COMPASSES

[FALPhil]

Dimitri wrote:

Bushy, Don't you also got to figure out longditidue that isn't done with the sextant as that figures lattitude ?? Dimitri

Actually, what a sextant gives you, once you do the math, is a line of position (LOP) based on spherical trigonometry. Plotted with a straightedge on a chart, it is actually a chord of a circle which represents all the places on the surface of the earth that you can measure a particular angle from the horizon (corrected for sea level) of a particular celestial body (sun, moon, star, planet). Now, if that celestial body is due north or due south, such as Local Apparant Noon (LAN), latitude can be derived. Angular altitude of the Pole Star generally translates directly to Latitude with minor corrections. Azimuth assists in refining the LOP.

When you have two LOPs that cross, you have determined your position. Depending on your plotting chart, this could be latitude and longitude, but other systems could be used. That means you have to measure the angle between the horizon and two celestial bodies. The angles must be corrected for time of day, altitude above sea level, and various and sundry factors, which are very predictable. We always shot a minimum of three LOPs for accuracy sake.

In the old days, the US Hydrographic Office used to publish volumes of lookup tables to assist in celestial navigation with a sextant. These days, if you really want to use a sextant, you can download programs into your PDA to compute everything.

[Dimitri]

Oh neat Phil,

I just know of John Harrison's H4 watch to calculate longitude at sea by comparing local time with Greenwich or "Zulu" time, which should have won the prize outright but he never did get it.

Dimitri

[Bushmaster]

Thanks FALPhil...Saved me a lot of remembering not to mention, a lot of typing. I haven't used a sextant for so many years I doubt that I could do it anymore...

Come on Big D...Yer fallin' behind here...(2721/2744)

[Dimitri]

Sorry Bushy, come November your going to have even a bigger lead on me as I'm going to be gone for the 2nd week of November and unless I find a way to get Internet access on my little trip I'm going to fall back even farther.

Dimitri

[watchmaker]

Nobody is pulling any legs, the compass needle is adjusted for any of the 5 zones of dip in the world, a compass bought in Australia will stick to the card in Canada, because the balancing of the needle was done for zone 5 and Canada is zone one.

This post of mine will explain what dip is.

COMPASS DIP

This will be of interest only to the people that travel to locations around the world, and are in need of using a compass to get their bearings.
I first encountered compass dip, many years ago, in a trip to Bariloche, Argentina. I was there to fly-fish for trout in the Nahuel Huapi Lake and climb the Cerros Tronador, Catedral and Lopez.
I took my regular base plate compass, which I had used extensively in North America, and I found there that the needle was sticking to the card providing false readings.
I was baffled until my guide explained that most climbers from the states had compasses that stick and that I needed a compass with the needle balanced for the area. In my compass, the pull from the forces of the magnetic north made the south end of the needle dip and stick to the card.
I found later that when compasses are made, they are balanced for the zone that they are going to be sold, and that the manufacturers have indentified 5 zones of dip.

MAP OF THE FIVE ZONES





Compasses sold in North America are adjusted for the zone one, and where I was in Argentina was considered zone four. Compasses sold over there by the sporting good stores where adjusted for that zone.
If you have opportunity to travel to Australia, you will be in zone five and the dip of the needle will be even more pronounced.

ENTER THE GLOBAL NEEDLE

Suunto has come out with a couple of traveler’s compasses with a global needle. Brunton has at least one in their line and maybe other manufacturers are doing the same.
This is a needle that is optimized to be use in all places (that is why they are called Global compasses). Brunton is making the 8096 AR (a racing compass) with the global needle, and it makes sense as the runners don’t have to stop and level the compass perfectly to take readings as the global needle can work with even a 20 degree tilt.

Climbers can benefit from a global needle as they have more latitude to take a reading from a peak that is too close, as sometimes bearings have to be taken using the imaginary center line of a mountain instead of a peak when using the regular compasses, as the tilt upward will ground the regular needle. With the global needle the chances that you can still use the peak for your target are increased if the angle is less than 20 degrees.

So we should welcome the development of the Global needle and hope that more choices will be made available in the different models of compasses.
All the best
Watchmaker

[FALPhil]

Bushmaster wrote:

Thanks FALPhil...Saved me a lot of remembering not to mention, a lot of typing. I haven't used a sextant for so many years I doubt that I could do it anymore...

I'll tell ya, Bushy, with a sextant, a good automatic wrist chronometer, and H.O. No. 9, I would feel comfortable navigating around the world. I loved it. I'd probably still be doing it if Jimmy Carter had never been elected. The U.S. Merchant Marine went down the tubes on his watch.

[twofifty]

Thanks for your research Watchmaker.

[sniper]

Dimitri wrote:

Vince, With me if something has batteries they will fail when I need them even if they are new! Murphy really hates me when it comes to batteries, Dimitri

Yeah that, Dimitri! Just one more thing to fail.

When I lived in Eurpoe in the early 60s, the Swiss army hd a compass that was really neat! About the size of a matchbox, and it slid in and out of an attached cover. a'la matchbox. I always wanted one, but they were really expensive, and my old Silva seems good enough to get confused by.

[Dimitri]

Sniper,

Check this out:

www.thecompassstore.com/51dp65.html



Its a "Global" compass too!.

Dimitri

[twofifty]

A Swiss company, Recta, makes a 'matchbox' compass almost identical to this Suunto you found, with the option of having degrees, radians, or mills; one model is 'global'. Had a chance to handle one indoors at a trade show and liked the quality.

See www2.recta.ch/FMPro?-d...ge=e&-find

What I did not like so much is the small size of the graduation (degree) marks on the bezel, which made it harder to read for my eyes. I also found that the open compass is quite short which makes it harder to get map bearings, compared to the long baseplate found on the Type 15 Sylva or similarly built Suunto.

[Dimitri]

Twofifty,

I think ether Suunto or Recta out source the manufacture to another or simply both have the rights to make it the same for one reason or another (perhaps a common goverment contract ?? ), if you look carefully there the same compass and the companies both list it as model "DP 65".

Dimitri

[sniper]

Thanks, Dimitri. I had no idea thery were still making those things.

[watchmaker]

SILVA RANGER 515 CL COMPASS

A mirror sighting compass that I use when I want the field reading to be as accurate as possible. The mirror can be tilted 45 degrees to observe the needle for proper positioning on the gate at the same time that a bearing is taken with the sight.

With scales of 1:24,000, 1:25,000, and 1:50,000 , the Silva Ranger 515 CL sighting compass makes plotting easy no matter which type of topographic map you're using.

Other versatile compass features include a 0-360 degree bezel with 2 degree increments, clinometer, 1/20-inch scale, millimeter rules, silicone feet for map gripping, adjustable declination, sighting mirror with vee notch, and lanyard with adjustable slider.

Dimensions: 4" x 2-1/2"
Overall Weight: 2.3 oz.



Price of the compass is about $60 USD it is one of the top notch compasses for map work.

Suunto also have a similar model, and Silva an others also have smaller mirror compasses without declination scale or clinometers



Best
Watchmaker

[Elvis]

Heres my 5cents worth. Ive always carried a silva brand compass when hunting and find the best thing is to set the "wobbly" before leaving camp/vechile so if I plan to hunt in a say mainly westerly way then my compass will be pointing east when its time to come back later. to do this you need a backstop feature eg a road \river.to catch you on return heading then you only have to options up or down stream etc. My good wife bought me a garmin etrex gps this year ands its great. I went to the other end of country to hunt this year .set the hut as waypoint and never felt even slightly challenged as to how to get back. was even better to put in clearings in bush crossings that deer were using and wallows. when going back into the area days later ask gps where they were and slow down when getting close. great tool.

[whittling]

I use just a map .. print one off the internet ... use a Map to find where I am...
OK ..survival experts HERE are the facts ..Moss grow THICKEST on the north side of trees That are out of the sun ... the top of tallest pine trees point south or southwest usually ...