Author Topic: Types of compass  (Read 356 times)

GoneWest

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Types of compass
« on: 18:45:05, 09/09/20 »
There have been quite a few topics on compasses but not much has been said about the different types. Here's my take on them:-

The baseplate compass is a "standard" for walkers and other land navigators, when used in conjnction with a good map. Its good points are:

1) It's very light and makes a very good portable geometry set for map work, assuming that there aren't significant alignment errors in manufacture (paying a high pricce is no guarantee of this, I've found).

2) It's "set and forget". Apart from allowing for Grid Magnetic Angle (GMA, often erroneously referred to as "declination" - see my other post(s) about that), you just set the bezel on the map and follow the heading obtained. You don't ever even have to know the actual bearing, unless sharing it with others. Some compasses even allow you to preset the GMA so that you don't ever have to bother with actual numbers of degrees (or mils) while on the move.

3) It's a tolerably good marching compass - easy to use and quite good enough to keep a course to about 5 degrees, which is realistic.

The baseplate compass is really poor, though, if you want a reasonably accurate EP or fix (I'm thinking backbearings and sighting maybe distant landmarks). Specifically, you can't sight the target (at eye level) and set the bezel at the same time. You have to move the compass, without rotating it, to do each thing, or you have to blue-tack the aligned compass to a stable level surface (if you can find or make one above ground level) and move your viewpoint.

The mirror compass solves the major part of the sighting problem by allowing you to see the target and the compass arrow at the same time. However, they are in different focal planes and it's still difficult to manipulate the bezel while trying to maintain a steady sighting.

The types of compass above fulfil most walkers' needs most of the time but if you care about having the right tool for the job, then the baseplate compass and its "mirror" version don't really cut it when it comes to sighting objects. The prismatic compass is the longstanding favourite sighting compass for perfectionists. Unfortunately, this type of compass has fallen out of favour for use specifically on land. Decent models are either antiques (very expensive) or high-grade surveyors' instruments (even more expensive). Fortunately, they are still widely available for marine use and work just as well on land! I recently bought a Plastimo one for about 50. OK, even that price is a bit self-indulgent! Unlike normal walkers' compasses, a prismatic compass does not have a bezel. Instead, the compass needle carries with it the entire circular, calibrated compass "card". The compass bearing is read directly and there is no setting to do by hand. Thus, such a compass can be described as "read and remember", rather than "set and forget". The really important thing about a prismatic compass is that it allows you to view the target and the bearing together, with both simultaneously in focus.

The ultimate prismatic compass comes combined with binoculars but, for most of us, that's overkill on weight and/or cost grounds.

« Last Edit: 18:48:37, 09/09/20 by GoneWest »

archaeoroutes

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Re: Types of compass
« Reply #1 on: 06:59:40, 10/09/20 »
It is incredibly rare that taking a sighting is needed. I mean, vanishingly rare. You'd be having to fix a position in a featureless plain surrounded by visible distinctive features to need and be able to do it.
Certainly in Britain, it has very little use. If nothing else, when you can see well enough to use it, you don't need it!
I am talking about walking. Surveying is different, but then I'd assume theodelite etc. And of course, laying in guns...
Walking routes visiting ancient sites in Britain's uplands: http://www.archaeoroutes.co.uk

richardh1905

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Re: Types of compass
« Reply #2 on: 07:36:47, 10/09/20 »
You forgot about the compass (and GPS based navigational system) built into most people's mobile phones, Gone West.
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GoneWest

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Re: Types of compass
« Reply #3 on: 09:48:19, 10/09/20 »
It is incredibly rare that taking a sighting is needed. I mean, vanishingly rare. You'd be having to fix a position in a featureless plain surrounded by visible distinctive features to need and be able to do it.
Certainly in Britain, it has very little use. If nothing else, when you can see well enough to use it, you don't need it!
I am talking about walking. Surveying is different, but then I'd assume theodelite etc. And of course, laying in guns...


I have to agree with you, on the whole. Like everyone else, I look to GPS (or whatever other GNSS system my device decides upon)first when I want to know my postion, and that works pretty well on the featureless plain! Dense woodland, though, is a place where it may not work well, and that's where compass back-bearings would be most useful. Admittedly, the targets for these tend to be quite close, but accuracy still counts, to minimise cumulative errors along the route.

To be honest, my humble mirror compass is my fallback, in case of loss of GPS (or the power to it). I bought the Plastimo prismatic partly out of nostalgia for my sailing days but mainly because I do, indeed, indulge in some amateur surveying, sometimes, when out walking. If, for example, I want to ascertain the position of the latest wind turbine to have popped up in the locality - and possibly beat OS to it - I am unlikely to be able to stroll right up to it with my GPS so triangulation is the only option. However, I draw the line at lugging round a theodolite; I can't afford one, anyway. ;)

GoneWest

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Re: Types of compass
« Reply #4 on: 10:00:47, 10/09/20 »
You forgot about the compass (and GPS based navigational system) built into most people's mobile phones, Gone West.


I did, indeed forget that one, mainly because I haven't ever had a compass in any of my (PAYG) phones. Expensive phones have compasses but whether "most people" have these (either on subscription or bought outright), I wouldn't know. :-\

A three-axis magnetometer is definitely the way to go, though, for an accurate all-purpose compass. However it is likely that the instrument implementing it will also be providing GNSS capability, as you point out, making the compass, for the most part nice-to-have, rather than essential.

Has anyone come up yet, I wonder, with a phone app that exploits both the magnetometer and the accelerometer to provide a complete inertial navigation system? Could be useful in jungles, caves and mines. 8)

ninthace

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Re: Types of compass
« Reply #5 on: 14:33:52, 10/09/20 »
Has anyone come up yet, I wonder, with a phone app that exploits both the magnetometer and the accelerometer to provide a complete inertial navigation system? Could be useful in jungles, caves and mines. 8)
Why would you need a magnetometer and an accelerometer?  AFAIK modern inertial systems just use accelerometers, possibly combined with a miniature quartz "gyroscope" for pitch roll and yaw.  Mind you it is has been a while - things may have changed.

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GoneWest

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Re: Types of compass
« Reply #6 on: 17:19:27, 10/09/20 »
Why would you need a magnetometer and an accelerometer?  AFAIK modern inertial systems just use accelerometers, possibly combined with a miniature quartz "gyroscope" for pitch roll and yaw.  Mind you it is has been a while - things may have changed.


Of course, you are correct! What was I thinking? ::)

archaeoroutes

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Re: Types of compass
« Reply #7 on: 18:39:41, 11/09/20 »
Like everyone else, I look to GPS
Well, not everyone. I use satnav when driving because I can't look properly at the map. Other than that, I rarely use it at all. I have it in case, but I don't personally see the point in it as a primary navigation tool for walking in Britain.


When out walking, I tend not to lose contact with the map so I don't have to relocate anyway. Of course there are time I didn't think I'd have to navigate or have had other more pressing things to do at the time (like running backwards and forwards in the dark being shot at etc). Then I have to relocate, but even then, taking bearings off things is an unlikely thing to do - looking carefully at the map for features (esp contour features) seen now and in getting here is the first thing, then perhaps bearings along linear features, then aspect of slope, and only then back bearings to intercept a linear feature and finally triangulation. I can do all of them and teach them as appropriate, but really the ones further down the list are unlikely to be needed.


Fundamentally, in Britain, if you know where you started and have had your eyes open, you should be able to know where you are on the map without needing back bearings.
Walking routes visiting ancient sites in Britain's uplands: http://www.archaeoroutes.co.uk