A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2013 Mar 3, 19:10 -0500
My apologies. I misunderstood your error term. 5" of error from "true" horizon would be just fine for a dip measure.
Sorry....maybe not clear......if I use the EDM to measure 400 ft horizontally, my measured electronic output would be 399.90 ft. Error of 0.1 ft. I used my calibrated steel tape to layout "exactly" 400.00 ft. But of course there is error in my tape layout!With many EDMs, distances of 2,000 ft or more can be measured with the proper reflecting prism (or even without a prism).I know my vertical angle measurements are accurate to within a few seconds, maybe +/- 5 second error because of my check against a surveyor's recently calibrated total station.But based on Bill's 2012 measurements with a 1 second theodolite (probably really good to a couple of seconds +/-), some dip measurement data is available.Bruce
----- Original Message -----From: Brad MorrisSent: Sunday, March 03, 2013 4:54 PMSubject: [NavList 22598] Re: Re: Re: Measuring (and Calculating) Dip
+/- 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!)
BradOn Mar 3, 2013 1:46 PM, "Bruce J. Pennino" <bpennino.ce---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---netBruce
----- Original Message -----From: Örjan SandströmSent: Sunday, March 03, 2013 10:57 AMSubject: [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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