Transit Compass

[Wayland]

I've just checked out a more up to date edition of my local 25k map, Mine was printed in 2002.

There is a 40' difference between the magnetic declination on the new map to the calculation based on annual change on the old one. that's a bit of a shock actually.

I've just recalculated my results using the new data and it puts the church back where it should be.

I think I'll be relying on the BGS for this data in the future

 

[Martyn]

I've just checked out a more up to date edition of my local 25k map, Mine was printed in 2002.

There is a 40' difference between the magnetic declination on the new map to the calculation based on annual change on the old one. that's a bit of a shock actually.

I've just recalculated my results using the new data and it puts the church back where it should be.

I think I'll be relying on the BGS for this data in the future

That is a lot, but maybe it's a sign of where we live, that we expect our maps to be accurate to within a few feet or less? Besides, even though 40' is a lot, churches are pretty big, you wouldn't miss it.

I love hyper-accurate instruments, but for general purpose use, we dont really have much of a need for them.

 

[EdS]

Just a thought - the florescent dye in the bubble level might well be flourescein: I use this to examine the eye for corneal abrasions. When doing this I use blue light, not UV. Might be worth trying.

fluroscein works well under UV light -- I use it by the big styel at work testing drainage systems. When the "water" is very murky UV light often shows it up when other lights fail.

 

[ged]

I've just checked out a more up to date edition of my local 25k map, Mine was printed in 2002.

There is a 40' difference between the magnetic declination on the new map to the calculation based on annual change on the old one. that's a bit of a shock actually.

That shouldn't have been a shock. OS maps show the approximate annual change in magnetic variation alongside the actual variation at the time of printing. On a pseudo-random Explorer Map (number 260 dated 2006 but variation figures given for July 2008) which happens to be on my coffee table the annual change in variation is given as 9'E. Than means in the eight years that gave you a change of 40', the change in Sherwood Forest would be nearer 72'. Of course the change isn't constant from year to year.

When I started Duke of Edinburgh's Award in 1967 the first thing they told us about maps was "A map is out of date as soon as it is printed."

 

[Wayland]

"2002 Declination 3°46' west, Annual change 13' east" 13' x 8 = 1° 44' east: Final result 2° 2' west.

As you can see, I had already calculated according to the map for the annual variation, but their formulae was still incorrect by 40' compared with the actual declination.

That is the inaccuracy that surprised me.

 

[ged]

I had already calculated according to the map for the annual variation, but their formulae was still incorrect by 40' compared with the actual declination.

That is the inaccuracy that surprised me.

I understand. As I said, it isn't a constant annual change. Maybe this will help.

 

[Wayland]

As with all estimates you expect a slight variation, I was just surprised at the size of it.

I have often used the map calculations without thinking much about it. I shall be less casual about it from now on.

 

[big_swede]

I hate you guys so much right now. Just bought the dampened version.

 

[Wayland]

Let me know how you get on with it.

 

[Laurentius]

My problem with old OS maps is that in the meantime between my being to mean to by a new one, someone has excavated a quarry across what used to be the path in my old map.

 

[Wayland]

I've been playing around with one of these to make up for the lack of a built in protractor.

I have to say that with the addition of a piece of thread from the centre hole these far out perform a baseplate compass because I can measure angles right across the page.

It's also just the right size to fit into the compass case with the transit.

 

[ged]

I haven't seen exactly that pattern of square protractor before, where did you find it? The closest I could find was the military protractor, but it doesn't seem to have a central hole:

http://www.survivaliq.com/navigation/direction_par5.htm
http://www.deltagearinc.com/Navigation/Protractors/SquareMilProtractor.htm

When it's used with a bit of string like that it's called a Sutherland Plotter:

http://www.starpath.com/catalog/accessories/1852.htm

That one and some others have the central hole, but they don't have the scales that yours has:

http://www.pooleys.com/prod_detail.cfm?product_id=98
http://www.thomasgunn.com/onlinesho...pLevelCategory=IN&CategoryTop=&ProductId=3848

You can draw a course line from this:

http://marinestore.co.uk/PL10560.html

Where the Bretton plotter just gets in the way:

http://chandlery.marinestore.co.uk/search/index?filters[category][]=Navigation+Instruments&filters[category][]=Navigation+Instruments

Some of them can get a bit silly:

http://www.sailgb.com/p/three_arm_protractor/

I've always preferred using just a couple of triangles:

http://marinestore.co.uk/MD290290.html

But still no central hole.

 

[Wayland]

There is a link on the word "these" above, but they are from an American company called "Maptools" that do a great range and will post to the UK.

 

[nickg]

Ordered mine on the 19th August - as of today its awaiting delivery of new stock

So folks dont rush to buy one just yet

Annoying as i wanted to try it out on a trip up the pen-y-fan this weekend

c'est la vie or as the french say thats life

cheers
Nick

 

[ged]

There is a link on the word "these" ...

:argh: I totally missed that. I've been thinking I ought to do something to make links like that more obvious because with my crappy laptop screen and my even crappier eyesight I often miss them, so I guess I just got some added impetus! Thanks. Did you get yours from America?

 

[Wayland]

I ordered mine directly from their site.

The postage was pretty cheap and fairly fast.

 

[johnboy]

I haven't seen exactly that pattern of square protractor before, where did you find it? The closest I could find was the military protractor, but it doesn't seem to have a central hole:

I made mine by doing a quick google search for 'Printable Map protractor' saved the file and then took it along to a copy store where they printed them off onto Acetate sheet and then laminated them..

I took along a 1:50K and 1:25K map to check the scaling was right after copying.

Result a pile of useable flexible protractors for $10.00.

Pop a hole in the middle add in string and you're sorted for most mortal navigation tasks..

They work well for entertainment value with the M73 I have and also were ok with the USGI M2 and USGI lenstatic I had...

The Square US Army style protractor is better to use then the RA type IMHO...

Have fun...

John

 

[jamesraykenney]

It just measures an angle, although as you've pointed out fairly accurately. A measurement gives you a position line, and like any other position line you need more than one line to get your position.

http://www.amazon.co.uk/Sextant-Handbook-Adjustment-Repair-History/dp/0070052190

The tricky bit isn't using it to make a measurement, but applying all the corrections* and doing the sums to get your position lines**.

* Instrument error, refraction, dip...
**Arcs of circles, which in the absence of errors intersect at your position, but if they really do intersect in practice then it's grounds for suspicion.

I wouldn't waste money on calibrating an antique. You can get perfectly acceptable results when you get to know the instrument and you're never going to do much better than a few miles unless you're on land.

He could make a cheap artificial horizon and learn to use it on land easily... A clue... You can use water instead of mercury for the liquid... Though I would LOVE to have one of the old ones with the mercury reservoir.
I remember when my father was taking Celestial Navagation school courses by mail. He got certified and taught Celestial Navigation classes for the Coast Guard Auxiliary.
I HOPE that those textbooks and workbooks did not get thrown out!
The thing is, that the easy way to do it requires the use of the Nautical Almanac for the current year.
If you want something a little more universal, you can get the Astronautical Almanac for the current decade and with a little spherical trigonometry, you can do the same thing...
If you have a programmable calculator you can do the equations quite easily though... Even if you do not know spherical trigonometry.

 

[jamesraykenney]

Well as an experiment I've just taken my first star shot of Polaris from my back yard.

It wasn't easy, in fact I had to make a slight estimation as I couldn't really see the star and illuminate the level at the same time.

Although the UV led light worked well enough I don't think my eye was dark adapted enough.

The reading I got from the scale was 53° or maybe just a smidge under.

My location here according to the 25K OS map on Memory Map is 53°34'8"N 2°5"02.1"W So roughly 39 miles out on a first try. Make of that what you will.

I have an idea that might improve that, but I'll have to try that another night.

You need a detailed star map of the north polar region of the sky... Polaris is NOT at North. It changes a little every year, but not much. The finder scope on my telescope has a circular grid that you use a circular slide rule device to calculate where in the eyepiece on the grid that you should see Polaris to polar align the telescope. You put in the time of day and the year and a little dot appears in a circular slot corresponding to the grid in the eyepiece. The year setting moves the dot inward and outward in the slot and the time and date moves it around the slot. Made polar alignment a lot easier.

 

[jamesraykenney]

Price wise the M88 can be found for about £200 and the Brunton Composite at about £170 at the time of writing.

And I thought my K&E(Keuffel and Esser) Pocket Transit for around $99.00 to $120.00 I thought the Brunton was better built but the K&E had the altitude scale vernier on the bubble arm tapered so that the scale had very little parallax error as the scale was only about a mm or less from the base scale. The Brunton had the edge of the scale something like 5mm above the base scale and you had to make sure that your eye was RIGHT in line with the center of the vernier.