Magnetic deviation question??

[WDP]

I live in an area with a 13* west deviation. Now that I am into astromony, everything is set up referenced to true North. How do I convert my compass reading to a true reading?? Please keep it simple, add or subtract is tough enough.Thanks in advance.

WDP

[+parkrrrr]

There's lots of mnemonics for these things, so I'll just pick one out of the hat:

 

"Declination East, Compass Least"

 

Now, you have a west declination, so the compass (magnetic) bearing is greater than the true bearing. Therefore, to convert a magnetic bearing to a true bearing, you want to subtract 13°.

 

If your magnetic bearing is less than 13°, you might find it easier to do the math if you add 360° before subtracting the 13°.

 

[Edit: note that you should be careful where you get your declination number from. Some maps, like the one at this site get it backwards. My declination is 4.9° W, but that site would have me believe that it's 4.9° E. Heaven help any Washingtonians who use that map to set up their satellite dish; they'll end up off by 40°!]

 

[Another edit: Since you're on the east coast, you really do have a west declination, so you can ignore the previous note.]

 

 

[This message was edited by Warm Fuzzies - Fuzzy on December 17, 2002 at 08:00 AM.]

[+Brown Dwarf]

You didn't say if you were using a compass in your GPS -- or a real compass. Your GPS can -- almost certainly -- be reset to give you a true bearing. As for compasses, the more expensive ones can also be adjusted to read out true bearings.

 

True bearings are a better bet for someone who's hunting in pretty much the same area. Just remember to check the declination if you are hunting far away from home. Declination out here in the PNW is about 20East, for example.

[shrekTBA]

I have a Brunton Eclipse GPS compass. You can adjust the declination on the back of the compass so that it is correct. This compass is really cool to transpose coordinates from your GPSr to the map or from the map to a GPSr. Just have to use a good map. The electromic compass in my Vista does this automatically. As for a satillite dish pointing, with Dish network, you enter your zip code and it tells you where to point it, no need to adjust for declination, they must have accounted for it so no one would get confused.

 

It's not a sport unless there is something dead in the back of the truck when you get home.

[WDP]

Thanks, the main problem is setting up the telescope or camera to properly align to veiw Iridium flares. I think I will pick up a better compas that will do the mental part for me. The GPS is set for auto mag as most of the caches in the area are map, compass and GPS work outs, quite complicated, thank you Metzabad of Delaware!!

 

WDP

[+Brown Dwarf]

Sounds like you may have a Garmin unit. I kept mine on "automag" until I realized it was giving me magnetic headings, not true. Most of the caches I've run into specify true bearings, rather than magnetic -- so I've switched the unit over.

 

One way to check would be to drive a road that goes due east/west, and see what the gps compass says you are doing. Should be real close to 90/270 if the unit is giving you true bearings.

[Curious George]

Just remember CADET

 

Compass to True Add East - and vice versa

[Geonavigator]

Magnetic declination is the angular difference between the Magnetic North Pole and the True North Pole. To convert Magnetic direction to True direction ADD West declination to the magnetic compass heading, for East declination subtract declination from the magnetic compass heading. Do the opposite to convert True direction to Magnetic direction.

Deviation is error within the compass that can be measured and a correction can be applied.

Many types of hand-held compasses have a built in correction scale permitting the compass direction to give TRUE direction instead of MAGNETIC direction for convenience in using maps or charts that are True North-up. As a rule use True direction on maps and charts.

[+sbukosky]

At least thats what I was taught when I got my pilot's license. Stop by an airport and buy a ''sectional'' map for your area. Besides some interesting topo information, there are the lines of magnetic deviation on them. Anyone interested in maps will like them.

 

Steve Bukosky N9BGH

Waukesha Wisconsin

[+Brown Dwarf]

Yup, George got it right: -add- east declination to convert from magnetic to true; -subtract- west declination to convert magnetic to true.

 

If you want to convert from true to magnetic, reverse the procedure above.

[+Alan2]

Off topic, but when I was a kid, before Iridium sats, I looked up into the clear night sky in upstate NY. Suddenly a bright light slightly bigger than let' say Venus, lit up. It got brighter cahnging colors ending in a bright emerald and then shutting off. It never moved. Lasted about 5-7 seconds as I recall. What do you think it was?

 

Alan

[+EraSeek]

quote:

---

Originally posted by Alan2:

Off topic, but when I was a kid, before Iridium sats, I looked up into the clear night sky in upstate NY. Suddenly a bright light slightly bigger than let' say Venus, lit up. It got brighter cahnging colors ending in a bright emerald and then shutting off. It never moved. Lasted about 5-7 seconds as I recall. What do you think it was?

 

Alan

---

 

Don't know, but I think it was headed right for you!

 

[+Poindexter]

Brown Dwarf wrote:

One way to check would be to drive a road that goes due east/west, and see what the gps compass says you are doing. Should be real close to 90/270 if the unit is giving you true bearings.

 

You need to be at 50' or more away from your car to get an accurate reading from a magnetic compass due to magnetic interference from the steel and other ferrous metal of the car. Even metal framed eyeglasses can throw off a compass that you are using to "sight" a bearing.

 

Poindexter

[RossOlson]

There are three norths, true north(north pole), magnetic north(hudson bay) and map/grid north. Declination is how you compensate for using a flat map in a round world.

 

Declantion is the adjustment you make between map/grid north vs. magnetic north. Don't spend to much time thinking about True north unless you use a globe to navigate.

[+Poindexter]

Declination is the angle between true north and magnetic north and changes annualy due to the constant movement of the magnetic north pole, which is not located in the Hudson Bay. Declination has nothing to do with the differences between a "flat" map and a globe. Grid declination is the angle between grid north and true north. Whichever north you use is a personal preference and not related to using a "flat" map versus a globe. Sorry, but your just adding to the confusion.

 

Poindexter

[RossOlson]

Take a flat map of the country and draw a line straight up from Los Angles and one straight up from New York, that would represent map/grid north. These lines would stay parallel. If you draw a line from LA to the north Pole and one from NY the lines won't be parallel, this would be true north/polar. If you take a measurement with you compass you will get magnetic north and if you were to draw lines from LA to the reading you got from the compass after you corrected for declanation the lines whoud meat at magnetic north, hudson bay. The declanation correction is how you compensate for this. You don't use polar (true north) you don't have anyway to measure it. How could true north even come into play?

 

I think the confusion is in the labeling of true north vs grid/map north. I have been taught trained in differnet terms. As W.F.F. pointed out one error in his exapmles I believe that there using true north as Map/Grid north is another error. I understand true norht to be polar and Map/Grid north on a flat map.

 

The important thing tounderstand is your declanation angle is how you use your compass and map together.

 

[This message was edited by RossOlson on December 23, 2002 at 08:51 AM.]

[RossOlson]

quote:

---

Originally posted by Poindexter:

Declination is the angle between true north and magnetic north and changes annualy due to the constant movement of the magnetic north pole, which is not located in the Hudson Bay. Declination has nothing to do with the differences between a "flat" map and a globe. Grid declination is the angle between grid north and true north. Whichever north you use is a personal preference and not related to using a "flat" map versus a globe. Sorry, but your just adding to the confusion.

 

 

Poindexter

---

 

Another question? If magnetic north changes annually what day do we set our compasses? I am sure it slowly changes over long long periods of time just like everything else but nothing we have to adjust for annually.

 

[This message was edited by RossOlson on December 23, 2002 at 08:18 AM.]

[+Olar]

This is the declination chart from a government topo of a section of Southern Ontario near Toronto.

Grid lines and Grid North are displayed for the use of those that require very accurate navigation working from map and applying to field compass. I've yet to see a cache hunt that demands that kind of accuracy. On the other hand, magnetic declination from true North in most areas is enough to justify paying attention to it and learning how to adjust both on map and with compass/GPS in the field.

This subject, to me, has got to be the most confusing and misunderstood aspects of navigation there is. Thank goodness my GPSR finds most caches for me.

Cheers, Olar

 

"Today really is Yesterday's Tomorrow"

[RossOlson]

Hopefully this chart helps. I deal with this almost on a day to day basis. It is only important if you plot your points on a map and than use a gps or a compass to navigate with. If you plug you point into the same gps that you will navigate with it doesn't matter what you use for a north.

[+Poindexter]

RossOlson wrote: "Take a flat map of the country"

 

In what projection? There really is no north pole on a mercator projection and if your talking about a conformal projection, then what do you mean by drawing a line "straight up"? I use true north many times to physically orient my topo maps to compare features on the map with what I see around me. As for your other question, yes, the variation changes slowly but not as slowly as you may think. The topo map of the area where I live was last updated in 1979 and shows a magnetic variation of 8°30' and today it is 11°8' It currently changes about 4'/year. You should never use the variation from USGS topo's as many of them are quite outdated. Also, grid variation on these topo's can be as much as 2° near the edge of a zone while the grid line at the center of a zone is the same as true north.

 

Poindexter

[RossOlson]

quote:

---

Originally posted by Poindexter:

RossOlson wrote: "Take a flat map of the country"

 

In what projection? There really is no north pole on a mercator projection and if your talking about a conformal projection, then what do you mean by drawing a line "straight up"? I use true north many times to physically orient my topo maps to compare features on the map with what I see around me. As for your other question, yes, the variation changes slowly but not as slowly as you may think. The topo map of the area where I live was last updated in 1979 and shows a magnetic variation of 8°30' and today it is 11°8' It currently changes about 4'/year. You should never use the variation from USGS topo's as many of them are quite outdated. Also, grid variation on these topo's can be as much as 2° near the edge of a zone while the grid line at the center of a zone is the same as true north.

 

Poindexter

---

 

Again, I think we are confusing terms. WDP is dealing with maps for astronmy. I am not familar with these maps so maybe true north is important, but on a topo you work with grid north, the map, and magnetic north, the compass. On a topo I don't undertand why you would concern yourself with true north. If your topos are 2deg diff form one side to the next your maps must be large and that would be something to keep in mind.

 

If trying to navigate with different maps and equipment it is important to understand the diff norths, otherwise it is not important.

[+Poindexter]

Well, if you don't want to elaborate, then don't bother trying to explain. My maps aren't "2deg diff form one side to the next", if you don't understand UTM zones then don't worry about it. Like I said, it's a preference thing. Some people like their moving map displays track-up, some like them north-up. Some work with bearings on their topo maps using true, some magnetic.

 

Poindexter

[RossOlson]

I am sorry for taking this discusion off-course. The original question was about coverting to a true north reading. I just wanted to make sure WDP was cleaf that there were different "Norths".

 

As far as using UTM if you can play battle ship you can figure it out. Reading a map was not the question.

[+writer]

quote:

---

Originally posted by Warm Fuzzies - Fuzzy:

There's lots of mnemonics for these things, so I'll just pick one out of the hat:

 

"Declination East, Compass Least"

 

Now, you have a west declination, so the compass (magnetic) bearing is greater than the true bearing. Therefore, to convert a magnetic bearing to a true bearing, you want to _subtract 13°_.

---

 

[Major Edit Here] I realized that you were right in some circumstances and that I was right in some circumstances.

 

The adding and subtracting depend on whether you are trying to take a magnetic bearing in and of itself and adjust your reading to what it would be in true north, or if you are working back and forth between compass and map. That is because the circumstances determine whether your bearing has been taken east or west of your reference (magnetic or true north), and that depends completely on what you are doing.

 

Let's start with western declination, where magnetic north seems to lie west of true north. (That is, an unadjusted compass pointing "north" is actually pointing to a spot that is west of true north.)

 

Say you take a bearing and want to transfer that bearing to a map with a true north grid. (Again, you have not adjusted the compass at all.) Then you must SUBTRACT the declination value from your bearing to the the true north bearing that will be correct on the map. That is because your original reference (magnetic north) is west of your final reference (true north), and you have to remove that distance to have a bearing that will work with the map.

 

If you measure a bearing on a map that has a true north grid and want to transfer it to your unadjusted compass in the face of a western declination, you must ADD the declination to your bearing from the map, because your original reference (true north) is east of your final reference (magnetic north), and you have to make up the difference to have a bearing that will work with the unadjusted compass.

 

Similarly, you want to take a bearing on an unadjusted compass and transfer it to a true north grid. In this case, though, the declination is east. Now your original reference point (magnetic north) is east of your final reference point (true north), and you have to ADD the declination amount to the bearing to have something that will correctly work with the map.

 

And if you have an eastern declination and are going from the map to the unadjusted compass, your original reference is true north, your final reference, magnetic north, lies east of it, and you have to SUBTRACT the declination from the bearing.

 

(I can't believe I'm doing this on Christmas Eve.)

 

But the easiest way to do all this is to buy a compass with a declination adjustment, use it, and then not worry about whether to add or subtract.

 

[This message was edited by writer on December 24, 2002 at 04:40 PM.]

[+Poindexter]

Declination east, compass least: means compass bearing is lesser than true bearing so you ADD to compass to get true.

 

Declination west, compass best: means compass bearing is greater then true bearing so you SUBTRACT from compass to get true.

 

Poindexter

[+parkrrrr]

It's really simple to derive the correct action from first principles. We know that the magnetic north pole is somewhere other than at the rotational north pole. Therefore, it's somewhere south of the rotational north pole, and in fact we know that it's somewhere in the vicinity of Hudson's Bay. Disregarding secondary effects like iron ore and crust density and so on, that means that if you're east of the longitude line that passes through the magnetic north pole, but south of the magnetic north pole itself, your compass needle will swing to the west of the actual north pole. That's west declination. So, let's say you're somewhere where you have a 13° west declination. That means that when you line up the needle on the compass with the 0 on the compass dial, a landmark that's due north of you (0° true) shows a bearing of 13° magnetic. (Before you feel the need to correct me on this, get out a compass and try it.) To convert the magnetic bearing - 13° - to the true bearing - 0° - you obviously subtract 13°.

 

[+Poindexter]

The north magnetic pole is now located in the Arctic Ocean out past the Queen Elizabeth Islands.

 

 

Poindexter

 

[This message was edited by Poindexter on December 24, 2002 at 06:19 AM.]

[+Olar]

quote:

---

Originally posted by Warm Fuzzies - Fuzzy:

It's really simple to derive the correct action from first principles. We know that the magnetic north pole is somewhere other than at the rotational north pole. Therefore, it's somewhere south of the rotational north pole, and in fact we know that it's somewhere in the vicinity of Hudson's Bay. Disregarding secondary effects like iron ore and crust density and so on, that means that if you're east of the longitude line that passes through the magnetic north pole, but south of the magnetic north pole itself, your compass needle will swing to the west of the actual north pole. That's west declination. So, let's say you're somewhere where you have a 13° west declination. That means that when you line up the needle on the compass with the 0 on the compass dial, a landmark that's due north of you (0° true) shows a bearing of 13° magnetic. (Before you feel the need to correct me on this, get out a compass and try it.) To convert the magnetic bearing - 13° - to the true bearing - 0° - you obviously subtract 13°.

---

 

OK, this is starting to sink in to my thick skull. So to expand on Fuzzy's explanation, if I take the above info to the map,(13° magnetic less the 13° west declination), I can spot that landmark on the map at 0° true.

Conversely if I wanted to navigate to a landmark shown on the map at 0° true then I would add the 13° west declination to get the magnetic compass bearing required to find the landmark in the field.

Cheers, Olar

 

"Today really is Yesterday's Tomorrow"

[RossOlson]

quote:

---

Originally posted by Olar:

 

I can spot that landmark on the map at 0° true.

Conversely if I wanted to navigate to a landmark shown on the map at 0° true then I would _add_ the 13° west declination to get the magnetic compass bearing required to find the landmark in the field.

Cheers, Olar

 

"Today really is Yesterday's Tomorrow"

---

 

You do not spot anything on a map at 0deg true, it would be 0deg grid if you are plotting it on a map.

 

If you are using a map you don't want to correct for true north and magnetic you want to correct for grid north and magnetic. Look at the chart, declanation diagram, that you posted and you will see the diff norths. True North, Polar, is marked as T.N. on the declantion diagram you posted and is different than Grid North. Check on your gps, I have a legend, and it should somewhere under settings you will be able to find away to correct for the different norths. The legend says true, grid, magentic, and user. You use this to compenstate. If you head out with compass and use the wrong north to correct form you will end up at the wrong place.

 

Trust me on this, use your gps to see it, look at you declanation diagram you posted earlier or do a search and you will understand the diff.

 

An easy way to see this is with a flat map that covers a large area, a protactor and a glope. Draw two lines straght north from opposite sides of the map, that will be grid north. Find those same to places on a globe and draw a straight line from each to the North pole, that will we true north. On the globe draw a straight line frome each plce to magnetic north, hudson bay area, that will be magnetic. Take you protractor and measure all three anlges. That is the difference between the norths.

 

The terms are not interchangeable.

 

[This message was edited by RossOlson on December 24, 2002 at 09:51 AM.]

[+Olar]

quote:

---

Originally posted by RossOlson:

 

You do not spot anything on a map at 0deg true, it would be 0deg grid if you are plotting it on a map.

---

 

Yes I understand that. However for what I need topos for and the difference between grid north and true north being less than 1 degree, I can afford to ignore it and keep it basic and out of the equation.

Besides, the majority of maps we cachers work from (Streets&Trips, Mapsource, Mapsend, roadmaps, and some digital topos, etc.) do not even reference grid north so the only choice is to consider grid lines on those maps as True north. Not necessarily correct but accurate enough.

Olar

 

"Today really is Yesterday's Tomorrow"

[crr003]

As everyone else is adding their variation on declination, here's something I prepared earlier in another thread -

 

Say you take a bearing from point A to point B on a map and it’s 69Deg – relative to the grid lines on the map. (Don’t worry about Grid North/True North if it’s quite a small difference – look at the info on the bottom of the map (otherwise you’ll need to add/subtract to get the true north/mag north declination).

 

Now you want to make your compass point you towards point B.

 

From the info on the map you see the Declination is say, 12 Deg East (and the map is printed this year so don’t worry about yearly changes).

 

12 Deg East means the compass is already pointing 12 Deg east (right) of where you want to go if you just put the 69Deg bearing straight onto the compass you’ll really be walking at 81Deg (map brg) – so you have to reduce the bearing you put on the compass by the 12 Deg dec = 57 Deg.

 

For West dec it’s the opposite – the compass is pointing left of where you want to go, so you need to add the dec angle to get you on the right track.

 

Now, this I think is the normal situation – taking bearings from a map and putting them on the compass. Sometimes you need to take a magnetic bearing with your compass and transfer it back on to the map to find out where you are etc.

 

It's the same logic as above, but in reverse – if your dec is East you need to ADD the dec angle to the mag brg to get the correct bearing on the map.

 

East Dec: translate map brg to mag brg subtract dec, mag brg to map brg add dec

West Dec: translate map brg to mag brg add dec, mag brg to map brg subtract dec

 

Having read the thread again, this is pretty much what jolyrog said!

 

Nil Satis Nisi Optimum

 

[This message was edited by crr003 on October 05, 2002 at 11:06 AM.]

 

Nil Satis Nisi Optimum

[+sygyzy]

I am a bit confused on the terminology here. If you add East declination to magnetic to find true, this means that you are increasing the coordinate reading (long, lat). But if you are increasing it, it means you are moving from east to west. So does an east declination mean that true north lies east of magnetic north and by adding to magnetic you get true? That is the only way it makes sense to me.

 

Peace!

Sygyzy

[+Groundhound]

Positions (latitude, longitude) don't really have to do with the subject of declination. Whether you add or subtract depends on this - Do you know true north and wish to determine magnetic north, or do you know magnetic north and wish to determine true north? What to do, for me at least, is to picture the map of where the magnetic north pole is in relation to a line drawn between me and the true North Pole. If the magnetic pole is west of that line, then the declination is west, and my compass needle is pointing west of true north. It's easy for those of us with a GPS, just set a waypoint for the true North Pole and set your GPS to give magnetic bearings. The bearing to the true North Pole is then equal to your magnetic deviation. If my declination is 4 deg. west, my GPS gives a bearing of 4 deg. to the true North Pole, therefore 4 deg. magnetic is equal to 0 deg. true.

 

...I think:-)

[+GEO*Trailblazer 1]

http://www.ordnancesurvey.co.uk/literatu/info/cr12.html

http://www.astro-fengshui.com/fengshui/truenorth.html

When all else fails Geotry again. http://www.geog.port.ac.uk/webmap/hantsmap/hantsmap/cmprose.htm

 

[This message was edited by Trailblazer # 1 on January 10, 2003 at 10:07 PM.]

 

[This message was edited by Trailblazer # 1 on January 10, 2003 at 10:09 PM.]

 

[This message was edited by Trailblazer # 1 on January 10, 2003 at 10:12 PM.]

[+Alan2]

Try this.

 

Alan

[Cracker.]

Alan,

I live in Western NY (Southern Tier), and I see emerald objects all the time....

 

Once, I saw a BIG one moving UPWARDS, and it then started slowing down and getting brighter, then it exploded.....VERY weird looking stuff....

 

Art

 

www.yankeetoys.org

www.BudBuilt.com

http://www.ttora-ne.mainpage.net/

[+Poindexter]

Alan2 wrote: "Try this"

 

Don't try it, it's wrong. With 14°West declination, 0°magnetic equals 346°true. You SUBTRACT west declination from magnetic to get true. ADD east declination to magnetic to get true.

 

Poindexter

[+Cachetrotters]

Originally posted by Trailblazer # 1:

 

http://www.astro-fengshui.com/fengshui/truenorth.html

 

QUOTE]

I hope you didn't think that site was really going to help, did you? "From this we can see that there is actually no such thing as the true north. It varies with time." Because of precession, eh.

Now that's what I call "spin".

 

don

[+Alan2]

quote:

---

Originally posted by Poindexter:

Alan2 wrote: "Try this"

 

Don't try it, it's wrong. With 14°West declination, 0°magnetic equals 346°true. You SUBTRACT west declination from magnetic to get true. ADD east declination to magnetic to get true.

 

Poindexter

---

 

You're right. I better go check to see how i set the declination on my compass. Never use it,(I have a Vista) but I might have too.

 

Tks

 

Alan