Map & Compass:

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At the Board of Review we always have a few Scouts struggling to figure out the use of these tools individually and especially when they need to be used together. There is a lot to remember here - so practice!

Compass:

A compass is a simple device that tells us where magnetic north is.

Most also have a graduated dial that allows us to set any angle from magnetic north and some have sighting devices that make it easy to sight from a magnetic bearing angle to the terrain in front of you.

In an orienteering course this might be enough - if all the bearing angles you are given to follow are 'magnetic' or you are given a map with a grid oriented to magnetic north - but in the real world you will usually have to relate these angles to a standard topographic map...

Another compass usage option is the reverse - to sight a bearing angle from a terrain feature (such as a distant peak) and find its angle to magnetic north to relate back to a map. Base plate orienteering compasses (including sighting types) allow the compass to also be used as a protractor on your map.

Maps:

The best way to learn about maps is to get one to look at - preferably one for an area you are familiar with - perhaps the area around your home or a place you often visit.

Learn how the map symbols work - look at the map's 'Legend' (in the border) to understand how features like streams & rivers, roads and footpaths, buildings, churches, schools, boundaries, power lines & vegetation are represented on the map. At a Board of Review you can always look at the legend to decode features on the map - that's what it's for! and anyway different scales & type of maps use slightly different symbols.

Look at the scale (in the border) to determine approximate distances on the map - especially compare this to the grid line spacing on the map.

Look for contour lines usually in brown that indicate areas on the map that are at the same elevation level. These lines when they are seen close together indicate steep slopes, and when further apart indicate more gentle slopes.

Look for signs of which way is the higher elevation... streams, rivers and lakes are usually at lower elevations while peak  height indications or viewpoints are located on the tops of hills or mountains. Look also for elevation label numbers on the contour lines and compare these levels. The contour interval - the elevation difference between the lines will be marked in the map legend.

Grid North:

The vertical grid lines on the map are parallel to each other. They point approximately to true north but based on regional map making master grid meridians which align to true north a specific map's grid north lines may be slightly different than true north and will vary slightly in error across the map. For USGS maps this variation is also shown on the maps declination key (error to the middle of the map grid). Other map makers do this differently - e.g. by specifying a grid error for each corner of the map allowing you to accurately estimate the actual error in a given spot of the map  If you are to use the compass and map together you will need to convert between the grid north that is marked on the map and magnetic north pointed to by your compass.

The Three Norths:

We have already discussed Magnetic North that your compass needle point to, Grid North that the Map has marked on it; so the remaining north of the three is True North or the actual north pole of the earth.

In fact surprisingly the only North you really don't use with a Map and Compass is True North!. Everything you do will be relating Grid North on a particular map to Magnetic North on your Compass.

For most practical general navigation purposes you can also just assume all the vertical Grid lines on the map are True North - it will usually be very close.

Learn more about Declination - the differences in different parts of the world and on different maps between: True North, Magnetic North and Grid North.

Orienting a Map using a Compass:

To orient a USGS map to the terrain in front of you - set the compass ring to 0/360 degrees, hold the compass edge parallel to the Magnetic North (MN) line on the map declination legend and turn the map and compass together as a unit until the red magnetic north compass pointer is aligned to the compass base arrow (and so to the magnetic north line on the map)

Alternatively for any map with a grid you can set your compass to 0 degrees + the value of the Grid-Magnetic Angle from the declination key and align the side of the compass with a grid line on the map

The map is now oriented to the physical terrain.

This means as you look at the terrain and and map they should be aligned so that terrain features you see (hills/mountains, roads, buildings etc) should appear in the same relative positions on the map.

Using Map & Compass together

Once you have Oriented the map you can do several things:

• You may be able to establish your position directly from the terrain features, or at least approximate it by simple observation & matching.
• You can establish your position accurately from 2 or more specific terrain features you can identify on the map using cross bearings from your compass

• You can measure a grid bearing from the map from your current position to a desired location so you can travel to this location using your compass, (even though you may not be able to see the final location yet...)

• You can sight a magnetic bearing from a terrain feature you can see and relate this back to the map (e.g. to plot is as an intermediate point on your path). 

Sighting a magnetic bearing on the compass and translating to a grid bearing:

If you use the compass to sight a bearing from a terrain feature you will have a magnetic bearing. To translate that to the map you will need to Add the Grid-Magnetic Declination Angle from the map declination key then you can plot this angle on the map using a protractor. In Arizona this means you need to add about 11 degrees.

Measuring a grid bearing on the map and translating to a magnetic bearing:

You can use a base plate compass, a geometry/trig protractor or better still a purposed designed map protractor to measure Grid Bearing angles from the Grid North lines on a map. A map protractor makes this easier because it is marked all the way to 360 degrees. You should align 0/360 degrees with the grid north lines on the map. If you use a regular 180 degree geometry/trig. protractor you will need to add 180 degrees to any angles you measure over 180 degrees. 

Using a map protractor to measure a bearing  on the map you will have a grid bearing. to convert this to do a sighting to terrain on the compass you will need to Subtract the grid-magnetic  declination angle from the map declination key - then you can use the resulting angle directly on your compass to sight an angle. In Arizona this means you need to subtract about 11 degrees.

How do you remember these relative directions: (Put this on a sticker on your map or compass or chant it to yourself...)

From Map to Compass -  Subtract the Grid Magnetic Declination Angle

From Compass to Map -   Add the Grid Magnetic Declination Angle

In Arizona this is easy because our magnetic declinations are east of true north and thus are considered positive angles. You may travel to locations where the magnetic declination is west of true north and therefore considered negative.

If you need to remember where the Declination angles are Positive & Negative.

"In the East Declination is Least" (e.g. negative)

In the east although the above rules still hold true: subtracting a negative angle is the same as adding the angles absolute value and adding a negative angle is the same as subtracting the angles absolute value - confusing (Just remember the above rules and hike in Arizona!).

Marking a compass for a specific Grid Magnetic Declination Angle:

A convenient alternative to adding or subtracting the Declination when moving between compass and map bearings is to mark the compass ring with a dot or alternate alignment arrow at the value representing the Grid Magnetic Declination Angle so that if you align the compass needle with this dot rather than the normal base arrow - you will have measured a bearing to Grid North that is directly usable on a map with that exact Declination (usually Grid Magnetic Angle).

Of course if you need to use the compass in a different area or with a different map with a different declination you will need to move the location of this dot. Personally  I like to use sticky alternate arrows on the compass base plate - they are easy to use & align and still allow the compass to be used as a protractor.

ACM 1/2007

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