*Topo Map Question*
Asked By Thug

Question:

I am studying Topographical maps in one of my classes this semester, and we are going through them pretty fast. I am very interested in these maps and would like to know a little more than he would like to cover. There is a specific part of this subject the book just mentions, but I am not sure exactly what they are talking about. Does anyone know anything about Magnetic Declination? It says something about 2 different North Poles. This really confused me. Just a curious College boy.

Answers:

Snowdog

Let me try to explain this detail for you without sounding like a complete idiot. There are two norths, true north (which is the North Pole) and magnetic north (which is in the Hudson Bay some where). When you look at a topo map and plot a course on the map you are plotting true north. In order to hike across the ground and get to the spot you picked you need to convert it to magnetic north (because your compass points to magnetic north) or you will never get there! So, here is where the declination thing comes into play. Look down on the bottom of your map and you will see two lines with true N and the magnetic declination. Now you plot your course and say it is 270 degrees true. Look down on you map and it says 13 degrees. You subtract the 13 from the 270 and get 253 degrees.

Now you point your compass to 253 degrees and you are on your way.
 

War

West Coast or East Coast etc decides whether you add or subtract for declination/ inclination... and most commercial topo maps now are drawn to Magnetic North...

Military maps can be drawn to Grid north OR Mil north OR Magnetic North, depending on only God knows what criteria.
 

JW

To the best of my knowledge topo maps are printed for TRUE north. The dark lines on the sides of the maps, sometimes referred to as "neat" lines are "true" north/south lines.
Refer to the "declination chart" printed on the bottom of the map. According to the topographical standards, there should be 3 arrows. The one with the "star" denotes true north. The one with the arrowhead denotes "Magnetic North" and is usually labeled "MN" and the other refers to "GN" or Grid North".

We find the simplest way is to draw a pencil line that indicates Magnetic North directly on each map. USUALLY the declination chart indicates the number of degrees west or east depending on what part of the country you are in. There are a couple of areas where they're the same, so read the "marginal" print carefully and using a GOOD protractor set your MAGNETIC north line on the map. Then all ya have to do is orient the map to that line using your MAGNETIC compass, then plot the course. This way you donít have to remember the adding or subtracting...it's already done.
 

War

MOST commercial Topo hiking maps are drawn to magnetic North... as in the example from a random map I have on my previous post, Magnetic North is what the map is drawn and aligned to, most hikers don't have the equipment to find true North, or the understanding to calculate simple declination/inclination.

I don't like it, but it makes sense, Hikers carry magnetic compasses and won't calculate inclination or declination, so the maps are made "idiot proof"... more "dumbing down" of the masses, but that's what they have gone to, magnetic North aligned at center of sheet.

The "magnetic North pole" is linked to both the Earth's mass and magnetic properties. As the earth rotates, this "magnetic pole" slowly moves a short distance (maybe 100 miles or so) over a period of years... for travel purposes over a short hiking distances this effect is negligible and non existent when you orient your topo map to the current magnetic north (more on this in a sec). For Engineering purposes, this little bit of deviation is not good at all, therefore "true north" or "grid north" points to an imaginary spot at the North Pole that's universally accepted to be the "perfect North point" considering the shape and mass of the earth.

To "orient your map" you simply lay your map on a flat surface, lay your compass on top of it and line the side of the compass up with the grids on your map, then rotate the map until the compass reads north. Now your map is aligned to magnetic north, and if you point to a mountain range on the map from where you stand, then if you look up and follow your pointing finger you should see the mountain range... as long as you always align your map to the same point, you'll always hit your target area dead on. Most common hiking topo maps are already drawn so that "magnetic North" is what the grid lines on your map are pointing to. It's hard to explain why, but you'll always be dead on as long as your compass is lined up with the magnetic north grid lines and your map is pointing to magnetic north.

There are a few articles on this on this in the site. Here are couple of good ones:

 http://www.alpharubicon.com/restrictedinfocache/advsgroups/mapreadings.htm

http://www.alpharubicon.com/restricted/advsgroups/skill5.htm
 

Hawk

The hood of the car is not a good flat surface! LOL
 

pathfinder

Thug, The "two north poles" refer to magnetic north and geographic north which is the same as true north. Magnetic north does not sit under the geographic North Pole. Remember that the earth is canted 22 1/2 degrees and rotates on this axis. This is geographic (same as true north) north. Magnetic north is a little south of geographic north. Depending on where you are, (say in US) you have to figure in the difference in the two in order to properly align your topo, i.e.- to geographic north. Does this help or are you more confused? Get yourself a topo and a good compass and play with it for a while.
 

Justme

On the legend of your map, you will see a number of degrees with an angle symbol. This is the difference between true and magnetic north. True being the North Pole and Magnetic being a spot in northeast Canada. The East Coast of the U.S. is almost zero difference but the West Coast is about 13-15 degrees difference between True and Magnetic north.

You need to know the difference because your compass points at Magnetic north, somewhere in Canada and your map lines up to the North Pole. You can either tilt your map to line up with your compass or subtract the difference of degrees on your compass to line up with your map. The best way to understand this is to get a navigation book with PICTURES. (Yes, pictures are good.)
 

Cavtroop

You usually see these on military maps. I haven't noticed it on any civilian maps, but then I've spent most of my life using the military ones.

Toward the bottom edge of the map, you should find a diagram, called a declination diagram that shows what grid North, true North, and magnetic North are on that map. If memory serves, the way you use this is to get an azimuth from where you are to what you're looking at. Then, using that declination diagram, you'll know how many degrees to add or subtract from your compass azimuth to line up with the correct map azimuth.
 

Freya

Here's a web site that seemed to have a reasonable explanation, as well as information on how to set compasses, exceptions, and lots of other things. Perhaps a little scientific, but that's my background!

It's a very important concept, as being 12 degrees off of where you think you are can make you very, very lost.

http://www.cam.org/~gouletc/decl_faq.html
 

Intrepid^NV

Wow, a question I can answer. Magnetic north points towards a spot located in approximately north central Canada. It is quite some distance from the real North Pole. Your compass points at the magnetic north pole, so there is a difference here, for most parts of the world. This is the magnetic declination. It is just the difference between true north and magnetic north. The map you are using should list it.

A couple more things. Magnetic north actually drifts, so a map from the 50s could have a magnetic declination several degrees different than what exists today. On most maps there is also a grid north. This is the difference between how the map is drawn and true north, and is usually small enough to ignore.

There are lots of books that go into detail about topo maps. Bookstores and libraries will be able to help for more details. Don't hesitate to ask, if there is something else we can help with.

Thug and Others


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