# NavList:

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

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Re: H.O. 249: rounding Ho
From: Paul Hirose
Date: 2020 Mar 20, 11:43 -0700

```On 2020-03-18 7:02, Gary LaPook wrote:
> Your final result is only as accurate as your least accurate input. Since HO
249 only provides data to the integer minute it make no sense to combine it
with the more precise HO to a tenth of a minute. Just round the HO to the
nearest minute, combine with HC and plot the intercept to the whole nautical
mile (minute of arc.) If you want more precision in your celnav then you need
a more precise method of sight reduction, HO 229; HO 214; HO 208; HO 211; or
various computer apps.

I have a somewhat different attitude. The error of the final result is
the sum (not a simple arithmetical sum, however) of a multitude of
errors. These include uncertainties in dip and refraction, instrument
error, assumptions in the assumed position intercept method, rounding
error in the reduction table, and rounding error in the computations.

That last is under your control, and can be reduced practically as low
as desired. If observed altitude is rounded to the nearest minute, the
error thereby injected can reach a maximum 0.5′, and half the time it's
0.25′ or more.

That can be significant if other error sources have been carefully
minimized, the best accuracy is desired even at the cost of a little
extra paper and pencil work, and a more accurate reduction method is not
available.

increase in error for simplified computation, and work in whole minutes.

Regarding HO 211, my experiments found a 0.59′ root mean square altitude
error. To attain that accuracy it's necessary to interpolate the table
in some cases (less than 5%) where the mathematical leverage of the
method is weak.
```
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