Converting Magnetic Azimuth to grid Azimuth

[pollux]

Hello guys, newbie here.

It's a question that I just can't figure on my own, so I need a little help with this. The area where I live has a westerly GM angle. As per the map diagram, to convert a magnetic azimuth to a grid azimuth, you substract the gm angle. I don't get why you need to substract your compass value when making the conversion to grid when grid north is actually on the right side of your magnetic north ? Instinctively, I'd be inclined to add it.

I hope my question makes sense. I am able to just use the formula and get away with it, but it's counter intuitive for me.

Thanks !

 

[roadrunner]

Clear as mud?

http://www.outdoorsfather.com/2015/02/map-reading-true-north-magnetic-north-grid-north/

 

[Rokbar]

When I was taking a land nav course in the Army I was told this would vary depending on what part of the world you were in. The instructor informed us that here in the US it really wasn't relevant. Now, how true that was I don't know. I never got lost and would find all the points giving by getting close and following the trails to them. Good question though. With the rise of technology, true maps, compasses, and land nav will be a thing of the past. We used the earlier GPS's called sluggers.

 

[MaraviaDave]

If your west of the agonic line...an imaginary line that runs vertically, west of the Great Lakes and west of Florida (Google agonic line for more detail)....you can use the acronym M.U.G.S. and G.O.M.A when applying the magnetic declination of the geographical area you’re in.


A bearing that has been identified on your map, that you’ll apply to your compass to determine the direction of travel - use MUGS
Map
Unto
Ground
Subtract (the declination)

A bearing you’ve taken through triangulating (or point line intersect), that you’ll apply to your map to determine your location - use GOMA
Ground
Onto
Map
Add (the declination)

If your East of the agonic line...then you’d use MUGA and GOMS...same principles applied.

 

[Boog304]

You can bypass all the calculations by purchasing a compass that allows you to set your declination. Here in the East it is about 7degrees west declination. Suunto makes a good mirror compass that allows you to set it Silva makes one too, foresters use these types as it makes things a bit easier.

 

[CrzyTrekker]

Look up "LARS." Left Add, Right Subtract. If you're using a regular map with a declination diagram this will always work. Doesn't matter if you're in the northern or southern hemisphere.

 

[robtattoo]

I was always taught it as:
"Grid to mag, add. Mag to grid, get rid"

 

[Traveler]

To start, what is your goal? If you are surveying or doing something other than land nav for hunting, this may matter. If you are just navigating to hunt or recreate I would recommend not getting wrapped around the axle on this. General directions, imagery and hand railing features is a more effective way to navigate.

To your specific question, it always helps me to picture a compass on top of the map. In an ideal location you have grid north = magnetic north so start there. Imagine you are in a location where grid north is aligned with magnetic north. Everything is 0 degrees and when you align the compass north with map north you have a straight line up. Now say you go to Utah and the declination is 15 degrees easterly. Keep the map north/top aligned with the compass north and the map is now pointing 15 degrees east ("to the right"). So to move the map to straight up/north you have to hold the compass in place but rotate the map counterclockwise to upright, which is adding grid degrees.

This can be complicated when a map (which is not specific enough) says the declination is "X degrees westerly" instead of "-X degrees". with westerly declination the declination is a negative number by convention. So if it says "westerly declination 7 degrees"...that is wrong.... you don't add 7, you add -7. some maps are get this right and some don't,

 

[Desk Jockey]

value can vary 15-20* in the lower 48. Alaska is more from what I recall. Depending on what your doing it can matter a lot. Flying a plane for instance. For map based orienteering, where you can Terrain Assoicate, maybe it matters less.

https://www.ngdc.noaa.gov/geomag/img/DeclinationMap_US.png

Magnetic Declination Varies Considerably Across The United States | U.S. Geological Survey

www.usgs.gov

 

[Ozarkansas]

The reason you need to subtract a value from grid to get magnetic is that there are actually multiple “norths”. There is true north (the northern axis around which the earth rotates), magnetic north (the direction compasses point, which changes slightly every year) and grid north (the somewhat arbitrary point where all of your grid lines would converge if your map could extend all the way to the North Pole. Since we don’t navigate by stars much anymore we really only care about grid north (what the map says) and magnetic north (what the compass says). “Declination angles” or “GM angles” are the difference between these two alignments, so you add or subtract your angle as needed to get them to “match up”, so that you can plot a point on a map, and then shoot an azimuth to it with a compass. If it were the other way around (adding the GM angle instead of subtracting in a western state) you would just be INCREASING the difference between your grid and magnetic orientations, rather than making them the same.
In most of the western US you can use this simple mnemonic device: You have to promote a Major to a General, so M-G you add. You have to demote a General to make them a major, so G-M you subtract. That becomes the opposite of you’re pretty much east of the Mississippi though

 

[Krispy]

Hi all,

I am just getting into map reading with a compass for my hiking and all. I am in the PNW Pacific North West (Issaquah) area. This area has a magnetic declination of 16 degrees (rounded)
So... I get my map and compass and have them aligned & pointing north. Say, from one plotted point to another there are 25 degrees to the east. My conversion from grid to magnetic would be 25 degrees - 16 = 9 degrees is my azimuth (right?), so I start walking at an angle of 9 degrees.

However -and here comes my question- again, after getting my map & compass aligned and pointing north, my two plotted points are 25 degrees west. That means, 335 degrees to the west. My conversion from grid to magnetic would be 335 + 16 degrees = 351 degrees OR would it be 335 - 16 = 319 degrees ?

Thank you!

 

[Rokwiia]

Remember, when taking a reading of a mountain then transfering it to your map, you must do the opposite of what you did when you took a reading from your map. That's why I believe the adding and subtracting methods are the wrong solution that can be completely avoided.

More importantly, you can get you into real trouble especially in a survival situation. If you're suffering from hypothermia, not thinking correctly, and you add the declination rather than subtract--you're toast. The same result if you confuse the rules.

I believe the best way to avoid issues completely is to draw magnetic north lines on your map, an inch apart. You can do this in the warmth and comfort of your home before you ever leave for the field. Once done, you'll never have to "orient the map" in the field nor consider declination again. At point, it's no longer important because the lines you drew on your map solved the problem.

Once done, you can be holding the map, then take a reading on the fly, and move on. Moreover, you never have to worry about putting down your map on something flat or your compass on something metal. You could take a reading while the map is on a powerful elctromagnet. It doesn't matter at all--the magnetic needle is never used when the compass is on the map. That aids in being able to take a quick reading on the fly.

You'll never use the add-or-subtract method again because it's fraught with danger and is slow to use.

All of my maps have magnetic north lines drawn on them. How this method works is pictorially shown at the very bottom of this article:

https://www.adkhighpeaksfoundation.org/navagation.php

Here is a video showing and discussing how it works:

 

[Pony Soldier]

I really hate to stir the pot but--. Most of the declinations are listed on each USGS quad but from an accuracy issue they change so what year is your quad. Your current quad will be drastically differant from your 1950s quad.

In the west the geology can severely tweek your compass. Magnetic ore bodies can alter the direction by multiple degrees. I have been in a couple spots where the needle just kept going around.

If you are serious about this you need to take foreshots and backshots occasionally to see what is happening. Most of this was done on my job with a surveying compass (brunton).

I try to use sun angle, time of day, topography and whatever else I can find to double check all my toys. I rarely trust just one. I still carry a compass to be used with my gps, altimeter and topo quad. I rarely get lost but occasionaly get confused in widely fluctuating terrain in a jungle where the fluctuations are less than the contour lines on the map.

I really don't wish to confuse you but rather make you aware of other things in addition to the problem at hand.

 

[Krispy]

Thank you for your replies guys! That explanation on drawing lines on the map is definitely very useful. I'll be doing it.
I am going to look into backshots and foreshots too. I am wanting to become very familiar with landnav and if I have a couple of different methods up my sleeve so much the better.
I am still going to need someone to please answer my two questions in my previous post and thus let me know if/which of my calculations make sense.

I appreciate everyone's help.

 

[dieselchessy]

Thank you for your replies guys! That explanation on drawing lines on the map is definitely very useful. I'll be doing it.
I am going to look into backshots and foreshots too. I am wanting to become very familiar with landnav and if I have a couple of different methods up my sleeve so much the better.
I am still going to need someone to please answer my two questions in my previous post and thus let me know if/which of my calculations make sense.

I appreciate everyone's help.

Not sure your question can be answered. When you say you aligned map and compass north. What north and how?
I’m of the opinion that if you understood enough to align your map and compass north, you’d understand the answers to your question.

Buy a map compass, turn the dial so N is properly delineated, then use the azimuth line on the compass to orient the map. Then the dial degrees will be proper. No math needed.

 

[Krispy]

Not sure your question can be answered. When you say you aligned map and compass north. What north and how?
I’m of the opinion that if you understood enough to align your map and compass north, you’d understand the answers to your question.

Buy a map compass, turn the dial so N is properly delineated, then use the azimuth line on the compass to orient the map. Then the dial degrees will be proper. No math needed.

To answer your question: I align it magnetic north. I place the compass on the map and align it with the longitude and latitude lines and have the map face north per the compass' north. So both compass and map are facing the same degrees north, basically, 0 or 360.
I am using a lensatic compass.

Does this help answer my question or shall I rephrase it a different way hoping I make myself clearer in what I am asking?

 

[dieselchessy]

Ok, but your longitude lines on your map don’t follow magnetic north/south, depending on your location.

You say your in PNW and 16 deg. declination. So your compass north needle will point 16 degrees east of your maps grid lines. (Give or take due delta between grid and true north)
Orient your map to grid north and then plot your points and the compass degrees will be correct.
Grid north on your compass should be about 344 degrees.