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Peter Hakel, you wrote:
"We can therefore plot the LOP at the right angle to the azimuth line (at the 
intercept distance) because Mercator mapping used in plotting charts is 
conformal, i.e. it preserves angles."

There's an interesting historical point related to this. When Sumner first 
proposed his method, he described it in terms of plotting on a Mercator 
projection. In fact, this is not necessary or even desirable. Any simple 
chart including plane graph paper (homemade will do) is all that's needed to 
find the intersections of two Sumner LOPs. It was not until the first 
editions of the twentieth century that this was specifically mentioned in 
that, then latest, revision of Bowditch. And that's a big deal. Plotting 
charts are not easy to come by, even today, and nobody likes drawing on their 
expensive printed charts. The fact that you can work two Sumner lines for a 
fix should have caught on much more quickly. The emphasis on conformal 
Mercator charts may have stalled that.

-FER




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