Tag: Compass

This is a very complex subject, but very simple in application, if you follow a few simple rules and have a basic understanding of how to read a map.

Map Terminology

• Contour – Imaginary line on ground, all points of which are at the same elevation above or below a specific reference surface.
• Coordinates – Linear and (or) angular quantities that designate the position of a point in relation to a given reference frame.
• Declination – Magnetic declination is the angular difference between magnetic north and true (geographic) north at the point of observation; it is not constant but varies with time because of the “wandering” of the magnetic north pole.
• Elevation – Vertical distance of a point above or below a reference surface.
• Grid – Network of uniformly spaced parallel lines intersecting at right angles. When superimposed on a map, it usually carries the name of the projection used for the map, that is, Lambert grid, transverse Mercator grid, universal transverse Mercator grid.
• Latitude – Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.
• Legend – explanatory table of symbols used on a map or chart.
• Longitude – Angular distance, in degrees, minutes, and seconds, of a point east or west of the Greenwich meridian.
• Map – Graphic representation of the physical features (natural, artificial, or both) of a part or the whole of the Earth’s surface, by means of signs and symbols or photographic imagery, at an established scale, on a specified projection, and with the means of orientation indicated.
• Orientation – Establishing correct relationship in direction with reference to points of the compass; the state of being in correct relationship in direction with reference to the points of the compass.
• Overlay – Printing or drawing on a transparent or translucent medium intended to be placed in register on a map or other graphic and which shows details not appearing or requiring special emphasis on the base material.
• Relief – Elevations and depressions of the land or sea bottom.
• Relief shading – Technique for making topography on a map appear three-dimensional by the use of graded shadow effects. Generally, the features are shaded as though illuminated from the northwest.
• Ridge – A spur of land that is higher than the land on either side.
• Topography – Configuration (relief) of the land surface; the graphic delineation or portrayal of that configuration in map form, as by contour lines.
• Triangulation – Method of extending horizontal position on the surface of the Earth by measuring the angles of triangles and the included sides of selected triangles.
• Valley – A spur of land that is lower than the land on either side.

Tools of the Trade

Basic land navigation only requires three tools. A topographic map, a compass and a pencil or dry erase marker, if your map is laminated or sealed in plastic.

Finding your Grid Coordinates

All topographic maps have grid numbers listed on the top, bottom, and both sides. To find your grid coordinates you simply find the numbered line to the RIGHT of the grid square you are in. Then you find the numbered line on the BOTTOM of the grid square you are in and you can get the four digit grid coordinate which will give you the 1 km square area you are in. Remember you read the map RIGHT AND UP.

To get a more exact fix on your location you first break the two sides (from right to left and bottom to top) into ten equal parts each. To get the next set of numbers for your coordinates you first go from the right and figure out which of the ten parts you are in, then do the same from the bottom up. This will give you a six digit grid coordinate, which will give you your location within 100 meters. To get your coordinates within ten meters of your location you divide each of those ten square you mentally created in half, you would use either five for half way between those lines or zero for directly on a line. It takes practice, lots of practice to “eyeball” an eight digit grid coordinate but it can be done.

Orienting your map

To orient your map first lay your map on as level a surface as you can then find the North Declination line, usually located in or near the maps legend. Lay your compass on the map and line the edge of your compass up with the MAGNETIC NORTH line. Then simply turn your map until the North seeking arrow is pointing in the same direction as the Magnetic North Line on the map.

You can now use the compass to find a bearing to any terrain feature, road, building, or grid coordinates on the map.

Resection – finding where you are

First orient your map and look for a prominent terrain feature you can see to the LEFT of your location. Then find that terrain feature on your map. Place your compass on the map, with the edge of the compass running through the center of the terrain feature and draw a line back towards your vantage point.

Next find another terrain feature you can see to the RIGHT of your location. Find that terrain feature on the map, place your compass on the map with the edge running through the center of that terrain feature and draw another line backwards towards your vantage point.

Where the two lines cross is your location. You can now get the grid coordinates to your location.

Reading the Map – What does it all mean

The legend is going to tell you everything about the map except how to read it. Here are the two most important things to know.

Distance traveled – this is a huge one, as it is difficult to keep track of your location if you don’t know how far you have traveled. The best method that does not depend on anything battery operated is a pace count. You should know your pace count to distance traveled ratio, for example mine is 110 steps to 100 yards, walking normally. Now I can pace off 100 yards using 100 steps, learned this from golfing, but that is not my relaxed pace and that is what you need to know. Go to your local High school track and walk the 100 yards with a relaxed pace, now you know your number. Remember a mile is 1760 yards. A good practice is to carry a string of pace count beads, an easy way to manage your counts and you can make your own.

Contour lines – you know from the glossary what these are, and what they mean, but can you read them. The simplest way to look at it is, if the line curves toward a higher elevation, it is a valley. If the line curves toward a lower elevation, it’s a ridge. Best place to find water is in a valley. When contour lines are far apart, it is a gentle slope. When contour lines are very close it is a cliff or at least very steep. If the contour lines form a circle, it is either a peak or a pit, based on the elevation change.

Now that you have the basics, get out there and practice in a safe place, with limited risk. I found that going out to some local Orienteering events is a great way to become intimately familiar with a map and build your navigation skills.

as Always,

Stay Vigilant and Be Prepared