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GeoffK 9 Nov 2009 19:43

Celestial navigation
 
Does anyone out there still use a theodolite or sextant to keep themselves found?

Does anyone pack a sextant/theodolite as a back-up should the GPS fail?

Geoffrey

Threewheelbonnie 10 Nov 2009 08:32

I'm sure someone will, but to be honest it'll be a bit of a hobby. I have a sun compass and have used various methods to find position in addition, but GPS is now so reliable and honestly cheap you'd be better off carrying a spare GPS for any practical purpose. Now a compass and map, that's a different story, the map is a lot easier format than most GPS units and the compass takes up zero space. If you knew where you were a few hours ago and roughly which direction you were going, you'd need to be a long way from anywhere for a simple compass bearing not to bring you to safety.

An interesting subject though, astronomy to a practical purpose.

Andy

GeoffK 10 Nov 2009 19:28

CN - hobby? Old technology? Or still worthwhile?
 
I would agree that using a compass and map to keep track of progress - rather than just following the arrow on the GPS as it points to the next waypoint - adds much to the experience of travelling through a strange land. Maps are what we start with when planning a route and using a map to chart progress gives one a sense of relationship and contact to the land in a way that "passing through" using a GPS does not.

Using a sextant or theodolite to provide an astro-fix to check the dead reckoning requires a skill of a different order which, with the cheapness and reliability of GPS receivers, is now probably redundant. Once again though, I would argue that as the accuracy of the CN fix will only be of the order of a kilometre or so at best, so figuring out just exactly where you are on the map within that kilometre does make you look at the land around you in a much more objective and intense way and your memory of it will that much better. And, it does force you to at least look at the sky in an informed way - and the night sky in the Sahara desert is magnificent.

But there again, it was the advent of the GPS which really opened up the deep desert to tourist travel. Before 1990, a sextant/theodolite was a necessary piece of kit, along with the skill to use it. Now....? Well, suffice to say that the prospect not making it back to the black top road is much reduced on many levels.

I wondered if there was many others out there who enjoyed using the old methods to stay found.

Geoffrey

Threewheelbonnie 10 Nov 2009 19:45

Quote:

Originally Posted by GeoffK (Post 263541)
I wondered if there was many others out there who enjoyed using the old methods to stay found.

Enjoy yes. Would bet my last litre of water against very under developed skills, no.

I think the time I was 170-odd degrees out (the lack of Indian Ocean was a clue that there was a plus that should have been a minus or a cosine that wasn't) convinced me of that :(

Is it cricket to use a calculator though? Two digital watches and a casio scientific and you've still got more electronic brain power than you need for a moon shot. It's tables and a pencil if you want to do it properly. ?c?

Andy

lambchop 11 Nov 2009 06:04

I have no first hand experience of this but I have been looking into celestial navigation as an altenative to GPS. It is my understanding that to get the most accurate reading possible you need to have an up-to-date nautical almanac to list all the daily variations in sun and star positions (relative to us) due to semi-eliptical orbits etc.. This information is only published for maritime areas as far as I can tell. With the information in the nautical almanac it is possible to get your position to within 2 nautical miles.. Without it you would be doing well to get to within 10.. Thats pretty good when your at sea and looking for a land mass but not much use on land when, more than likely, you could be reading a map to provide much greater accuracy..

GeoffK 11 Nov 2009 06:14

Use a calculator? Oh sure! I carry a programmable calculator, but use a book almanac. I feed in the figures from the almanac, as the program requests, and the intercept distance and azimuth for the position line pop out at the finish. I could put an electronic almanac inside the calculator (as part of the program) but if the calculator should decide it has had enough of life, there is always the printed almanac from which I can do pencil-and-paper calculations. A printed book is almost indistructable. It can be chewed by dogs, pee'd on by camels, run over by Toyotas and even chucked on the fire (but not for too long) and it will still be usable.

Were you using a theodolite or sextant Andy?

Geoffrey

Threewheelbonnie 11 Nov 2009 08:04

Quote:

Originally Posted by GeoffK (Post 263595)
Were you using a theodolite or sextant Andy?

Geoffrey

Sun compass with lashed up means of measuring the shadows length. You know local sun time, Greenwich sun time from a watch, the height of the sun from the height of the gnomon the length of the shadow and simple trig and voila you have the means to calculate long and lat from the tables, so long as you can estimate a rough position and hence use the right bit. Take five readings over 10 minutes and it's accurate to a mile or so on a good day.

I tried adding a torch for use at night, but it doesn't seem to work too well!

Andy

Caminando 11 Nov 2009 13:03

Quote:

Originally Posted by GeoffK (Post 263404)
Does anyone out there still use a theodolite or sextant to keep themselves found?

Does anyone pack a sextant/theodolite as a back-up should the GPS fail?

Geoffrey

I assume you don't wear the Tshirt which says "I don't know where I am, but I'm not lost"? :mchappy:

Your sextant would certainly cause a stir if you hauled it out of your panniers (or would you bar mount it?) and began to take sightings. You'd have to ignore the Cap'n Birdseye jokes...

grizzly7 11 Nov 2009 14:32

Can anyone tell me specifically what tables would be relevant for North Africa? I've looked at various chandelry sites but I can't find one that says suitable for Mali ;)

GeoffK 11 Nov 2009 21:37

"Lambchop" and "Grizzly7" ... Almanacs for celestial navigation purposes give the positions in the sky of a selection of the brighter celestial bodies (sun, moon, visible planets, the brighter stars). The Nautical Almanac - despite its title - can be used anywhere on earth, not just at sea. Most (if not all) currently published almanacs for navigational purposes also include a set of 'sight reduction tables' which enable you to turn an observation of celestial body(s) into a position on a map. So - you also need a map, marked with lines of latitude and longitude, on which to plot your position. In the Sahara, I use the 1:200,000 Soviet World Series maps of the area.

I generally use an American A-12 bubble sextant, made for aircraft navigation in WWII, for measuring the altitudes of celestial bodies. It is compact and much easier to stow than a theodolite. I can generally get a fix within 3 or 4 kilometres of my true position (two nautical miles) which is quite good enough for most purposes in the Sahara desert, given that you can usually see that far and so identify a campsite, wadi or other geological feature which may require positional knowledge of greater accuracy.

Geoffrey

Calvin 12 Nov 2009 01:46

These 2 frenchmen may be of some interest to you, if you are interested in adventure travel!

Duckworks - The Atlantic with no compass and no watch!

Cal

grizzly7 12 Nov 2009 19:46

GeoffK thankyou for that, I almost purchased a second hand Davis sextant a few months ago but not knowing if a plastic one was essentially just for practice I didn't. At that price in retrospect I should have done!

Unfortunately I should have been more specific with my query, do nautical almanacs have the suns position suitable for a sun compass too? I'd really like a Wild T2E if anyone has one gathering dust :)

Threewheelbonnie 13 Nov 2009 12:52

Quote:

Originally Posted by grizzly7 (Post 263822)
, do nautical almanacs have the suns position suitable for a sun compass too? :)

Yes.

A sun compass is really only meant to allow a metal bodied vehicle to stay on course though. It's a side effect that you have a sun dial (for local time) and an accurately measured gnomon height and hence can get to long and lat. You are reliant on watching a shadow move 5 minutes on a 24 hour=360 degree scale and measuring a meter long shadow with a steel tape. Compared to a sextant or theodolite, a sun compass used in this way is a bit like using half a brick instead of a hammer, fine if you've got the means to sort the mistakes.

Andy

grizzly7 13 Nov 2009 14:25

Cheers!
I have a Howard Mk II as yet unused sun compass for interest really, and am trying to suss how to use it.

One small point, you could use your brick, chocolate cake or anything else that came to hand to measure your meter long shadow, cos it'll be a meter long either way :smartass:

GeoffK 13 Nov 2009 19:52

Quote:

Originally Posted by grizzly7 (Post 263822)
GeoffK thankyou for that, I almost purchased a second hand Davis sextant a few months ago but not knowing if a plastic one was essentially just for practice I didn't. At that price in retrospect I should have done!

Unfortunately I should have been more specific with my query, do nautical almanacs have the suns position suitable for a sun compass too? I'd really like a Wild T2E if anyone has one gathering dust :)

Plastic Davis sextants are fine in temperate climates, but in the heat of the desert the index error will be uncertain to say the least. Left out in the sun, the thing may well become unusable. Too, with a marine sextant, you will need an artificial horizon of some sort. Then, when doing star sights, identifying the particular star you want to shoot can be a problem unless you pre-compute the altitude of the star in question and pre-set the index angle so that you can be reasonably sure of getting the star in the field of view of the sextant telescope - all of which adds to the time taken to do the round of sights.

A bubble sextant or a theodolite really is the way to go as then you have little trouble in acquiring the star directly. Unless you have a suitable filter to put over the object lens of the theodolite's telescope, you will be limited to star sights at night. (The LRDG used theodolites, but never took sun sights, confined themselves to star sights at night.) Then, you need to ensure the theodolite has some means of illuminating the cross hairs. Shining a flashlite somewhere around the telescope object lens so that scattered light inside the telescope tube shows up the cross hairs will do the job, but it is a regular pain in the bum doing this while looking through the telescope at the same time. A theodolite with internal illumination is much to be preferred.

I used a homemade sun compass on my last two trips into the Sahara. The answer to your question about whether a Nautical Almanac will give you all the necessary information to use a sun compass, is yes. You will need the azimuth of the sun at any given time so that the shadow of the sun cast by the gnomen on the compass plate can be converted into a compass bearing.

Geoffrey


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