A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
CEKCTAH CHO-T [Re: lack of manufacturer's non-adjustable error info...]
From: Jared Sherman
Date: 2003 Oct 4, 22:51 -0400
From: Jared Sherman
Date: 2003 Oct 4, 22:51 -0400
Courtney- The vagarities of email sometimes make "reply" go to the sender, other times to the list as a whole. So in the interest of getting you the best information from the most sources, I'm posting a reply back to you here. <
> Russian Navy surplus sextants have been very common in the west in recent years, perhaps one of the list members has the information you need for this model. < > The inherent inaccuracies in machining the arc or scale calibrations would be undetectable to the eye. << I'm currently in Atlanta and my boat in Oriental, NC. Given that, where would be the nearest convenient benchmarks suitable >> I'm not sure where I found the URL, if no one else has it you could start with a search on "USGS" or "US Geological Survey Benchmarks" and drill down through their sites. From one of them, I had found a page dealing with matters of survey benchmarks and from there, a list which itemizes every benchmark on every USGS topological survey chart. They provide the lattitude and longitude, the benchmark number, the year of survey, and a short description which will tell you exactly what and where the benchmark is. These are often small (4"?) granite blocks, or concrete blocks, embedded in the ground with a bronze (2"?) circle in them that is graven with the benchmark information, so they are easy to spot in the field. Sometimes the text will say something like "North east corner of brick building..." or "center peak of roof" and those can be of mixed use. On most topo maps you will see a + type of mark indicating a benchmark's mapped position, so you can start with something like the www.maptech.com online map server. Drill down to the topo for your area, look for convenient benchmarks on it, and then try to locate them on the web page, which can be searched by lat/lon and other criteria. I know, I SHOULD have that URL benchmarked...The USGS is also very responsive to email so if you can't find it, ask them directly. < > Not that it is unsuitable at all, especially these days with degradation disabled! I just wanted to eliminate or cross-check the position by going to a source list. In theory the GPS position on the ground should match the survey position within 20 feet. But since my GPS position was differing from the position shown on the topo charts, I wanted a third reference to check with. I thought, who needs GPS when there's a convenient chart and a building to use as a mark...and that's where I found the local latitude and longitude had moved by 1/3 of a mile in between two surveys. Not knowing which one was the base for the topos I found online, I went to the source. That's a bad flaw in using the Maptech online map servers, by the way. There is *no* way to confirm which maps they are using, or charts, so there is no way to confirm which base (WGS 87, etc.) applies to anything you view on their server. Ooops. < > I don't know that you can differentiate the two readily. Let's suppose the instrument is made with +-20" of error throughout the arc, like Celestaire's new Chinese models are. Take your sights, reduce them by the means of your choice. let's suppose that position comes up 1-1/2 miles off from reality. Now run a couple of the sight reductions again, using an observation that is +20" off, and then -20" off. You may find that the +-20" change reflects in a 2 mile change in your position...If it brackets reality by that much in either direction, you might assume that the "instrument+observer" has less of an overall error than 20". But if your reading, regardless of the +-20", is still five miles away from reality, then you can assume the greater error is coming from something you are doing--not from the instrument's arc error. I know that's still vague, but I'm not sure you can refine it any further without having someone else, or another instrument, to compare things with. If you ask around the docks, or simply stay in open sight with the sextant visible for a while, you may find other local navigators will find you and help out with that. < > That comes back to my tongue-in-cheek replies to Dan's questions about a basic FAQ. If the instrument is built comparable to the Aries/Astra type, which I expect it may be, that should put the instrument capable of placing you within a fairly small circle. (Perhaps two miles.) That's plenty good enough to take a small boat around the world, given the three+ mile visible horizon down "at" sea level. Arguably all you will get, because of a small boat tossing, local horizon distortions, and other factors. On the other hand, there are people on this list who would tell you that's grossly unsatisfactory and they can easily get 1/4 mile of accuracy. With better eyes, larger craft, and more experience.