# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**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"To: 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 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.