Why am i 12 Degrees off???

[yegdod1]

Taking sight bearings and relaying them to a map....I sight an object & line up compass lines. When i put compass onto the map and draw a line back i am about 12 degrees out . I was using a rooftops map which did NOT have any info about DECLINATION...Is this the problem?,,,,if i adjusted compass for correct DECLINATION would that solve my problems???...I can cross reference my LONGITUDE & LATITUDE on my gps but i am trying not to use it.......


THANKS ALL FOR THE HELP AND ADVICE..I HAVE SORTED MY ISSUES... WENT OUT A COUPLE DAYS AGO AND THERE WAS NO NEED FOR THE GPS... ...

[alliecat]

Yep - it's the magnetic declination or variation that's messing you up. A reasonable summary is at the bushwalking.org FAQ. In short, your compass is not pointing at "grid north" as shown on your map - it is pointing at roughly 12 degrees or so to the east of north. (The exact variation depends on where you are - it should be shown on any decent map.)

There's a nice map showing the declination across all of Australia at Geoscience Australia

People use various mnemonics to remember how to convert from grid to magnetic and back. Personally, I just remember that the compass points 14 degrees east of grid north and adjust accordingly. Some compasses have an adjustment ring so that you can set the declination and use the compass as if it pointed to grid north.

Hope that helps!

Cheers,
Alliecat

[hikingoz]

I stuffed this up on a recent trip and cut a bearing about 20 degrees in the wrong direction.

The trick that I should have used was "Grand-ma sucks". or "Grid to Magnetic Subtract". I added by mistake.

Hope that helps.

[alanoutgear]

GrandMother Sucks - Grid to Magnetic subtract - now that's brought back some memories from National Service in the early '70s. The corrollary is MGA (the car) - Magnetic to Grid add. Used GMS a few weeks ago going across the Tassie central plateau and on to Lake Antimony - worked a treat and navigated to within a few metres of Hut 5. Thank you Sgt Kanga Griffiths - I wonder where you are now?

[ILUVSWTAS]

Hahaha wow, this is awesome!! I CAN use a compass (somewhat) if i must, just find it so much easier to use the GPS....

[Nuts]

maybe there's a problem with the compass?
anyhow.. What map, perhaps someone could give you the variation, i have an inclination to think that's a lot declination?? (been a long time since ive taken any notice..) it's not the same (for you) on all maps is it ?? checking against the gps should be an easy fix no??

[South_Aussie_Hiker]

In aviation we use:

Variation East - Magnetic least
Variation West - Magnetic best

Might mean when you bushwalk overseas where magnetic variation is west, you won't adjust the wrong way!

[yegdod1]

Hahaha wow, this is awesome!! I CAN use a compass (somewhat) if i must, just find it so much easier to use the GPS....

Gotta learn somehow.. ..of course the gps is easier,,,but as is said,,,,tryin to not use it....

[doogs]

Hahaha wow, this is awesome!! I CAN use a compass (somewhat) if i must, just find it so much easier to use the GPS....

Gotta learn somehow.. ..of course the gps is easier,,,but as is said,,,,tryin to not use it....

Be careful if you go to the Northern hemisphere as you need to add the degrees there!!

[Gippsmick]

Our method is "Crucial Mapping Advice" or CMA.
Compass to Map = Add.
In reverse Map to Compass = Subtract.

[blacksheep]

my car method- magnetic to grid north/ grid to magnetic..
MG + (good)
GM - (bad)

never forget that one!

[wayno]

a compass bought in the northern hemisphere is calibrated differently and won't point correctly in the southern hemisphere and visa versa

[durks]

a compass bought in the northern hemisphere is calibrated differently and won't point correctly in the southern hemisphere and visa versa

No, that's not correct. Any compass will always point the right way. What's different between such compasses is the compensation for the differences in magnetic 'dip'; that dip varies depending on where you are on the surface of the earth and, very roughly, is one way in the northem hemisphere and the other way in the southern hemisphere. See http://en.wikipedia.org/wiki/Magnetic_dip

You need the dip compensated for (by appropriate weighting of the compass needle) so that the needle lies flat. But you can still take an accurate bearing from an uncompensated compass if you're careful.

[durks]

As to that 'declination west, compass best' style of thing: I long ago gave up trying to remember such rules, because you can get yourself confused about what you need to do depending on whether you're going from map to field, or from field to map.

Instead, what I do myself is just visualise the magnetic grid lines on the map. (Indeed, you could actually draw some of these gridlines in with a waterproof pen, if you liked. to make it even plainer.) The adjustment you subsequently have to make with the compass to deal with magnetic declination is then quite obvious.

[ninjapuppet]

Ive got a basic Ranger 3 compass. Above a certain altitude, its got a bubble that I can use to level the compass to ensure there is no drag on the needle.
Back at lower altitude, the bubble dissappears. Probally will have to invest in one of those compasses that automatically adjust for the needle leveling, or get a compass that has that bubble at all altitudes.

[wander]

That bubble would be a fault in the compass, return it for a warrantee replacement. Take a photo of the bubble noting the elevation and include with the compass to explain the problem. Silva Australia sent our compass back to us because there was no bubble in it (at sea level) and they could not understand our problem until we did this.

[Gusto]

Teaching simple tricks/rules can be confusing. Better teach the foundations of the concept so that people understand how rules apply and also understand exceptions to the rules. The level of difference from Grid to Magenetic varies depending where you are in the world. And it also varies as to if you should add or subtract.

The following two maps show the same information but presented differently.

This uses Positive and negative numbers.




This one uses East and West notation.




It's worth noting that Magnetic declination is constantly changing. You'll notice this if you have a two maps of the same area but produced in different editions. Near the legend there should always be North arrows for both grid and magnetic.


This map gives some indication of what the rest of the world does.

http://www.solarpathfinder.com/images/spf/magdec-world-big.jpg


So does this one



If you wish to accurately know what is it is in your local area try this http://www.ga.gov.au/oracle/geomag/agrfform.jsp

Also if you want to know more. Wikipedia has some good info. http://en.wikipedia.org/wiki/Magnetic_declination

[ninjapuppet]

That bubble would be a fault in the compass, return it for a warrantee replacement. Take a photo of the bubble noting the elevation and include with the compass to explain the problem. Silva Australia sent our compass back to us because there was no bubble in it (at sea level) and they could not understand our problem until we did this.

Found this on the silva website. I have noticed that it remains accurate with the bubble, so i'm not too worried. ... (yet)

this is direct from their website:


http://www.silvacompass.com/detail.aspx?id=76#4


My Silva compass has a bubble in the fluid. What should I do?
We intend that our compasses are free of bubbles; however, if a small bubble forms in the liquid-filled capsule, it has no influence on the accuracy of the compass. Its appearance and disappearance are due to changes in temperature and atmospheric pressure. If a bubble larger than 1/4" in diameter appears, it is probably the result of a leaking capsule, usually caused by some form of shock damage, and the capsule will have to be replaced. In that case, just call our customer satisfaction department for a return authorization number and shipping address. From the U.S.A, call 1-800-572-8822 between 8 am-6 pm, eastern time. From Canada, call 1-800-263-6390.

[durks]

All Silva compasses seem to develop bubbles with age - and the bubble will grow, shrink, or even disappear depending on atmospheric pressure or altitude. Provided the bubble doesn't get too big, it doesn't interfere with accuracy, *provided* the compass needle is correctly compensated for magnetic dip. I.e. provided you're using the compass in the region of the earth it's been calibrated for.

What you definitely want to avoid doing is using a compass which has a bubble, in a region of the earth for which the compass hasn't been calibrated: the combination of having to tilt the baseplate to deal with the magnetic dip, and the corresponding movement of the bubble (and its physical interference with the 'high' end of the needle) is a recipe for utter frustration and badly inaccurate compass readings. In case you can't guess: I've had the experience!

[phan_TOM]

If anyones still confused as to the actual process of converting GMA or MGA heres a couple of examples taken from the map reading handbook, might help to visualise it in another way

Conversion of bearings (easterly variation)

Conversion of bearings (westerly variation)

back bearings

Teaching simple tricks/rules can be confusing

Not for me, I find mnenomics great for learning different things and once you practice anything long enough it usually sticks anyway. I use the car variation when converting bearings, its easy for me to remember MG's are good (+) and GM is crap (-)... but I remember mnemonics from school, the navy, uni biology, chemistry, geology, music etc etc

edit:

It's worth noting that Magnetic declination is constantly changing

Yes but in this region magnetic north only moves west by less than 0.01° per annum, less than 0.1° every decade or less than 1° every century so not really a concern for bushwalkers

[South_Aussie_Hiker]

Teaching simple tricks/rules can be confusing

Yeah, I don't really get this either. I don't understand how:

Variation east, magnetic least and variation west, magnetic best

could possibly be confusing or result in additions the wrong way. The comment of it not working in different parts of the world is bollocks. The only thing you need to understand is that maps are true and compasses are magnetic - and I don't think any adult would get this wrong - we all know a compass is a magnet!

Some examples

Your map shows a direction of 135 degrees to your destination and the variation is 10 degrees west - since the map is true, and remembering variation west magnetic BEST, 135T is 145M on your compass

Your compass shows a bearing of 280 to a summit, the variation is 5 degrees east and you wish to plot it on your map - since the compass reading is magnetic, and variation east magnetic LEAST, 280M is 285T on your map.

The mnemonic works if you are taking compass readings and plotting on a map, or vice versa, in any part of the world. It's not that hard.

[durks]

I don't understand how:

Variation east, magnetic least and variation west, magnetic best

could possibly be confusing or result in additions the wrong way.

Some examples

Your map shows a direction of 135 degrees to your destination and the variation is 10 degrees west - since the map is true, and remembering variation west magnetic BEST, 135T is 145M on your compass

The possibility for confusion comes from the fact that the necessary addition/subtraction is reversed, depending on whether you're going from map to field, or from field to map. (By 'addition/subtraction' in this context I mean the actual mechanical adjustment of the standard Silva-style orienteering compass.) So, in your example:

o Map to field: the map gives a bearing of 135 grid - so I mechanically *add* 10 to the compass dial so that the compass points the correct direction in the field.

o Field to map: I take a field bearing of 145 magnetic - so I mechanically *subtract* 10 from the compass dial so that the bearing on the compass (now being used as a protractor) correctly matches the grid bearings of the map.

If you know what you're doing then you won't go wrong, but if someone simply learns a mnemonic without understanding why then there is a possibility that they could get confused.

Personally, I don't find the mnemonics for this of any value - but each to his own.

[South_Aussie_Hiker]

The possibility for confusion comes from the fact that the necessary addition/subtraction is reversed

I understand what you are trying to say and I do know some people prefer to see things in front of them, but the beauty of this technique is it takes the field to map or map to field argument out of it - it doesn't matter.

I guess as long as we each find our own way that works that's all that matters!

[climbingone]

what I do is draw magnetic north lines on my map before i leave home disregarding the grid north lines, easy to do with a protractor and T square saves time in the bush
climbingone

[Redtail]

Brand new, from NOAA's Geophysical Data Center

http://www.ngdc.noaa.gov/geomag-web/#declination

[Gusto]

The possibility for confusion comes from the fact that the necessary addition/subtraction is reversed

I understand what you are trying to say and I do know some people prefer to see things in front of them, but the beauty of this technique is it takes the field to map or map to field argument out of it - it doesn't matter.

I guess as long as we each find our own way that works that's all that matters!

The reason I'd rather promote the actual core concept rather than promoting a simple trick rule is because all the rules suggested previously in this thread have been wrong are with the exception of South Aussie Hikers. Yes, some of these suggested rules work in some locations. But unless they work in all locations they are simply wrong. Only teaching someone a rule like those suggested would mean you are teaching people the wrong thing. If a simple trick or phrases is misinterpreted or forgotten then it is difficult to work out what it should be. Where as if people understand how to interpret the information presented on a map properly then they can't forget what to do.

The trick that I should have used was "Grand-ma sucks". or "Grid to Magnetic Subtract". I added by mistake.

GrandMother Sucks - Grid to Magnetic subtract - now that's brought back some memories from National Service in the early '70s. The corrollary is MGA (the car) - Magnetic to Grid add. Used GMS a few weeks ago going across the Tassie central plateau and on to Lake Antimony - worked a treat and navigated to within a few metres of Hut 5. Thank you Sgt Kanga Griffiths - I wonder where you are now?

In aviation we use:

Variation East - Magnetic least
Variation West - Magnetic best

Might mean when you bushwalk overseas where magnetic variation is west, you won't adjust the wrong way!

Our method is "Crucial Mapping Advice" or CMA.
Compass to Map = Add.
In reverse Map to Compass = Subtract.

my car method- magnetic to grid north/ grid to magnetic..
MG + (good)
GM - (bad)

never forget that one!

[Rob A]

-32 x5/9. *&%$#! cold right now though.

[darsenator]

Hahaha wow, this is awesome!! I CAN use a compass (somewhat) if i must, just find it so much easier to use the GPS....

Gotta learn somehow.. ..of course the gps is easier,,,but as is said,,,,tryin to not use it....

Be careful if you go to the Northern hemisphere as you need to add the degrees there!!

Hi Doogs....when you say about adding the degrees in the northern hemisphere...what exactly do you mean?

[GPSGuided]

Hi Doogs....when you say about adding the degrees in the northern hemisphere...what exactly do you mean?

Doog went to the US of A.

[darsenator]

Hi Doogs....when you say about adding the degrees in the northern hemisphere...what exactly do you mean?

Doog went to the US of A.

...figured as much...

But converting between magnetic to grid and grid to magnetic should use the same formula from a east(positive) declination , if in fact ones region lies within a east-declination isogonic line (no matter which hemisphere the location is in) ?