NavList:

You are absolutely correct.  The better ways to check your chronometer are all today's ways!

The period when this method would be popular would be after chronometers became prevalent and before technology provided a better answer.  Like the telegraph!

Worsley used this method on Elephant Island because there was no alternative.  No internet, no GPS, no WWV radio, no passing ships, no telegraph, etc.  The uncertainty of location of Cape Belsham on Elephant Island led to uncertainty in the rating of the chronometer.  Which brings us full swing back to Eamonn's question.

Eamonn's question was about "harbor markers".  There are a few references to them in books.  Apparently, they were used to provide the exact known location for this observation.  He wanted to know about them and if any still exist.  We on NavList often head right into the weeds and miss answering the question.  Let's at least try!

A harbor marker almost sounds like a surveyor's monument, as it is land based with a precision definition of location. Similar to a geodetic monument, or perhaps even the same thing.  

I consulted the 18th edition of Wrinkles in Practical Navigation, Lecky, published in 1919, to see what he had to say about "harbor markers".  In the topic "Principle of getting the correct G.M.T. by observation on shore", Lecky describes the exact process of the observation, mentions a few fixed locations on the Suez Canal, but does not mention the phrase harbor marker. Lecky does speak to the known geographic location, but does not use the phrase harbor marker.  Odd that he would ignore the phrase if it were in the navigator's common parlance, in consideration of the vast breadth of Lecky.  That would be odd except if it wasn't actually in use!  If not in use, then why would Lecky mention it?  

Brad