NavList:

Re: Abhav vs S-Tables
From: Axcel B.
Date: 2016 Sep 22, 07:59 -0700

Ok, so can I conclude from this that all the methods have the problem of becoming inaccurate for an LHA around 90° as they are all some kind of Ageton Tables? Except for Sadler?

Considering the accuracy and practicability, which table would you recommend? As I said, I read good reviews about Pepperday but apparently the values obtained from it are not that accurate in comparison to Ageton. But it might make up for that in ease of use?

Oh, and how about ABHAV? Ist that also a modified Ageton-Table?

I would appreciate a clarification/categorization of these tables to bring me up to speed...  :-)

Axcel

As near as I can figure ABHAV is not a derivative of  Ageton though it might be related.  I haven't looked closely but if it uses an AP instead of DR then is is probably not related.  I have no opinion on the statistics of how much its results deviate from the "true" values for altitude and azimuth.  PaulH may be interested in pursuing that.

Pepperday and Bayless have the same error rates and magnitude of errors, they are fundamentally the same tables with the same precision of tabulated values.  Pepperday does some things with table headings that require paying closer attention but also makes it less confusing to deal with values between 90^o and 270^o.  Pepperday's headings allow the use of LHA rather than t which some people may prefer.   Bayless includes the Sadler technique in the second edition which provides improved accuracy very near to 90^o.  There is no reason I can see that Sadler can't be used with the other tables.

I don't have the time this evening to go thru PaulH's posts, but if I remember and have the time this weekend I wil dig out the statistics for Ageton & Bayless/Pepperday so comparison can be made.

Ageton provides some accuracy improvement over Bayless and Pepperday at the expense of longer tables by tabulating with twice the precision.  In addition Ageton does not wrap the tables so they are 4 times as long, 36 pages instead of 9 pages.  It would trivial to produce a table that retains the precision and accuracy of Ageton but wraps the table so it is only 18 pages.

Ignoring accuracy the tradeoff between Ageton and Bayless/Pepperday, Ageton has larger table size (though not large compared to the most used methods) which results in a greater number of table openings to extract values but, B/P requires more attention to column headings and has greater potential for error.

Comparing A/B/P to HO29/249.  

ABP 4 big advantages

1]  MUCH smaller tables

2]  A single DR position can be used for all sights which eliminates some book openings when doing multiple bodies and makes plotting easier.

3]  If you make an error in the value lookups or the math it is much more likely to be apparent in the final results.

4] Because you are using DR instead of AP to plot from you can use a smaller plotting sheet.

HOs have 4 big advantages

1] Fewer  page openings to extract values.  Even with lookups for interpolation there are fewer book openings for a body.  I suspect the HOs still have an advantage for multiple stars but not as much.  Haven't counted.

2]  No regions with significantly degraded accuracy.

3]  Decreased likelihood of making an error.

4]  Wide spread adoption which makes it easier to get help.

As far as reccommendations go it depends on what you are looking for and whether you want to compare just ABP or ABP, 229 & 249.

If you are looking for backup navigation on a cruising sailboat then any one of them will do as all provide more accurate results than can generally be acheived with a sextant on a small boat.  As a backup you probably want to minimze the storage space required so 229 is out on those grounds.  I feel that the tradeoff between the increased likelihood of error with ABP is a wash with the increased likelihood of not realizing you've made the error with 249/229.  

To me the final decision comes down to whether you can live with the region of unaccepably degraded accuracy with ABP vs the full volume size of 249.

If you can get a round of 5 stars you can proably tell if there is an ABP accuracy problem.  But if you only get 1 to 3 bodies with 1 or more having t or K near 90^o can you live with the uncertainty or will that uncertainty gnaw at you to distraction.  That's a personal awareness issue you have to know about yourself.

If you want a small sight reduction system that is very easy to use look up the Brown-Nassau Navigation Computer.  It is probably the easiest system to learn and the fastest to use but result accuracy is about the same or not as good as what you can expect from a sextant generally.  You can download the graph sheets from some earlier threads and then you would have to home brew the actual device.

Every system has a tradeoff.