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    Re: Mary Blewitt vs M.J. Rantzen CORRECTION to correction
    From: Hewitt Schlereth
    Date: 2011 Mar 14, 10:29 -0400
    Gary, to make this clear to myself:  To any 218 Hc, I should:

    1) Add the NA refraction value for that Hc, (this means reversing the sign of the NA value); then:

    2) Apply the value for "Height in feet 0" as printed (i.e., -1') in Table VI in the back of 218 -  Adjustment to Refraction?

    Further, looking down the NA refraction table, it appears that for 218 altitudes above 45° 34' the net of these two steps goes from positive to negative for surface navigators.

    Example: For altitudes between 60° 26' and 65° 06', the NA refraction is -0.5'. Reversing the sign makes it +0.5'. Applying 218 Table  VI value of -1' would give a net correction of -0.5'.

    Have I got this right?

    Hewitt

    On Mon, Mar 14, 2011 at 1:37 AM, Gary LaPook <glapook@pacbell.net> wrote:

    Never mind. I sent the last message without access to the tables and I had forgotten that there was the expected differences between H.O. 249 and H.O. 218 tabulated Hcs.

    I did a more thorough comparison using H.O. 214 to look for the effect of rounding in both H.O. 218 and 249. Using all of the combinations for latitude 34 and declination of 19, I added the refraction correction from the Nautical Almanac to the values from H.O. 214 (which which are tabulated to one-tenth of a minute of arc) and found for all cases with Hc greater than 13 that they then totaled to the Hcs tabulated in H.O. 218 as expected. For Hcs of 10 to 13 adding the additional one minute of arc as required in the auxiliary table produced the correct value.

    So it appears that H.O. 218, when the refraction correction is applied properly, produces the same computed altitudes as H.O. 249 which are also correct as rounded from H.O. 214.

    gl
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