# 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: Gary LaPook
Date: 2008 Jun 29, 21:30 -0600
Gary LaPook writes:

In fact, H.O. 214 did have tables for interpolation for LHA and a procedure for adjustment for minutes of latitude.  H. O. 214 predated H.O. 249 which copied 214's table arrangement, pages for latitude, columns for declination and rows for LHA. In addition to 249's "d" column of interpolation factors for declination, 214 had another column of "t" correction factors for LHA (angle "t"). If you look at the "d" correction values  in 229 and in 249 you will see that they are simply the number of minutes between consecutive declination values (columns in 249 and rows in 229) which allows you to do straight line interpolation for intermediate values. H.O 214's "t" values also tabulated the differences between successive LHA values to allow straight line interpolation also. (H.O. 214 used 100 units instead of 60 units for "d" and "t" values but the interpolation tables worked the same as in 249 and 229.) Since neither 229 nor 249 lists such "t" values it is not so convenient to do this interpolation but you can determine the proper "t" factors to use to interpolate for LHA by finding the difference between successive values of LHA (easy with 249 since it is the next row down, more inconvenient with 229 since you have to go to another page of LHA values) and then use the same interpolation table you use to interpolate for declination. H.O 214 then provided a table to correct for minutes of latitude based on the azimuth.

These adjustments can also be accomplished graphically just draw the LOP through the AP and measure the perpendicular distance form this LOP to the DR position..
gl

Greg R. wrote:
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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