# NavList:

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

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Re: Why no interpolation for Latitude/LHA in sight reduction tables?
From: Thomas Kleemann
Date: 2008 Jul 08, 21:55 +0200

```It's a little late but I'll jump in here.

To achive a correction for the intercept from AP to DR
one can assume some simplification:

Because intercepts are usually short (less than 45 nm)
plane geometry is sufficiently applicable.

The following formula won't change the Azimuth or move your LOP at all,
only the intercept is corrected for the other position it starts from.

Delta I = Delta LHA * sinAz + Delta Lat * cosAz

It can be done by tables, a natural sine table has you left with two
multiplications, whereas a logarithmic sine table uses sums only.

Don't get confused by the term 'Delta LHA', thats the difference between
your longitudes AP vs DR, expressed by the minutes you round the LHA up
or down for AP. This way you spare one calculation.

/Thomas

P.S. I haven't looked after it, but, maybe the old traverse tables are
of some use here.

Greg R. schrieb:
> Here's something that I've wondered about for a while: Does anyone know
> why there aren't interpolation tables for Latitude and LHA in the sight
> reduction tables (as there are for Dec)? Seems like if there were, we
> could just use the DR position as the AP - which would also make for
> faster position-finding since there wouldn't be a need to plot
> individual APs for each of the sights.
>
> Might have something to do with the limited amount of computing power
> available when the tables were first created (as was discussed with the
> reason why Pub 229 is LHA-oriented), or maybe Latitude and LHA don't
> vary linearly between even degrees - but I was curious if anyone knew if
> there was some reason for it.
>
> --
> GregR
>
>
>
> >

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