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    Re: Horizontal distance off measurements
    From: Phil Guerra
    Date: 2003 Mar 24, 12:39 -0600

    I'm interested, so please share it.
    ----- Original Message -----
    From: "Royer, Doug" 
    Sent: Monday, March 24, 2003 12:15 PM
    Subject: Horizontal distance off measurements
    > On 03-17 @ 212624 hrs. Mr. Kenchington wrote that one cannot use sextant
    > horizontal angles to find distance off measurements or bearings.I must
    > differ with his opinion.Perhaps he did not understand  or misunderstood
    > I was trying to explain.
    > The following is a simple explaination of the proceedure taken from the
    > books "Elements of Navigation" pgs. 140-143 by W.J. Henderson and "Coastal
    > Pilloting"pgs.89-93 by Commadore R.P. Ferchew.Each book has multiple
    > examples of the proceedures ranging from a simple 90* angle solution to
    > complex solutions for acute and obtuse angles.
    > Using the simple example of one of the objects @ 90* from the observer and
    > table 31,Bowditch and a calculator I did the exercize.I then used a UTM
    > projection and using 10 digit coordinates measured the distance between
    > electrical towers.I then calculated the distance off 2 of the towers and
    > fixxed a position.To keep the evolution simple I kept the 3 objects as
    > to parrallel to my LOP as possible.Useing a handheld GPS I then went to
    > calculated position fixxed from the projection.Using a sextant I then took
    > the Horizontal angles of the towers.I also took the bearings of the
    > towers.After the calculations were complete I used a laser rangfinder to
    > measure each distance.The delta between the chart measurements,sextant
    > measurements and the laser measurements were 5 ft.on the longest leg
    > distance and 2* T on the bearings.To me that proves one can get accurate
    > distance off and bearing measurements from Horizontal sextant angles.
    > The example:
    >                                                   C                    A
    > B
    >                                                    .                     .
    > .
    >                                                                          O
    > Useing dividers measure the distance AB = 300yds. AC = 307 yds.
    > Useing the sextant find the angles AOB = 11* 32' 13". AOC = 11* 47' 33"
    > dist. off OB = 300yd./(sin 11*32' 13") = 1500 yds.
    > dist. off OC = 307 yd/(sin 11*47' 33") = 1502 yds.
    > Set dividers to 1500 yd. and swing an arc from B.Set the dividers to 1502
    > yd.and swing an arc from C.Where the 2 arcs intersect will be your
    > approximate position.Using parrallel rules find the bearings in * T of OB
    > and OC.Take the dividers and measure the line OA.This will be the distance
    > off A.One can calculate the dist. off A = 300 yd./(tan 11* 32' 13") =
    > yds.
    > A = 307 yd./(tan 11* 47' 33") = 1470.5 yds.
    > There is also a nice technique for finding the ballpark distance off an
    > object over terrain useing a mil type lensatic compass I have used with
    > results.If anyone is interested I will share it with you.

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