# NavList:

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

**Re: Why no interpolation for Latitude/LHA in sight reduction tables?**

**From:**Greg R_

**Date:**2008 Jul 7, 22:26 -0700

Gary LaPook writes: > In fact, H.O. 214 did have tables for interpolation for LHA and a > procedure for adjustment for minutes of latitude. Aha... I've never used 214 so I wasn't aware that the SR tables had those at one time - wonder if they were dropped in an effort to "simplify" the tables when 249 was released? > 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.. Correct... but then again that's also the "normal" way to plot LOPs. I was thinking it might make for faster/more accurate plotting if everything were interpolated first and just plotted from the DR position vs. the (several) APs. -- GregR On Jun 29, 8:30 pm, "Gary J. LaPook"wrote: > 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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---