M73 Prismatic Compass

[SCOMAN]

I seen a thread on here recently regarding the M73 Prismatic Compass, how nice they were and how expensive they were. I had one of these quite a few years ago I got for a song but it was in graduated in Militaires. I got it converted by a company called SIRS navigation to degrees and despite it's weight used it quite often. Regrettably I had to sell it to raise some cash but the thread on here raised my interest in them again. I bought one quite cheap of Ebay, again it was calibrated in Mils and had a bubble. SIRS no longer maintain the compasses but they did give me a number of a guy who could help. So at the minute my compass is winging it's way to the Kent area and the guy estimated the conversion to be approx £70. If so it's going to work out to be £130 for a fully working Stanley M73 Compass which is considerably cheaper than anywhere else. It came in a padded '58 webbing pouch but I'm looking at getting a frag grenade pouch from somewhere putting a bit of padding in it and hey presto I have a secure belt stowage for it. I'll see how my conversion goes and if anyone is interested I'll pass the info on.

[Sniper]

Why on earth do you want to downgrade it's accuracy by converting to degrees? The idea of these marching compasses were how accurate they are for moving about on foot and could acheive pinpoint accuracy in bearings down to a matter of yards instead of the traditional 100mtr you get with degrees. You only have 360 degrees to a circle whereas you have 6400 in mils therefore far better for bearings. Convert this to degrees and it becomes no better than the cheap plastic affairs you can buy anywhere for 3 - 4 quid instead of the original precision instrument worth in excess of £200, well done mate.

[Sniper]

Instead of wasting this instrument why not sell it to me for £70 and I'll send you a top of the range Silva compass in degrees. I hate to see great items of kit destroyed in this way.

[Phil562]

Tend to agree with Sniper on this one, why downgrade the dial from mils to degrees...

[DKW]

Why bother with mils, is my thought.

Lets face it guys, i have worked with mils on a battletank aswell as an infantry seargent, and used it every single day for orientation, fireguidance aswell as for sighting the gun in.

It is far more accurate, but who can honestly say they can hold a compass steady enough, for mils to be of civvy use? I for one surely can not, unless i am using one of those fancy 3-way gyrostabilized vehiclemounted sighting-compasses. And even then the vibration of the engine is sometimes enough to let the compass deviate to some degree.

As long as you're within 1-1½ degrees correct during a march using the 360 degree compass, youre on the right path, and will reach your destination nomatter how long your march is.
The danish issued compasses are also in mils, however on the backside, NOT the front wich you use when sighting and marching! The only thing we use it for, is when calling in for artillery-fire, and we are controlling the fire as observers. Otherwise there really is no point in using it, as stated above.

A question i do have though: Whats all the fuss about this sort of compass?
I have myself never found it practical, nor worthy of owning a spot in my inventory. Quite frankly i actually find that type of compass to be more of an annoyment, than of practical use.
Could i be wrong? Is it actually a bril. compass, i have just not managed to see it yet?

Tutor me, if you will, because i really would want to know what the fuss is all about.

[Sniper]

It's THE best precision compass ever, and the fact as you say DKW that it is used for fire control proves it's accuracy. It is primarily a marching compass though and if you have to find a particular track in a wood with many tracks the mils calibration is the only one capable. Don't get me wrong, you are correct in that a standard compass in degrees is plenty good enough for most people, especially in this country, I don't dispute that. But to take a Spitfire with Rolls-Royce Merlin engine and swap the engine for one out of a Piper Cherokee destroys the spitfire and degrades it to a much lesser aircraft. That is in essence what you would be doing by changing the compass here and in this manner. Oh it will work but why destroy a precision instrument, why not just buy the miriad of cheaper and less accurate compasses available. You would not buy a microscope when you just need a magnifying glass now would you?

[DKW]

I can't argue with that Sniper, apart from this little thing:

It's THE best precision compass ever

You state that you would give him a Silva aswell as the money for the m73, and thereby you stated that the silvas aren't as accurate as the m73 compass.

Now you did put this in, as if it proves the accuracy of that specific compass:

and the fact as you say DKW that it is used for fire control proves it's accuracy

Now the danish army-issued compass is, can you guess it?...................... A silva.

I believe the civvy name would be a silva ranger. We call them M83 compasses.

I have no trouble using the mils on a Silva for firecontrol. I do know however that you nomatter what you do cannot hold any compass steady enough for it to "justify" mils on it. Especially not for civilian use.
Think it over once more... Firecontrol is done firstly by giving a direction of observation, after the shot(-s) have landed, you start halfing your way in to the target. 200 meters left, 100 forward, fire..........100 meters right, 50 back, fire........ fire for effect....... you might even start on 500 meters, depending on what weapon you are controlling fire for.

No need to discuss military strategies, this was written only to prove a point.

A handheld compass simply isn't accurate enough to justify mils graduation on the scale. The primary source of error is the human holding it. Next on the line is the width of the magnetic needle. third is the graduation itself. Ill explain the last one now:

Yes. Mils are 6400 individual "marks". How many marks are on that compass again? Surely not all 6400 of them? (i honestly do not know this about the m73 compass)
On artilleryguns/sights, aswell as on tanks, there is a mils graduation sideways. This is mainly used to prepare fire on forehand. And it is great to do so. Just the sheer accuracy...........When you "zero in" through a VERY powerful optical system, on a target far, far away.

On artilleryvehicles this would be done in conjunction with other vehicles, AND with a very, very very accurate gps system. Sometimes in conjunction with an observer equipped with a strong optical system, and a good compass built specifically for the task. (as artys seldomly are placed in line of fire)

Handheld compasses are for footsoldiers, and when controlling fire, only for the crude and "from the hip" controlling where the suppression is more needed than the actual hit of a round.


But i will ask again, as i believe it got lost last, and probably will again:

What is the fuss with this specific type of compass about? What can it do that makes it so good?

[DKW]

Basically what i am trying to say is:
I seriously do not see the benefits of mils, if you are not looking through a gyrostabilized extremely powerful scope at the same time.

See. It took me some time, but i did manage to make a short version afterall

Now back to the: "whats good about the m73 compass, compared to say the silva ranger" (if that actually is its name)

[Jared]

Basically what i am trying to say is:
I seriously do not see the benefits of mils, if you are not looking through a gyrostabilized extremely powerful scope at the same time.

See. It took me some time, but i did manage to make a short version afterall

Now back to the: "whats good about the m73 compass, compared to say the silva ranger" (if that actually is its name)

Well it is kind of like why buy a Bugatti Veyron when a ordinary bog standard car does the same job.

The M73 (or SIRS G150) are I think acknowledged as one of the top compasses ever made. 5 Tritium vials so can use it in the dark. And lastly its British

[Sniper]

Absolutely nothing wrong with the Silva mate. I love them and have been using them for years, in fact I wouldn't go past them. Silva is a brilliant bit o kit, but the M73 is the top predator, ok if you can't hold it steady enough for an accurate bearing, and I have never found this, you could rest it on something to remove handshake in the same way you do with a camera. The point I am making is quite simply don't destroy or detract from a precision instrument to something of a lesser level. As I said it will do the job but it destroys the original. Don't take a Rolls Royce and swap the engine for a mini engine is what I'm saying, it will still get you there but not quite in the same fashion. I would love to own an M73, and I would use it and cherish it, might even wash and polish it on a Sunday afternoon, yes you can do the job perfectly well with a Silva or a Suunto and I would add you could do it with one of those Ranger plastic prismatics for about £2.00 at any market stall, but it's just not the same. Why would you buy a Silva instead of a £1.00 button compass, cos it's better right? Same as the M73 and the Silva, better made, better quality.

[DKW]

The M73 (or SIRS G150) are I think acknowledged as one of the top compasses ever made.

Must surely be because of the iconic appearance, i think.

5 Tritium vials so can use it in the dark

Good thing, i can relate to that

And lastly its British

Arh. At last dear matey... gnarrrrr...... Kind of like the thing brits have for british racing green then

[Jared]

Here's Kifaru's take on compasses... http://www.kifaru.net/compass1.htm

[Jared]

Must surely be because of the iconic appearance, i think.

Yeah, most definitely. Compared the US military compass it's a thing of beauty. Lots of lovely brass.

Look at

http://www.sirs.co.uk/g150spares.jpg

Proper bit of engineering, not some injected moulded piece of plastic

[DKW]

Well. Apart from the fancy lights (wich really are a very good thing)
i cannot tell wich one is the best. I have managed to find a little bit of info on the danish issued silva compass, however not a whole lot, i must say

http://www.wellingtonsurplus.com.au/...SS+-+MODEL+15T

I however have been braught up with one of these in my hand, and would never trade it out for anything else than that same model. Apart from when something better comes along


Brass is good, i do find it a bit stupid (in lack of a better word) that the m73 apparently lacks any ability to be used as a chart-plotter, to derive bearings, aswell as plot them.
It is mainly a sighting compass, and not much more.

The american version, however ugly, does have a proper flat side to put to use.

The silva 15 t we have issued here, has degrees on the front, mils on the back, a proper plate for plotting and deriving of bearings. It does have illuminative plates bevelled in, i however find them a bit difficult to read in full darknes, but then again: we do carry a monokular nightvision goggle, so problem solved professionally.

All in all i really only see one good thing in the m73: The tritium lights! (and the iconic awe of history and brass!!)
But then take the newer model, that has brass/aluminum, then half of the awe is suddenly dissapeared

Whatever works best for you, right.
I still stand by that mils are useless without a proper line of sight, magnification, and a rocksolid mount though.

[SCOMAN]

Why did I downgrade it to Mils and make it as accurate as a Silva. Well you hint on it in some of your later posts, it's a fantastic compass, accurate, brass and when you hold it you know it's a lovely bit of engineering. But when I go off walking do I need the accuracy provided by mils, in short no. I've done night navs across northern dartmoor with one of these, graduated to degrees, and been spot on at each waypoint. I will not be calling NGS or an airstrike so I don't need the accuracy of mils. When I'm out with friends walking or teaching my girlfriends kid about nav I can then talk about degrees with out explaining mils. Pretty much my choice chum, thanks. Of note by the way if you do want one get to e-bay and a guy is selling of 3 all spot on graduated to mils for £55 each, put in prismatic compass as your search.

[SCOMAN]

That should be downgrade it from Mils, doh

[Ben Trout]

Thanks for posting SCOMAN. I have one of these compasses and I looked into getting it converted a few years ago, with no success. Any chance of forwarding on details of who I need to speak to?

Mine is a Mils version. I learnt in degrees, whilst there are advantages to the Mils system I'm sticking with degrees. Much of my walking is leading groups of youngsters, so I don't want to make a stupid mistake by forgetting which system I'm using.

I'm intrigued by the discussion, so I've got the compass here. The sighting ring looks to be marked in 20 Mils (NATO) increments so 320 divisions, my Silva 4/54 sighter is in 1 degree increments, so 360 divisions. When marching on a bearing the Stanley probably has a bigger parallax error on the markings, so I would suggest that for walking high end Silvas are more accurate.

With no provision for a taking bearings from a map, I don't think I'd change to the Stanley as main navigation tool. It would be nice to actually use it though. The conversion looks to be a simple case of swapping the needle/scale card and bezel, so easy enough to reverse. Destroyed seems a bit harsh to me.

It is such a solid lump of compass that I'd like to have it as a user again. Such a waste of a great piece of kit for it to sit on my bedside table for evermore.

I'm not a military man but I am an engineer!

[Sniper]

philistines, antique wreckers
Seriously though they are great bits o kit, I suppose if you will use them it's better than having them lying in a cupboard somewhere, but yoos hae nae couth! LOL

[DKW]

I'm intrigued by the discussion, so I've got the compass here. The sighting ring looks to be marked in 20 Mils (NATO) increments so 320 divisions, my Silva 4/54 sighter is in 1 degree increments, so 360 divisions.

Nice info, and it was exactly what i would have thought..

Have just been looking at my Issued Silva T15. It is in 50 mils increments, so 128 divisions. Allthough its easy to do halfs, and ending up on 25 mils increments and 256 divisions.
Remembering that the mils are on the backside of the compasshouse, i flip it over and see the degrees. 360 degrees in 2 degree increments. thats 180 divisions. Easy enough to work in halfs, so obviously this also has 360 divisions.

I have a Silva sighting compass (don't know the name of the darn thing.)
Its also in degrees and in mils. Funny thing here though is, that it has 1280 divisions on the mils (5 mils increments), and 1440 divisions on the degrees. (quarter degree increments)

With no provision for a taking bearings from a map, I don't think I'd change to the Stanley as main navigation tool..

And i would say that that would be the biggest drawback.

I'm not a military man but I am an engineer!

Close enough

Tells me you respect the solid piece of equipment the m73 apaprently is.

Thanks for the numbers on the compass, i was hoping to get info like that

[charlesr]

Wow I just read this complete thread and was intrigued from start to end but at the finish i am still as ignorant about what youse are talking about as ever makes me want to pull out the books on navigation which in my younger days i was actually good at lol.

[Mr Adoby]

The main reason to use a compass with mils is that this approximates the 2 pi radians used in geometry. The circumference of a circle, remember?

http://en.wikipedia.org/wiki/Circumference

And the anglular readings in radians can be directly used for different navigational and distance estimations and calculations in your head, without the need to convert from different ways of measuring angles. This could be used to very quickly direct fire, for instance.

http://en.wikipedia.org/wiki/Radian
http://en.wikipedia.org/wiki/Angular_mil

But if all you need is to find your way through the woods, then I find it hard to see how it could matter if you use 360, 400, 3600 or 6400 degrees.

[malcolmc]

I’m a bit uneasy about some of the comments on accuracy of compasses in this thread. My perception is that, for a plain scale, accuracy is a function of the size of the scale irrespective of units; a 3” (75 mm) protractor is going to be less accurate than a 6” (150 mm) one. With compasses there are additional factors such as declination and local magnetic variation. Any time I’ve used a pocket compass to take bearings to establish my position there has always been an error of around a couple of degrees, it’s why 3 bearings taken from a fixed point never cross at a point. It’s not a problem so long as the error is recognised and taken into account, such as aiming off the feature you’re travelling to. Military types need mils scales for their own purposes (mind you, the though of using a pocket compass for gun laying really does scare me!) but civilians use civilian maps and they are marked in degrees - it makes very good sense for a civi to use a compass calibrated in degrees. To do otherwise increases your workload, probably fine in good conditions when you’re not tired, but not ideal at the end of the day when it's starting to rain.

Just my thoughts on the matter.

[DKW]

I’m a bit uneasy about some of the comments on accuracy of compasses in this thread. My perception is that, for a plain scale, accuracy is a function of the size of the scale irrespective of units; a 3” (75 mm) protractor is going to be less accurate than a 6” (150 mm) one.

Exactly.

Military types need mils scales for their own purposes (mind you, the though of using a pocket compass for gun laying really does scare me!)

They don't. Gun laying is done by the scale onboard the gun. Huge diameter scale, compared to a handheld compass. Normally the outcome is doublechecked with all other guns in the group/battery, aswell as checked with an observer or a firemissionsvehicle. ALl of wich are equipped with very accurate compasses specifically designed for the job, aswell as armytype gps systems.(accurate!!)

The handheld compass is merely for the footsoldier to lay out a firingorder. To do so the battery needs an approximate direction of observation, in order to shoot the battery in on the target. (they basically need to know what is forward, and what is left and right, as seen from your place.)
No need for 1 mil accuracy for that

Just my thoughts on the matter.

Good thoughts. I agree

[malcolmc]

...
They don't. Gun laying is done by ...

Very pleased to learn that .

[DKW]

Very pleased to learn that .

Im pleased to set your mind at ease about it

If they did do it though, even i would curl up in my foxhole

[BOD]

...but civilians use civilian maps and they are marked in degrees - it makes very good sense for a civi to use a compass calibrated in degrees. To do otherwise increases your workload, probably fine in good conditions when you’re not tired, but not ideal at the end of the day when it's starting to rain.

Just my thoughts on the matter.

Many years ago in the SW of West Australia we were sent on an exercise. We camped the were told our objective. Maps provided were in degrees and we made our route cards accordingly.

Just before setting out the next morning, the instructors came and gave us mils compasses.

No idea why it was done other than to mess us up and increase the stress. Since we were already sleep deprived and poor with mental arithmetic, people were lost all over the place.

And then we were ambushed