Go out some clear night with your compass. Look up and find the North Star. Draw an imaginary line from the North Star to the horizon, intersecting it at a right angle. Make note of some landmark at that point. Now take your compass and establish a bearing to 0 degrees, noting some landmark at that point. You’ll note, if you’re careful, that the landmark you noted from the position of Polaris is offset from the other landmark by a small amount, usually five to ten degrees. The difference between the two is called magnetic declination. It occurs because of a number of factors, including the location and movement of magnetic material in Earth’s crust and core.
If the magnetic north is east of the geographic north, then the declination is positive; if it is west of geographic north, it is negative. The reason we care about declination is that while our maps are drawn with reference to true north, our compasses always point to magnetic north. When using them together, we just need to add or subtract the declination to get the true bearing.
If your declination is 5 degrees west, and your travel bearing is 275 degrees true, then add five degrees from your magnetic bearing to get the proper reading for the compass (280 degrees).
If your declination is 5 degrees east, and your travel bearing is 204 degrees true, then subtract five degrees from your magnetic bearing to get the proper reading for the compass (199 degrees).
Finally, NOAA’s National Geophysical Data Center gathers data about magnetic declination and maintains a database here. Get in the habit of checking your maps against their data periodically, and updating as necessary.
If you are out in the field, how do you know where you are? If you have a map and compass, you have the tools, but you may not know how to use them together. We all know that compasses point north, and that maps show a section of land on paper. But to locate yourself on the map takes something called resection.
An orienteering compass has a rotating bearing that sits in a plate; its cousin, the lensatic compass, has a folding housing. Both types can be used for in the field, but the lensatic compass is recommended for this purpose, as it is much more accurate.
To find yourself on a map, pick two landmarks, preferably 90 degrees apart as seen from your position – the closer to 90 degrees, the better. Measure the bearing to one of the landmarks and write this number down. Add or subtract the magnetic offset for that region, then add 180 (if the number is between 0 and 180 degrees) or subtract 180 (if it is between 180 and 360) to find what is known as the back azimuth. This represents a line that can be drawn on a map, intersecting the landmark and the back azimuth bearing. Repeat the process for a second landmark. When the back azimuths are both drawn on a map, the intersection is where you are.
Example: the bearing to a water tower is 310 degrees, the bearing to a cabin is 230 degrees, and the local magnetic declination is 4 degrees West. This gives
310 + 4 -> 314 degrees – 180 = 134 degrees rev. az. 1
230 + 4 -> 234 degrees -180 = 54 degrees rev. az. 2
(If the magnetic declination is east instead of west, then subtract instead of add.) Place a protractor over the water tower on the map and mark 134 degrees relative to that point, then draw a line from the tower through the 134 degree mark. Repeat for the cabin. Where the two lines intersect is where you are.
This is basically what radio direction finding equipment does when it attempts to locate your position from your transmission, but in reverse. Where the rdf equipment uses two locations to make observations on a single point, you are using one location to make observations on two points. Either way, the math works out the same.
So this weekend, while you’re testing out your cold weather preps that Arctic Patriot has given an excellent set of lessons on, make sure you take a map and compass along and see if you can find your position with those mittens or gloves on.
Might as well practice two things at the same time, if you can.
OAKOC is an acronym used in terrain analysis. It is used for describing the military aspects of a piece of land in a way that clearly identifies the locations necessary to defend or seize a particular area.
O = Observations and Fields of Fire – Where would you place your troops to gain the maximum visibility and fire cover? How would you overlap them?
A = Avenues of Approach – Where would troops normally come from when approaching the position? Where would they expect resistance?
K = Key Terrain – What must be held and what can be given up?
O = Obstacles – What natural or artificial barriers to movement exist, and what can be constructed?
C = Cover and Concealment – What can be used for cover, and what for concealment? What caliber would be necessary to defeat each? If no natural cover exists, what can be improvised?
When using the OAKOC method, you analyze a piece of land in terms of each aspect for both sides – what are your observation posts, what are your enemy’s observation posts, etc. etc. These points of analysis will have an effect on each side, which can be used to determine what action and counter-action to take.
This week’s homework: from the map you developed in the past two weeks, pick a point near one of the military crests/reverse slopes and examine it, evaluating the OAKOC aspects of the position.
reverse slope – Any slope which descends away from the enemy. – FM 101-5-1, Operational Terms and Graphics
An exercise in local terrain analysis, using the map generated in last week’s exercise, where you drew lines through the topographical crests of local hills and identified the miltary crests along those lines. Pick the most prominent of those crests, or the one closest to your residence, and do the following:
- Identify the reverse slope that lies on or closest to the line through the crest, and calculate the distance from the middle of the reverse slope to the topographical crest from the map. Verify your answer in the field.
- Identify any local elements (such as rocks, trees, or buildings) that could be used for concealment, and those that could be used for cover*. Specify what caliber weapon would be necessary to defeat whatever you identify.
- Identify the following paths from your residence to the reverse slope: fastest, stealthiest (defined as the one where you are least likely to be observed), and the one passing by the nearest residence belonging to a member of your regular training group.
- Identify the most likely avenues of approach to the reverse slope along the line set at 90 degrees from the line you drew through the crest (that is, the flanks of a position on the reverse slope whose fire is directed towards the crest).
* There is a difference between the two.
military crest – An area on the forward or reverse slope of a hill or ridge just below the topographical crest from which maximum observation and direct fire covering the slope down to the base of the hill or ridge can be obtained. (See also topographical crest.) See FM 21-26. – FM 101-5-1, Operational Terms and Graphics
An exercise in local terrain analysis:
- Obtain the 7.5 minute quadrangle USGS map that includes your residence.
- Identify all the elevated areas of significance on the map.
- Using your residence as the center of a circle, draw lines radiating out from your residence that intersect the topographical crests of these elevated areas.
- Identify the military crests that lie on those lines, if they exist.
- Conduct a field reconnaissance of these crests to determine if your map analysis matches reality.
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