# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding

**Position from intercept & azimuth by calculation.**

**From:**Sean C

**Date:**2012 Oct 2, 15:02 -0700

I was reading through the "Sight Reduction Procedures" chapter of the N.A. today and I came upon the section dealing with how to calculate the position of a fix without a chart. The method is straightforward and I believe I have a firm grasp of it. I was puzzled by one thing, though.

In the preceding section ("Position from intercept and azimuth using a chart.") it is explained that, "The position line of an observation is plotted on a chart using the intercept and azimuth with origin at the calculated position (Long, Lat) at the time of observation..."(p.282) This "calculated position" is determined using the fomulae:

Long = Lf + t(V/60)sin T/cos Bf

Lat = Bf + t(V/60)cos T

...where t = the time interval in hours from the time of fix to the time of observation, V = speed, T = course, Lf = Longitude of fix and Bf = Latitude of fix.

Fair enough. I get that. However, in the following section, dealing with calculating the position directly, it states, "...an improved estimate of the position at the time of fix (L1,B1) is given by

L1 = Lf + (AE-BD)/(Gcos Bf)

B1 = Bf + (CD-BE)/G"

(p.282)

Also fine. But a few paragraphs later, when suggesting that running through a second iteration will yield a better fix, it mentions that, "In each iteration the quantities Ho, GHA, Dec, and t do not change."(p.283)

Now, I understand that Ho, GHA and Dec are needed to find Hc, Z and "p" (intercept). But since L1,B1 is calculated directly from Lf,Bf, and "t" is not used in this particular equation, then why even mention it at all?

Thanks in advance for any insight.

Regards,

Sean C

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