Guessing distance off.
It is notoriously hard to guess a vessel's distance offshore.
One method that may be of use when offshore of a straight section of coast is a follows:
Estimate a line directly to the shore and one at 45 degrees to it. The distance between those two parts of the coast is the same as the distance offshore. All you need to do is guess how long the piece of coast is!
In practice, if there is a conspicuous point directly inshore of you and another about 45 degrees ahead, a quick look at the chart will give you the distance they are apart. This is about the distance the vessel is off the coast.
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