Maps/Navigation #4, 11/26/99: Orientation, Part 4, Using Compasses with Maps, continued

	Using Your Compass As a Protractor (Maps/Navigation #4, November 26, 1999)
Direct Method	The orienteering compass, with its adjustable azimuth circle and large baseplate with straight edge, was designed to use as a map protractor. Since this use does not involve magnetic measurement it is simple and accurate:
	Adjust the compass azimuth circle so the lubber's line points to the declination error angle; place the compass on the map with the capsule orienting lines or boxing mark aligned with a N/S grid line or the N/S border of the map; the lubber's line and the compass straight edge now indicate net magnetic north on the map. Orienteering compass as a protractor. Note the orienting lines in the capsule.
Magnetic Method	If your compass doesn't have an adjustable azimuth circle or a straight edge you'll have to improvise a protractor using what you have: a magnetic needle or card and a lubber's line.
	Lay your compass on the map with the lubber's line aligned with the map N/S grid. Turn compass and map together so: the compass needle points to the error angle, OR the lubber's line indicates the correction angle on the compass card. Mark the map with the positions of the N and S ends of the needle. The line between these marks indicate net magnetic north. Lensatic compass as magnetic protractor How to Apply the Magnetic Correction The easy way is good, and there are several easy ways.
Adjust the Map	Draw parallel lines on the map at angles equal to magnetic declination. Use these magnetic grid lines instead of the map grid lines and continue to mark your map directly in uncorrected magnetic angles. This takes a lot of preparation, but in the field it is very fast to use. Draw magnetic north/south lines on the map
Adjust the Direction	Use the map N/S grid lines without additions and simply apply the offset every time you transfer an angle to or from the map. This is easy to do, but also easy to forget.
Adjust the Compass Declination scale in capsule	Some adjustable compasses have small declination correction scales printed over the boxing mark in their capsules. When boxing the needle it is easy to turn the compass (lubber's line) an additional amount to apply the correction to the direction of travel. Other compasses have adjustable azimuth circles or capsules. The effect, though applied in a different way, accomplishes the same thing—preset the compass with an offset equal to the correction required. There are many ways to correct regular magnetic errors; but remember: correction is necessary only when transferring angles from field to map or from map to field.
To orient the map...	Lay the map flat on the ground away from any metal objects. Lay your compass on the map with its centerline or straight edge along one of the N/S grid lines or borders (don't use the UTM grid lines). Turn the map and compass together so:
With simple compass...	on a needle compass, the N end of the needle points to the error angle; OR the lubber's line indicates the correction angle on the compass card. On an adjustable compass, apply the correction angle and box the needle.	...with card compass...
	The compass centerline or straight edge now points to true north (the needle or card always indicates net magnetic north). With true north on the map grid aligned to true north, landmarks you can see on the ground around you should be in close correlation to the corresponding map objects. Where am I on the map? (Plotting Magnetic Bearings) Begin by orienting the map and selecting a landmark with a corresponding map object. Next, place either the top or bottom end of the compass centerline or straight edge on the map object.
	On a needle compass, rotate the compass (but not the map) about the map object end so the needle points to 360° minus your measured bearing: 360° -270° 090°. On a card compass use the magnetic bearing indicated directly by the card (270°). On an adjustable needle compass, set the bearing at the lubber's line and box the needle. Mark the map at the other end of the compass body centerline or straight edge. Connect the map object symbol and the mark—you're on that line. To plot a fix, select a second landmark and map object and repeat the process. Your location on the map is where the lines cross. You already knew that, but now you can see it on the map! You can now use your DMS, DM.M, or UTM coordinates using the methods described in Part Two. By connecting map fixes or map object positions you can determine course and distance from one to another, or to other destinations.
Landmarks and Map Objects	It's one thing to use a distant tree or hilltop as a landmark while you are walking a compass course. It's another to find suitable landmarks with corresponding map objects to use to plot a fix on a map. On the map of the Perdiz River wash, near the mouth of Spider Cañon, is a map object named "grave." Forest rangers told me that the grave had not been seen for a number of years; perhaps it had been obliterated by vandals. Where is the grave? When I traveled into the cañon I used my GPS receiver to locate the grave landmark; but, of course, that only got me close (±100 meters—maybe!). A close visual search showed no clues at all in the wash. I needed a fix to find the spot; and I had hiked in that day without a compass. At that place the Perdiz River does a series of sharp "S" bends. I should have no trouble getting a good multipoint fix when I returned. But when I examined the map and compared it to the terrain I found no really good map objects for the map fix. None of the prominent peaks on the map were visible from the wash; they were cut off from view by intervening cliff edges and overhangs, which weren't easy to recognize on the map. I would have to use a couple of indirect fixes—fixes made from other landmarks which I could mark and use as references for a final fix on the grave. I call these "amibguous" landmarks. They're there in the terrain, and they're on the maps; they're just not pronounced enough to be reliable. They're kind of "fuzzy." Good landmarks are sharp, unique objects: tops of named peaks, clean cliff edges, ponds, cliff dwellings, isolated buildings or structures, things like that which are also represented clearly by map objects. Unfortunately unambiguous landmarks are not always easy to find. And some maps have errors—be careful.
	Using Map And Compass with GPS Map and compass do a lot more than back up a GPS receiver; they complement it powerfully. In simple terms, here's one way to use them together:
	Plot a destination on your map and enter its UTM coordinates into your GPS receiver. Check your receiver's arrival warning limit. Reset it to a more convenient distance if necessary. Use your GPS receiver to find the location. When the receiver says "arrived" or sounds its alarm you'll be within 100 meters of the arrival warning limit you've set for your destination (most of the time). At your GPS destination use your map and compass and several unambiguous local landmarks to locate the precise destination fix. Mark the fix with something visible. You can construct really good visible marker "wands" from 1/4-in. x 3-ft. green bamboo garden stakes, flagged with a couple of feet of fluorescent engineer's tape. Don't use the newer plastic stakes—they're too heavy.	Wands make good markers
	What Kind of Compass Should I use? I try to stay versatile enough to be able to use a variety of hardware; but there are suitable and less suitable compasses for map work. First look at what we need to work with a map: Useful map tools a long straight edge for connecting points; scales for measuring map distances; a protractor for measuring angles; sights for taking bearings.
Mirror sighting compass	The adjustable orienteering compass with mirror sight has all these features in one relatively inexpensive instrument. It's obviously the most practical choice in a single instrument; though, as I mentioned last time, I find it a little hard to use as a sighting compass. A good example of this compass is Silva's "Trekker." My second choice is the adjustable orienteering compass. It has all the features listed except the mirror sight. It does a creditable job taking bearings without sights. Good examples are Silva's "Starter," "Polaris," and "Explorer."	Silva's "Polaris" orienteering compass
Spider Cañon Sidekick	The lensatic sighting compass is superior at taking precise bearings to landmarks. Unfolded and laid flat it has a long base and a clearly marked centerline—but no straight edge. Since the adjustable bezel on the lensatic compass is for reference, and not directly calibrated, it can be used as a protractor, but only effectively as a magnetic protractor. I'd call the lensatic compass a third best for map use. It should be supplemented with a six-inch straight edge and scale. The simple compass is my last choice for map work. It can do the job; but with a shorter baseline, no straight edge, no provision for use as a protractor, and no scales, it's much less convenient—not great for tough situations like wind or bad weather. The best thing I learned in all this is that any compass that works is useful, and that having several is a safe way to work. In this article I've concentrated on using map and compass together, outdoors, for direct basic navigation applications. There are many other applications for map and compass, both direct and indirect. I'll cover them in the future. Now...	Lensatic sighting compass

It's Fun Time

Words, words, words. How about some real practice?

Continue frequent practice taking magnetic bearings, magnetic fixes, and noting the fixes on paper.
Investigate the effects of local magnetic errors by taking bearings deliberately near cars, culverts, bridges, etc. Compare those bearings with the same bearings taken well away from large metal masses.
Use your local map catalog and order a 1/24,000 USGS map of a local area you can reach. Use this map and your compass to practice:
- applying declination correction to your measurements and entering positions on your map.
- taking magnetic fixes on local landmarks and transferring them to your map;
- determining your declination directly from:
  - local landmarks
  - multiple landmarks
- Practice using your compass as a protractor:
  - directly, if you have an orienteering compass;
  - magnetically, with all other types of compass.

For many more interesting orienteering exercises see Be Expert with Map and Compass, by Björn Kjellström. It's available from our Online Bookstore , or by clicking its book jacket, below.

In this series of articles I've tried to express the basics clearly, as simply as I can. Here are some of the books I found useful:

Be Expert with Map and Compass

Björn Kjellström

Map Reading and Land Navigation

U. S. Army

Be Expert with Map and Compass, Björn Kjellström. Kjellström is a recognized pioneer in orienteering, and one of the inventors of the orienteering compass. His book contains many map and compass training scenarios suitable for orienteers of all ages.

Mountaineering The Freedom of the Hills, The Mountaineers. An excellent reference in all aspects of working and living in the outdoors; excellent section on map and compass.

GPS Made Easy, Lawrence Letham. Mainly about GPS, but contains an excellent scenario on what to do when GPS fails.

You can order these three books via our Online Bookstore, or by clicking their book jackets, here.

Army FM 21-26, Map Reading and Land Navigation. The updated version of this field manual is actually available online: FM 3-25-26.

"Silva Compass Information," The Silva Co. This is the simple, excellent brochure that accompanies each Silva compass.

For Internet tutorials on map and compass see the Navigation Education Links in our Reference Library. If you find other good books or links, or have any comments about this article email me at:

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See you soon for another installment.
enjoy

Mountaineering: The Freedom of the Hills

The Mountaineers

GPS Made Easy

Lawrence Letham

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