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My just-posted question on DR plotting actually arose while researching
a different topic:

When did plotting courses in True (vs Magnetic) become  commonplace?  In
fact, were courses in a plot originally labeled in True or Magnetic?

I suspect that with the advent of the gyrocompass on ships knowing True
courses became important.  But there were centuries and centuries of
navigation before the perfection of the gyrocompass that depended on
magnetic compasses and it might have made a lot more sense to plot in
Magnetic.

Of course, if navigators didn't keep a DR plot then the whole True vs
Magnetic issue becomes more difficult to answer.

The only reference I've found to True vs Magnetic is in browsing through
NOAA's on-line library of antique charts.  A very few of them have
courses between prominent objects which are labeled in Magnetic using
the traditional intercardinal point notation.  An excellent example of
this is a chart of Woods Hole (although the chart calls itself Woods
Holl, search for chart 348), a marginal note says "directions not shown
in brackets are Magnetic, those in brackets are True"   All directions
are unbracketed.

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