# NavList:

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

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
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 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
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)