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    Re: Measuring (and Calculating) Dip
    From: Brad Morris
    Date: 2013 Mar 3, 16:49 -0500

    Hi Bruce

    +/- 1 foot at 400 feet is +/- 51 arc seconds.  For low height of eye (at the beach), dip is only a few arc minutes.  Your error term will be an appreciable percentage of the total measurement.

    Consider, for Long Island Sound, I got 2 plus minutes.  Your error term is nearly 1 minute.

    I won't discourage you, but either the horizon reference must be better or, as I have done, chose a different reference (the other horizon!)


    On Mar 3, 2013 1:46 PM, "Bruce J. Pennino" <bpennino.ce@charter.net> wrote:

    Thanks for raising the point about using a total station (Electronic Distance Measuring) for setting a horizontal line.  I've thought about setting up my total station on my deck with the eyepiece at my height of eye. Then marking trees in my back yard so I could string a taut clothesline from tree to tree.....maybe a few streamers hanging down. Call that my horizon. Dip = zero?  Then I don't have to use an AH. Unfortunately the trees in my yard are close. Ideally any errors would be less if the trees were at least a few hundred ft away. I recollect my Topcon EDM reads directly to 5" and can estimate to +/- 2.
    I have not calibrated it recently, but a year or so ago I checked it for  vertical total accuracy against another professional land surveyors work and my results were scattered around his at +/- 0.02 ft at 300 -400 ft horizontal distance.  I know my horizontal distance measurement is short at  0.1 ft at 400 ft.  I used a calibrated steel tape to check the distance.  This accuracy is good enough for the volunteer work I do.
    If it is worthwhile, I can take my total station to a beach  this summer and measure the vertical dip  angle. I would do it on an east facing beach looking out over the Atlantic . Water is 50F in summer.  I could also do this at Mystic/Groton. There is a convenient beach or in Boston area. Could meet someone there? 
    If interested ,can discuss off list. My email:  bpennino.ce---net 

    ----- Original Message -----
    Sent: Sunday, March 03, 2013 10:57 AM
    Subject: [NavList 22589] Re: Measuring (and Calculating) Dip

    A properly set up transit or theodolite would be VERY good when checking dip from shore, they have higher magnification (usually 20-35x),most have crosshairs and they are accurate to within few seconds of arc.

    I am talking about something like a Sokkia DT240 electronic (accuracy better than 3"). 30x scope.
    it would - properly setup - provide a "check" on your reflecting circle.
    also something like the above would be hard to beat for accurate reading of any point of the horizon, likely better than your eye would be able to discern with even the best 7x or even 10x scope out there, especially on a hand held instrument.
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