Wilderness Navigation: The Basics
When you venture into the wilderness, plan the means for a safe return. Unless you sincerely intend to get back to nature for an extended period of time then you may need some map reading and navigation skills in order to return to the usual comforts of your everyday life. These days, with diminishing wilderness, increasing populations and the possibility of continuous contact with persons who can provide rescue or assistance through various electronic devices, the probability of becoming lost for very long is low. The severity of the consequences can vary with the length of time you are lost however, depending on your location. I think there is a big difference between being marooned on a desert island with an ample supply of coconuts, fresh seafood and a pet monkey and wandering a forest in near arctic conditions wearing nothing but your cargo shorts, T-shirt and sandals, trying to avoid becoming a breakfast snack for the next hungry grizzly that you spot. GPS, cell phones, radios are helpful but may fail at the worst moment. They should be used by all means but having some basic navigation skills should always be a backup. Following is a summary of information I’ve picked up over the past several years. It is relevant to topographical maps not nautical charts.
What do you need:
A map or map set of the area and know how to read it. What you want is a topographical map which shows elevation and preferably uses the UTM coordinate system. This added information is useful in spotting features in the terrain or when it becomes necessary to leave an established trail and travel cross country. A good source for paper maps is the US Geological Survey (USGS) http://topomaps.usgs.gov/. When I plan trips to my favorite wilderness canoe area I use Mckenzie maps which are plastic coated and water proof. http://www.bwcamaps.com/ . You should order paper maps for the wilderness trip you have planned, a waterproof plastic coating is desirable. A wealth of map resources exist online but a map printed from and online source may not include all the necessary information.
This uniquely identifies the map typically named for some prominent geographical feature in the area
A typical scale you may find, smaller numbers are more “zoomed in” and have more detail:
1:250000 – each map unit = 250,000 units on the ground
(i.e. 1 in = 250,000 in = about 4 mi)
1:62,500 – called 15 minute series – covers 15 min of latitude & 15 min longitude
(1 in = about 1 mile)
1:24,000 – called 7.5 minute series
(1 in = 2000 ft, or 2.5 in / mile)
Indicates when it was revised and field checked (usually 1950s). This is important if roads, trails or fences are identified, they may have changed since the revision date or they may no longer exist.
angular coordinate system
The earth can be divided into lines of latitude that run parallel to the equator (0 º) and lines of longitude that run N and S and intersect at the poles. (International Date line is 180 º)
Coordinates are expressed as Degrees:Minutes:Seconds of Longitude East or West of prime meridian ( 0º thru Greenwich, England) and Latitude North or South of the Equator.
1 degree = 60 minutes 1 minute = 60 seconds
Spherical trigonometry required for calculations using this system. It is still favored by pilots and sailors navigating over greater distances.
UTM – Universal Transverse Mercator – Square Grid coordinate system
This was developed by U.S. Army in 1947 for designating rectangular coordinates on large scale military maps. It is currently used by US and NATO armed forces.
The Earth is divided into 60 (numbered 1 – 60) zones each 6º of longitude wide, starting at the international date line, and extends from 80º S to 84º N latitude.
In polar regions the Universal Polar Stereographic system is used.
Each zone is divided into horizontal bands each of 8º of latitude wide are lettered from C to X starting at 80º S. Letters I and O are not used.
The advantage of this system is that coordinates are expressed in meters East and meters North within the zone. Distances on the map are obvious when looking at the grid. The central meridian of the zone is set at 500,000 meters. A map having a useful scale will typically be composed of a very small area within the zone.
Typical map coordinates:
10 S 0559741 559741m E.
4282182 4282182m N.
This is read as Zone 10 S Easting = 559,741 meters (East) Northing = 4,282,182 meters (North)
For locations North of the equator the equator is assigned 0 meters North
For locations South of the equator the equator is assigned 10000 meters North
Contour Interval – CI
This is the vertical distance or elevation between the thin brown contour lines. If CI = 50 then elevation increases (or decreases) by 50 ft between each line. Each 5th darker colored line is marked with actual elevation above sea level.
Contour lines that are close together means sharp elevation, or rapids/waterfall on a river.
This is the angular distance between True north and magnetic north or grid north and magnetic north.
True North – direction of the lines of longitude
The vertical lines that are the right and left margins of a map are almost true north(this is Grid North). Closer to poles these lines become less parallel to longitude lines. Use neat lines(smaller lines between the gridlines) to find true North.
Magnetic North – direction a compass points.
The magnetic north pole is several hundred miles south of the real pole. It moves constantly but slightly. Map may have and approximate mean declination, a date and annual change. You only need to adjust if it is a very old map.
Declination East – subtract from your map direction (compass least) or add to compass bearing
Declination West – add to your map direction (compass best) or subtract from compass bearing
This describes the model used to match features on the ground to coordinates on the map. There are known points or positions surveyors use for a datum. When using a GPS it needs to be set to the same datum as the map you are using. examples:
NAD27 - North American Datum of 1927 WGS84 - World Geodetic System 1984
Types of Compass
An optical sighting compass is probably the most accurate but I like to use an orienteering type with a base plate for map navigation like the Brunton 8010G. So how accurate can you be with a compass? An optical sighting compass will typically produce about 1º of error without practice, while an orienteering compass may be around 4º without practice. One degree of compass error means 92 feet per mile of ground error.
Navigation - Using a map and compass
Determining a Map Bearing
1. Place edge of compass base plate over starting point and the same edge over destination on the map.
2. Hold compass tightly in position, turn dial until needle housing points to True north on the map
3. While holding compass in front of you, with direction of travel arrow pointing away, rotate your body until the needle points to north on the dial.
4. You’re facing your destination.
Adjusting for declination
Use declination diagram to adjust your map or compass bearing or adjust the dial for declination if your compass has that feature.
Following a Map Bearing – Attack points
Once you’ve determined a bearing, find a visual point of reference (a tall tree, mountain, radio tower, or any distinct variation in terrain) along the bearing and proceed towards that, checking your bearing along the way. Once you’ve reached the “attack point”, find the bearing to the next attack point.
Your destination could be missed due to compass error if you navigate trying to exactly follow the true bearing. A better method is to aim slight away from your objective point for some geographical feature like a river, shoreline, road, tree line, etc., which intersects your objective point. Once you reach that feature follow it until you reach your objective.
Position by Triangulation
Use your compass to shoot bearings to two (or more) topographical map features you can identify on the horizon. Draw the back bearings on your map. Where the lines cross is your position.
Position by Free Triangulation
If you are along a road, railroad track, river, etc., shoot a bearing to a topographical map features you can identify on the horizon. Draw the back bearing on your map. Where the line crosses the road/river is your location.
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