# Why do most compasses come with magnifying glasses? **URL:** \ **Category:** Factual Questions\ **Created:** [May 7, 2009, 12:12am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609 "2009-05-07T00:12:44Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Diamonds02](https://avatars.discourse-cdn.com/v4/letter/d/b5a626/32.png) [@Diamonds02](https://boards.straightdope.com/u/Diamonds02)\ **Post date:** [May 7, 2009, 12:12am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/1 "2009-05-07T00:12:44Z")
I know very little about the outdoors, but I am trying to increase my knowledge about it since I am develping an interest in camping and floating. Today while I was wandering around in Cabela’s I took a look at some compasses. I noticed that most of them had magnifying glasses attached to them. Why? Also what are some less obvious uses for compasses? I know they are made to show whether you are going NSEW, but what else are they good for? ---
**Author:** ![JSexton](https://avatars.discourse-cdn.com/v4/letter/j/96bed5/32.png) [@JSexton](https://boards.straightdope.com/u/JSexton)\ **Post date:** [May 7, 2009, 12:17am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/2 "2009-05-07T00:17:43Z")
Ideally, they are used in conjunction with a map. Maps often have tiny print. Hence, magnifying glass. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [May 7, 2009, 12:21am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/3 "2009-05-07T00:21:20Z")
The magnifying glass is there to help read maps. You tend to use compasses with maps. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [May 7, 2009, 1:04am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/4 "2009-05-07T01:04:42Z")
Also for burning ants. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [May 7, 2009, 1:25am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/5 "2009-05-07T01:25:24Z")
The map enlargement is one reason, but most gear like that likes to add in multipurpose tools. A magnifying lens will have survival uses. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 7, 2009, 1:32am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/6 "2009-05-07T01:32:09Z")
> [@](#): > > Also what are some less obvious uses for compasses? I know they are made to show whether you are going NSEW, but what else are they good for? A half-decent compass (like, not one of the ones built into a watch band or zipper pull) will tell you to within a degree or better which way you’re going. It’s hard to explain how this works over a message board, but easy in person: Anyone at an outdoors supply store should be able to show you. ---
**Author:** ![Fubaya](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@Fubaya](https://boards.straightdope.com/u/Fubaya)\ **Post date:** [May 7, 2009, 1:35am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/7 "2009-05-07T01:35:46Z")
There are a lot of things you can do with a compass. One of the more basics is that if you can see two landmarks, you can “shoot an azimuth” to them and find your position. Basically, you use the compass to find that one landmark is at 45 degrees from you and the other is at 315. You then find the reciprocal heading for those by adding or subtracting 180 degrees so that 45 becomes 225 and 315 becomes 135. Then line your compass up on the map at those and draw a line. The lines will intersect at your position. Hmm, I could show you in 10 seconds, but that probably makes no sense to read… ---
**Author:** ![UncleBill](https://avatars.discourse-cdn.com/v4/letter/u/d26b3c/32.png) [@UncleBill](https://boards.straightdope.com/u/UncleBill)\ **Post date:** [May 7, 2009, 1:40am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/8 "2009-05-07T01:40:42Z")
> [@friedo](#): > > Also for burning ants. First ensuring the aforementioned ants are not actually ON your map. ---
**Author:** ![JFLuvly](https://avatars.discourse-cdn.com/v4/letter/j/ac91a4/32.png) [@JFLuvly](https://boards.straightdope.com/u/JFLuvly)\ **Post date:** [May 7, 2009, 3:13am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/9 "2009-05-07T03:13:12Z")
The lens can be used to start a fire, a mirror can allow you to see landmarks behind you and also be used as a signalling device, the ruler can be used with the map scale to give distances and above all else, the batteries never wear out in a compass!!! ---
**Author:** ![redshift](https://avatars.discourse-cdn.com/v4/letter/r/bbce88/32.png) [@redshift](https://boards.straightdope.com/u/redshift)\ **Post date:** [May 7, 2009, 3:22am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/10 "2009-05-07T03:22:32Z")
We really need to institute practical knowledge into schooling now. Why do we spend 8 years learning arithmetic over and over again when we can learn this. This stuff sounds good to know… I wish I knew myself. ---
**Author:** ![Silophant](https://avatars.discourse-cdn.com/v4/letter/s/a698b9/32.png) [@Silophant](https://boards.straightdope.com/u/Silophant)\ **Post date:** [May 7, 2009, 3:56am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/11 "2009-05-07T03:56:10Z")
> [@JFLuvly](#): > > a mirror can allow you to see landmarks behind you ! Out of curiosity, when would aiming a tiny mirror at a landmark behind you be more useful than turning around? I’m not saying there aren’t such times, only that I can’t think of any. ---
**Author:** ![ToeJam](https://avatars.discourse-cdn.com/v4/letter/t/dfb087/32.png) [@ToeJam](https://boards.straightdope.com/u/ToeJam)\ **Post date:** [May 7, 2009, 4:08am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/12 "2009-05-07T04:08:07Z")
> [@Silophant](#): > > Out of curiosity, when would aiming a tiny mirror at a landmark behind you be more useful than turning around? I’m not saying there aren’t such times, only that I can’t think of any. Gorgons. ---
**Author:** ![Oukile](https://avatars.discourse-cdn.com/v4/letter/o/13edae/32.png) [@Oukile](https://boards.straightdope.com/u/Oukile)\ **Post date:** [May 7, 2009, 11:29am UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/13 "2009-05-07T11:29:50Z")
> [@JFLuvly](#): > > a mirror can allow you to see landmarks behind you The mirror is here to allow you reading the direction on the compass on the same time that you are aiming at a landmark. See [here](http://www.thecompassstore.com/usmircom.html). ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [May 7, 2009, 12:15pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/14 "2009-05-07T12:15:44Z")
> [@Fubaya](#): > > There are a lot of things you can do with a compass. One of the more basics is that if you can see two landmarks, you can “shoot an azimuth” to them and find your position. > > Basically, you use the compass to find that one landmark is at 45 degrees from you and the other is at 315. You then find the reciprocal heading for those by adding or subtracting 180 degrees so that 45 becomes 225 and 315 becomes 135. Then line your compass up on the map at those and draw a line. The lines will intersect at your position. > > Hmm, I could show you in 10 seconds, but that probably makes no sense to read… For better accuracy, get a bearing from three landmarks, your resulting lines will cross over and form a triangle. Somewhere in that triangle is your likely position. The smaller the triangle, the more accurate your measurements were. ---
**Author:** ![Oukile](https://avatars.discourse-cdn.com/v4/letter/o/13edae/32.png) [@Oukile](https://boards.straightdope.com/u/Oukile)\ **Post date:** [May 7, 2009, 12:21pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/15 "2009-05-07T12:21:29Z")
[quote=“1920s Style “Death Ray”, post:14, topic:495609”] For better accuracy, get a bearing from three landmarks, your resulting lines will cross over and form a triangle. Somewhere in that triangle is your likely position. The smaller the triangle, the more accurate your measurements were. [/QUOTE] … and don’t forget that there are a few degrees difference between the geographic pole (the one on the map) and the magnetic north (measured by the compass). And that this difference changes with time. This is called [declination](http://en.wikipedia.org/wiki/Magnetic_declination), and for accurate results you should correct it. ---
**Author:** ![JFLuvly](https://avatars.discourse-cdn.com/v4/letter/j/ac91a4/32.png) [@JFLuvly](https://boards.straightdope.com/u/JFLuvly)\ **Post date:** [May 7, 2009, 12:25pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/16 "2009-05-07T12:25:05Z")
Yes that is what the mirror is for but the op was other uses, you can use the mirror to see a landmark behind you as you navigate forward. As I think about it, it is hard to explain, but if you are navigating between two landmarks and you lose your forward point you can align the rear landmark with with your bearing. It’s the same as navigating towards a landmark only in reverse. To be honest I have never had to try it but it was a part of SAR training. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [May 7, 2009, 12:46pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/17 "2009-05-07T12:46:04Z")
> [@Oukile](#): > > … and don’t forget that there are a few degrees difference between the geographic pole (the one on the map) and the magnetic north (measured by the compass). And that this difference changes with time. This is called [declination](http://en.wikipedia.org/wiki/Magnetic_declination), and for accurate results you should correct it. Indeed. And the difference can be quite significant. Where I presently work, in the north of Australia, the variation (aviation term for declination) is 3 degrees east and you won’t get into much trouble over short distances if you forget about it (it will put you about 3 miles off track for every 60 miles traveled.) When I was in New Zealand it was more like 25 degrees east which is much more significant. The saying “variation east, magnetic least, variation west, magnetic best” helps you remember whether to add or subtract the variation to/from true north to get magnetic north. There is a further factor called “deviation” which is a measure of how accurately the compass itself points to magnetic north. Any ferrous metals in proximity to the compass can cause it to deviate from magnetic north. In a vehicle installation where the surrounds of the compass can be controlled to some degree, the deviation can be measured and a card produced which gives corrected bearings. It may say “For 360deg steer 358” and may have a value for every 30 degrees. Electronic devices can also affect the accuracy of the compass, hence in some aeroplane installations, if the magnetic compass is to be used, there will be a card or section of the flight manual that specifies which avionics may be switched on while maintaining compass accuracy. We used to use the mnemonic “Can Ducks Make Vertical Turns” and the more juvenile “True Virgins Make Dull Company”, to remember that Compass +/- Deviation = Magnetic +/- Variation = True. The compass is also affected by acceleration and turning errors caused by the compass needle dipping as it tries to align itself vertically as well as horizontally with the magnetic flux lines (I think i have that term right, I haven’t thought about this stuff for a while.) There are rules of thumb to use to try and account for these errors but ultimately, when in a vehicle, it’s a matter of turning onto a heading, waiting for the compass to stabilise, then adjust the heading as required. There are far better direction indicators fitted to aircraft than the humble compass, and due to the various inaccuracies of the compass, it’s generally considered to be a minor emergency if you need to actually use it! ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [May 7, 2009, 3:00pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/18 "2009-05-07T15:00:09Z")
> [@Oukile](#): > > … and don’t forget that there are a few degrees difference between the geographic pole (the one on the map) and the magnetic north (measured by the compass). And that this difference changes with time. This is called [declination](http://en.wikipedia.org/wiki/Magnetic_declination), and for accurate results you should correct it. I am pretty sure that the maps I have used for orienteering back when I was a boy scout had the declination on the map. There was an line on the map pointing to magnetic north. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [May 7, 2009, 4:20pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/19 "2009-05-07T16:20:19Z")
> [@gazpacho](#): > > I am pretty sure that the maps I have used for orienteering back when I was a boy scout had the declination on the map. There was an line on the map pointing to magnetic north. That line moves. In some places it moves enough in a year to screw you up. You need current maps with current declination, or you draw lines on your map to make taking a reading easier. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 7, 2009, 4:41pm UTC](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609/20 "2009-05-07T16:41:50Z")
Note that while you can, in principle, start a fire with a magnifying glass, in principle, you need one a good bit bigger than you’ll find on any compass. One of those full-page Fresnel magnifiers (the plastic sheet with the concentric ring ridge pattern on it) works reasonably well. [Next page](https://boards.straightdope.com/t/why-do-most-compasses-come-with-magnifying-glasses/495609.md?page=2)