aspects of the boards design and construction.