Transit markers Durlston head

[Dadof4]

sailing past Durlston head at the weekend I noticed two sets of transit markers (the easterly one is just near Anvil point lighthouse) - huge white posts on top of cliffs


The easterly one is right near Anvil point lighthouse

Initially I was completely confused as they both lead directly onto the cliffs. - thought they may be something to do with Lulworth ranges
Then noticed that they are exactly 1nm apart (the chart says "1852m measured distance" ) - so my thinking is that theyre something to do with calibrating instruments or measuring tidal flow ??


Anybody know what theyre for and who put them there ?

 

[Hadenough]

It is a measured mile.

 

[Jonny A]

For pre-GPS speed trials.

 

[Praxinoscope]

Measured Mile, Interesting to know they are still there, used them years ago (at least 40 years) for checking accuracy of our log.

 

[jbweston]

Measured Mile, Interesting to know they are still there, used them years ago (at least 40 years) for checking accuracy of our log.

Now it can be used to check the accuracy of your GPS - or to give an excuse for the skipper to 'entertain' the crew with one of those stories that begins 'You young uns don't know how lucky you are. When I was a lad . . . '

So here goes . . .

When I was a lad my father and I used to go out to this very measured mile and run it both ways with our engines (in a motor cruiser) at different rev settings so as to draw up a table of revs - speeds. We had no GPS, Decca or even an electric log, so dead reckoning was based on the rev - speed table with occasional cross checks using a Dutchman's log.

 

[JumbleDuck]

They are common in ship-building areas. On the Clyde there were at least two: one now derelict by Skelmorlie and one still usable at the north of Arran.

 

[alan_d]

If you look on Admiralty Chart BA2172 you will see a dotted line between the two transits marked:
Course 083.5deg-263.5deg Measured Distance 1848.9m.
Guess what 1848.9 metres is in nautical miles?
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0.998326134, apparently

 

[LittleSister]

0.998326134, apparently

Yes, but no-one minds if their speedo over-reads. It's when it's the other way you get complaints.