# NavList:

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

**Re: Graphical method by Favet / de 'Lisle**

**From:**Gary LaPook

**Date:**2011 Apr 29, 11:12 -0700

I am attaching an explanation of how to use the Rust table to determine azimuth.

EXPLANATION OF AZIMUTH RULES

In most situations there is no ambiguity as to which

quarter the Zn lies since you know the approximate direction you are

looking when you take the sight. The problem arises because the azimuth

angle is limited to the range of zero to 90 degrees and when the Zn is near

east or west the correct Zn might fall either

side of the line so there is an ambiguity in converting from azimuth

angle to Zn.

One easy rule to apply first is that if the declination is greater than

the latitude then the azimuth can never be in the opposite semicircle.

To generalize this rule, if the declination has

the same name as the latitude and the declination is greater than the

latitude, then you start with the direction of the elevated pole (the

nearer pole) when converting from azimuth angle to azimuth (Zn.)

The second rule to apply is that if the declination is contrary then the Zn

must be in the opposite semicircle. To generalize this rule, if the

declination and the latitude have contrary names then you start with the

direction of the depressed pole (the further pole) when converting from

azimuth angle to Zn.

These two rules take care of most of the cases, especially for

navigators in low latitudes.

The remaining ambiguity concerns situations in which the declination is

the same name as the latitude but is less than the latitude. In this

situation the azimuth of the body will be both north and south of the

east - west line during part of each day. In this case use the

auxiliary table to resolve this remaining ambiguity.

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You can also make a flat Bygrave to compute Hc and Zn or use the Bygrave formulas

on a calculator. All the information for making a flat Bygrave are here:

https://sites.google.com/site/fredienoonan/other-flight-navigation-information/modern-bygrave-slide-rule

gl

--- On Fri, 4/29/11, Gary LaPook <glapook---.net> wrote:

From: Gary LaPook <glapook---.net>

Subject: [NavList] Re: Graphical method by Favet / de 'Lisle

To: NavList@fer3.com

Date: Friday, April 29, 2011, 9:20 AM

On Oct 8, 1:42 pm, Gary LaPook < glap...---net> wrote:

> Gary LaPook writes:

>

> Attached is Rust's diagram for computing the azimuth of the sight which

> was included in Weems "Line Of Position Book," 1927. It was developed

> using the sine formula. You enter on the left with LHA and go to the

> right to the declination then straight up to the altitude then to the

> right to the azimuth. Also included is a table used to determine in

> which quadrant the azimuth falls when the body is close to east or west.

> This diagram can be used with any set of tables, H.O. 208, H.O. 211,

> etc., and the extra steps used in those tables to compute the azimuth

> can be disregarded.

>

> The shape of these curves show the potential loss of accuracy using the

> sine formula as the azimuth approaches 90º as the altitude curves become

> almost vertical and only slight changes in altitude results in large

> azimuth changes.

> --

>

> Rust diagram.pdf

> 206KDownload

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