Bryan, I now understand better what you mean by winnowing down, but I don't necessarily agree with your description regarding either the ideal holes or the holes at NGLA.
1.
CBM's Ideal Holes. It is not as if CBM wasn't already familiar with many of the ideal holes. He had been playing them and discussing them for years. Remember the influence that the Great Hole debate at the beginning of the Century had over CBM's thinking, and remember also that he was already very familiar with many of the holes from his extensive experience prior to 1906.
Also, surely he was already forming and solidifying his list of which of the holes were his favorites while he was seeing them, and deciding which to document. And so when he returned he must have had some ideas on what holes he viewed as best. For example, when CBM returned in June 1906 he had already decided to include the Biarritz.
Imagine you played a bunch of courses on a trip, many of which you had been discussing and playing for years, and you even took notes and created diagrams, etc. Would you need months to decide which you liked best, or would you know when you got back?
Also, according to CBM holes on his list of 18 weren't even necessarily superior to others he had in mind:
"I have notes of many holes equally as good as a numtier of the above, but this list will convey to the mind of the reader a fair idea of what I have gleaned during the last few months as constituting a perfect length of hole consistent with variety." So I am not sure it is a winnowing down so much as a representative sample.
2.
NGLA. By CBM's description it sounds as if he found his key holes fairly early on. So we are really talking about his hybrids and his originals.
A lot of the ideal 18 made it into the design for NGLA. Some did not, and was replaced by other ideal concepts or his own original designs. The winnowing down of the ideal 18 as to what would fit on the real life property must have taken some time.
Coming up with his original designs and fitting them into the routing must have taken some time.
I think in envision this a little differently. Rather than coming up with the exact holes he wanted to use then trying to fit them on the property, I gather that, after placing his key holes, he found features that would with any of his many of his concepts. You seem to have it as the concept was the driving force, whereas I think that after placing the key holes, it was the land that was the driving force and he found concepts which would work with the interesting feature on the land. So his broad knowledge of great holes wasn't something he needed to winnow down before the design process, it was something that helped him make the design process more efficient.
It is a subtle difference but an important one. Where you suggest he had to "com[e] up with his original designs and fit them into the routing," I think he used his broad knowledge to find new and original holes (and hybrids) on the property.
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Looking at the relief map and the blueprint it is interesting to imagine how it might have happened. I don't think I ever understood why the Channel Hole is a Channel hole until I studied the blueprint.