Compass

Compass

• to orient a map
• to aid navigation to a specific location
• as a protractor to plot and measure the crosscountry path between 2 points
• as a guide to maintain bearing along a travel route
• as a means for determining location

The compass itself is a measuring instrument with five important features:

1. it is pocket-sized
2. it has a magnetized needle that points to magnetic north
3. it is a dial housing with azimuth (or quadrant) degree markings in 2º increments from 0º to 360º (or in the case of a quadrant compass 0º at north and south and 90º at west and east) around its circumference and needle orienting arrow
4. a baseplate with direction of travel arrow and various scales
5. a built-in declination adjustment.

Other features that a compass may have are liquid dampening to reduce needle movement, luminous features for use after sunset, clinometer to measure slope, a sighting feature, and a lanyard to reduce loss. Azimuth is the most prevalent designation on compasses today.

Global positioning systems (GPS) do not necessarily eliminate the need for wildland firefighters to carry a compass and a map. A GPS unit is usefull for identifing precise locations, quickly noting elevation, charting routes, and plotting record of travel, but it may be difficult to use under a tree canopy or in dense smoke.

Using Compasses with Maps

Probably the best map a wildland firefighter has available to use with a compass is a United States Geological Survey (USGS) 7.5 minute, 1:24,000-scale, quadrangle series (topographic) map. An example of this map series is shown below. For compass work this map provides numerous scales to measure distance and declination (magnetic norths relationship to geographic north). Since this map is printed with true north on top and a compass must be adjusted to read geographic north, this map provides the appropriate declination to correct a compass.

A compass and a map can not be directly used together because a map is oriented to geographic north and the needle of a compass points to magnetic north. Only in certain areas of the country is no correction is necessary to either map or compass because geographic north and magnetic north align. In most areas in North America, firefighters will need to either have magnetic north lines drawn across the face of a map or the compass needs to be adjusted for declination ito read geographic north. If the compass is adjustable it is easiest to change.

Orienting a Map with a Compass

Orienting a map is done with a compass. Adjust the compass for map declination. Align north on the compass housing to the direction of travel arrow. With the compass direction of travel arrow pointing toward north end of map, place the long edge of the compass adjacent to any north-south line on the map. Turn the map and compass as a unit until the north end of the compass needle is within orienteering arrow on the floor of the compass housing. The map will then be oriented to the landscape. With a map in this position firefighters can identify landmarks around him or her.

Finding a Location with a Compass and Map

With landmarks identified, a map and compass can be used by the firefighter to find their location. This is done by taking a bearing to a couple of identified landmarks that are about 90º apart and writing those bearings down. Place north on the compass housing and align it with the direction of travel arrow. Using the compass as a protractor (forget the compass has a needle for the time being), place the center of the compass over one landmark with protractor north oriented to the north end of the map. Subtract 180º from the bearing taken to this landmark. Make a small tick mark next to this new bearing heading. Use the straight edge of the compass to draw a line from the landmark, through the tick mark, back toward where the firefighter is located. Use the same process for a second landmark. The point where the two lines intersect is the location of the firefighter. It is always wise to double check by reorienting the map and deciding if the firefighter is indeed in the location they have plotted or following the above procedure from a third landmark to see if the three lines intersect in the same location. This technique is often referred to as triangulation.

Once the firefighter has determined his or her location on a map through orienting the map, or triangulation, they can now identify and plot a course to another location. Begin by identifying the destination point of the firefighter. Draw a line between the two locations (from where they are to where they want to go). Refer to the compass as a protractor again and place the center over present location. Make sure the north end of protractor is pointed to the north end of map. The bearing the firefighter will need to travel is where the line intersects the compass housing. Read the degrees and set those degrees to the back of the direction of travel arrow. Lift the compass off of the map, align north end of needle with north orienteering arrow on the floor of the compass housing and travel in the direction that the arrow is pointing.

The compass itself is very accurate and provides a mechanism for firefighters to navigate in unfamiliar territory. However, firefighters can make errors in reading bearings or carelessly setting a bearing on the compass that will lead him or her away from the location they are attempting to find. The cost of carelessly setting the bearing 1º off will lead to an error of 92 feet (28 meters) over the course of 1 mile.