Reading a Topographical Map.
If you are good at reading a topographical map you can get by without a compass but you have to have a good, accurate map. The map should include contour lines, roads, trails, streams, water, declination diagram, scale, and a legend to help you out with all the symbols. You can get maps locally at many outdoor sporting goods stores, online (just conduct a search with your favorite search engine), and sometimes you can even find them in the reference section of your local library
Before we can read a map we must first break it down to its basic features. The first thing you need to know about is the contour lines and how to read them.
Contour lines are the most common method of showing relief and
elevation on a standard topographic map. A contour line represents an imaginary line on the ground, above or below sea level. All points on the contour line are at the same elevation. The elevation represented by contour lines is the vertical distance above or below sea level. The three types of contour lines used on a standard topographic map are as follows:
- Index: Starting at zero elevation or mean sea level, every fifth contour line is a
heavier line. These are known as index contour lines. Normally, each index contour line is numbered at some point. This number is the elevation of that line.
- Intermediate: The contour lines falling between the index contour lines are called
intermediate contour lines. These lines are finer and do not have their elevations given.
There are normally four intermediate contour lines between index contour lines.
- Supplementary: These contour lines resemble dashes. They show changes in
elevation of at least one-half the contour interval. These lines are normally found where there is very little change in elevation, such as on fairly level terrain.
CONTOUR INTERVALS
Before the elevation of any point on the map can be determined, the user must know the
contour interval for the map he is using. The contour interval measurement is usually given in the legend and is the vertical distance between adjacent contour lines. To determine the elevation of a point on the map you must:
- Determine the contour interval and the unit of measure used, for example, feet,
meters, or yards.
- Find the numbered index contour line nearest the point of which you are trying to
determine the elevation.
- Determine if you are going from lower elevation to higher, or vice versa.
- Determine the elevation of a point. Remember, if you are increasing elevation, add the contour interval to the nearest index contour line. If you are decreasing elevation, subtract the contour interval from the nearest index contour line. For example, if you are going uphill and you determined you are past the 600 foot index contour line and halfway to the next intermediate contour line and know that the contour interval is 50 feet, then you will know that your elevation is 625 feet above sea level.
- Determine the elevation to a hilltop. Add one-half the contour interval to
the elevation of the last contour line. For example, if the last contour line before the hilltop is an index contour line numbered 700. Add one-half the contour interval, 25 feet, to the index contour line. The elevation of the hilltop would be 725 meters.
TYPES OF SLOPES
Depending on the route, you may need to determine not only the height of
a hill, but the degree of the hill's slope as well. The rate of rise or fall of a terrain feature is known as its slope. The speed at which you can move is affected by the slope of the ground or terrain feature. This slope can be determined from the map by studying the contour lines. The closer the contour lines, the steeper the slope; the farther apart the contour lines, the gentler the slope.
Gentle Slope: Contour lines showing a uniform, gentle slope will be evenly spaced and wide apart .
Steep: Contour lines showing a uniform, steep slope on a map will be evenly spaced, but close together. Remember, the closer the contour lines, the steeper the slope.
Major Terrain Features
Hill: A hill is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric circles. The inside of the smallest closed circle is the hilltop.
Saddle: A saddle is a dip or low point between two areas of higher ground. A saddle is not necessarily the lower ground between two hilltops; it may be simply a dip or break along a level ridge crest. If you are in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. A saddle is normally represented as an hourglass.
Valley: A valley is a stretched-out groove in the land, usually formed by streams or rivers. A valley begins with high ground on three sides, and usually has a course of running water through it. If standing in a valley, three directions offer high ground, while the fourth direction offers low ground. Depending on its size and where a person is standing, it may not be obvious that there is high ground in the third direction, but water flows from higher to lower ground. Contour lines forming a valley are either U-shaped or V-shaped. To determine the direction water is flowing, look at the contour lines. The closed end of the contour line (U or V) always points upstream or toward high ground.
Minor Terrain Features
Draw: A draw is a less developed stream course than a valley. In a draw, there is essentially no level ground and, therefore, little or no maneuver room within its confines. If you are standing in a draw, the ground slopes upward in three directions and downward in the other direction. A draw could be considered as the initial formation of a valley. The contour lines depicting a draw are U-shaped or V-shaped, pointing toward high ground.
Spur: A spur is a short, continuous sloping line of higher ground, normally jutting out from the side of a ridge. A spur is often formed by two rough parallel streams, which cut draws down the side of a ridge. The ground sloped down in three directions and up in one direction. Contour lines on a map depict a spur with the U or V pointing away from high ground.
Cliff: A cliff is a vertical or near vertical feature; it is an abrupt change of the land. When a slope is so steep that the contour lines converge into one "carrying" contour of contours, this last contour line has tick marks pointing toward low ground . Cliffs are also shown by contour lines very close together and, in some instances, touching.
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