# What is the forumla to find out compass declination using a map



## Scoobie Newbie (25 Feb 2005)

What are the numbers, where do you find them on the map and what is the formula you use to set your compass declination.  Thanks.


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## George Wallace (25 Feb 2005)

You should find that info on the right hand margin of the map sheet (UNLESS you have cut it off).  There will be a diagram and instructions.  The diagram will be dated, and then you take 'todays' date, calculate the difference and then use that to multiply by the amount of declination per year (as stated on your map) to get the required changes for your compass.  Just ensure that you go the correct direction, both on the map and on the compass when applying your calculations and corrections.

Every map will have different declinations because of: different years of printing; of course, the difference in latitude and longitude; etc.

GW


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## Scoobie Newbie (25 Feb 2005)

Does it sound something like this:


Use diagram only to obtain numerical values.
Approx mean declination 1993 for center of map.
Annual change decreasing 7.4


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## Scoobie Newbie (25 Feb 2005)

Alright I asked one of the boy's and I got it to work.  Thanks


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## dan476 (9 Mar 2005)

CFL,

Can you post an answer here please.


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## dangerboy (9 Mar 2005)

Here is an example using a Wainwright map.

              Current year                              2005
           -  year of map                              2000
              Total                                            5
             
              multiply by annual change              10.7' (decreasing)
              Total                                          53.5' (decreasing)

               magnetic north                           16(degrees) 26'   [convert to 15 (degrees) 86' for ease of math]
               minus Total                                53.5'   (if annual change was increasing you would add)
               Declination                                  15(degrees) 32' round to 15 (degrees) East 

              You get the east or west if your mag north is east or west of true north
Hope this helps.


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## ZipperHead (9 Mar 2005)

An easy way to remember which way your declination should be is: if you are east of Thunder Bay, the declination is to the west, and if you are to the west of the big Thunder, it is east (magnetic north is basically north of TB).

BTW, make sure you use as current of a map as possible, as the annual change isn't constant (ie. if you used a map from 1960 you are going to be off severely, as the annual change is (more than likely) different now than 45 years ago..... don't laugh, I have seen some pretty ancient maps used when training in civvy areas).

Al


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## Gayson (10 Mar 2005)

Look at the declinaion info on the right side of the map.  You need to find the declination at the year of publication, and the rate of change of the declination per year,  Then find the year of the map (should be at the bottom).

Subtract the year of the map from the current year.

Multiply the answer by the rate of change of the declination.

Take that answer and add or subtract it from the maps decination accordingly.

The number you get is the current declination you should use on the map.


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## dan476 (10 Mar 2005)

whats the formula to converts degrees and minutes into Mills?
thanks


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## Danjanou (10 Mar 2005)

You mean in this day of sooper dooper hi tech GPS we still have to resort to old map and compass skills?  ;D

Allan, funny when I was reading your post that old maxim re east/west came back to me and I don't want to think how long ago I took my first map and compass lectures.

Never ceased to amaze me how when guys started getting their maps ready for the field, mac tacking them, using yellow highlighters on the grid lines, cutting the borders off etc, that they usually cut off all the useful data like the date and declination info. Did it myself, once. After that I made sure it was glued somewhere on the map ( on the back or over a water feature).


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## dangerboy (10 Mar 2005)

1 degree equals 17.8 mils


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## TCBF (10 Mar 2005)

http://lfdts.army.mil.ca/ael/pubs/300-008/B-GL-382/005/B-GL-382-005-FP-001.pdf

(CFP 318-8)   Military Training Volume 8: Maps, Field Sketching, and Compasses

Remember, though we talk of declination (or deviation), what we always use is "Grid Magnetic Angle" : (The difference between Grid North and Mag North), not Magnetic Declination or Grid Convergence.

It's a good book that no one ever reads.

Tom


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## Gayson (10 Mar 2005)

When cutting a map the following bits should be kept.

Declination info
The Year
The map DATUM (especially if you are going to use a GPS, Meaford for example is NAD 83 / WGS 84)

I'm probably forgetting something else.


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## TCBF (11 Mar 2005)

You brought up a very good point.  If the exercise or Op is not in a trg area, and NTS maps are used, it is very important to capture the marginal info from the map bEFORE it is trimmed and glued to other sheets to make one big map.  A few years back we did this, and we had to make a matrix to show the different years, GMAs, contour intervals and datums of the 12 or so maps in each set.  Everyone got a set glued together, plus an 81/2 X 11 matrix showing the needed info.

Tom


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## LF(CMO) (11 Mar 2005)

I always liked 'Map and Compass' stuff, still do.

 What was one of the most dangerous combinations in Vietnam?

Ans:  A 2nd Lt with a map and compass in his hand!

Told to me a few weeks ago by Dan T as I was preparing for a Cadet FTX.  Dan served in the US Special Forces in Vietnam 1964-65.


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