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    Re: Dip Short
    From: David Pike
    Date: 2019 Mar 7, 08:12 -0800

    David C

    IMHO, if your doing celestial for fun, you can experiment as much as you want, including making use of GNSS.  The more you experiment the more you learn.  You could even argue Capt Sumner was experimenting when he hit upon Sumner lines although in his case he was acting out of necessity not fun.  I can think of four ways of getting your height of eye on a ferry. 

    1.  Ask the captain. 

    2.  Go to one of these live AIS sites on the internet.  Look for your ship; invariably there’ll be a waterline photograph and the ship’s length.  Measure the ratio of where you were standing to the ships length with a ruler from the screen (but make sure its a side-on photograph), and work out your height of eye that way. 

    3.  Before proceeding ashore, hold your GNSS receiver as close to your eye as possible at the point you were standing to take your celestial observation, and note the GNSS height.  When ashore, get as close to the sea as possible and take the GNSS height again.  Make an adjustment for your height and how close you are to the sea.  Although GNSS height is the least accurate GNSS value, differentially it’s not bad.  I’ve done this walking to the shore line, then walking up to a shelter, which is a much more comfortable place for taking observations.  After making an adjustment for my height, it worked quite well, and I didn’t even get my feet wet.

    4.  Tie an adjustable spanner or similar to the end of a piece of chord and lower it carefully down to the sea.  Make a mark on the chord and haul it up quickly.  After they’ve released you from prison, you could measure the string and complete your calculations.

    Incidentally, as dip is a function of the square root of the height of the observer above the sea or a cloud bank, the higher you climb, the less precise you need to be about your height.  That’s why airships such as the R34 and the Zeppelins could obtain passable observations even though, without regular updates to sea level temperature and pressure, they were never really sure of their exact altitude.  DaveP

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