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    Horizontal distance off measurements
    From: Doug Royer
    Date: 2003 Mar 24, 10:15 -0800

    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 what
    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 close
    to parrallel to my LOP as possible.Useing a handheld GPS I then went to the
    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") = 1469.7
    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 good
    results.If anyone is interested I will share it with you.
    
    
    

       
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