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    Re: Evaluation of H.O. 218
    From: Dave Walden
    Date: 2011 Mar 24, 21:00 -0700

    Attached are two spread sheets to generate HO218 tables. One does the star tables in the front. (In fact for any of the 57 navigations stars + Polaris). The other does the Dec tables from the back. Refraction is calculated using a two term tan ZD + tan ZD ^3 equation. The two coefficients are based on temperature pressure values from the 'standard' atmosphere at 5000 feet. (Method given in the spread sheet.) The Dec table seems to agree exactly down to altitudes of about 5 degrees or so, where it misses by 1. (I do only have 1 vol of 218, so limited checking.)


    The star tables take some more work, and I'm still playing. "t" values are calculated. (A version of the t interpolation table Gary gave is also included.) I seem to miss by 1 min every 10 or so. It seems 218 uses star data (RA, Dec, and PM) from the Boss General Catalog of 1937. I used modern values. I used modern precession values too. Since 218 is linear in time, it neglects nutation and annual aberration. Seems reasonable for "air" accuracy, since both are small with virtually no secular component.

    Interestingly, in using 214 to confirm, I find about 9 differences of .1 min in a single Dec column. (see spread sheet)

    ((Warning, an effort to amuse and entertain myself. Errors likely remain.))


    >Gary,
    >
    >I haven't had time to read all your posts on these tables, but since you've given >them a thorough review, could you rate them? Are they a B+ or an A, as tables go? >I haven't seen a complete set of them ever. Are they good enough for us to >consider re-generating them (easy enough today) and making them available for >small boat navigators and other navigation enthusiasts?
    >
    >-FER
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    File: 116008.glcaltho218.xls
    File: 116008.glho218.xls
       
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