•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 Interval. Before the elevation of any point on the map can be determined, you must know the contour interval for the map you are using. The contour interval measurement given in the marginal information of the map is the vertical distance between adjacent contour lines. To determine the elevation of a point on the map you must:
1.Determine the contour interval and the unit of measure used for your map (in the marginal data): for example, feet, meters, or yards.
2.Find the numbered index contour line nearest the point of which you are trying to determine the elevation.
3.Determine if you are going from lower elevation to higher, or vice versa. In the figure, point (a) is between the index contour lines. The lower index contour line is numbered 500, which means any point on that line is at an elevation of 500 meters above mean sea level. The upper index contour line is numbered 600, or 600 meters. Going from the lower to the upper index contour line shows an increase in elevation.
4.Determine the exact elevation of point (a), start at the index contour line numbered 500 and count the number of intermediate contour lines to point (a). Locate point (a) on the second intermediate contour line above the 500-meter index contour line. The contour interval is 20 meters (see figure on the previous page), thus each one of the intermediate contour lines crossed to get to point (a) adds 20 meters to the 500-meter index contour line. The elevation of point (a) is 540 meters; the elevation has increased.
5.Determine the elevation of point (b). Go to the nearest index contour line. In this case, it is the upper index contour line numbered 600. Locate point (b) on the intermediate contour line immediately below the 600-meter index contour line. Below means downhill or a lower elevation. Therefore, point (b) is located at an elevation of 580 meters.
6.Determine the elevation to a hilltop point (c). Add one-half the contour interval to the elevation of the last contour line. In this example, the last contour line before the hilltop is an index contour line numbered 600. Add one-half the contour interval, 10 meters, to the index contour line. The elevation of the hilltop would be 610 meters.
Slope. Depending on the military mission, soldiers may need to determine not only the height of a hill but also the degree of the hill’s slope. The slope is the rate of rise or fall of a terrain feature. The speed at which equipment or personnel 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 line. The closer the contour lines, the steeper the slope; the farther apart the contour lines, the gentler the slope.
Percentage of Slope. The speed at which personnel and equipment can move up or down a hill is affected by the slope of the ground and the limitations of the equipment. It is often necessary to have a more exact way of describing a slope.
•Slope may be expressed in several ways, but all depend upon the comparison of vertical distance (VD) to horizontal distance (HD). Before we can determine the percentage of a slope, we must know the VD of the slope. The VD is determined by subtracting the lowest point of the slope from the highest point. Use the contour lines to determine the highest and lowest point of the slope.
•To determine the percentage of the slope in meters between points (a) and (b), determine the elevation of point (b) (590 meters). Then determine the elevation of point (a) (380 meters). Determine the vertical distance between the two points by subtracting the elevation of point (a) from the elevation of point (b). The difference (210 meters) is the VD between points (a) and (b). Then measure the HD between the two points on the map. After the horizontal distance has been determined, compute the percentage of the slope by using the formula shown in the illustration.
TERRAIN FEATURES
All terrain features are derived from a complex landmass known as a mountain or ridgeline. The term ridgeline is not interchangeable with the term ridge. A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides, from which a total of ten natural or manmade terrain features are classified.
GROUND NAVIGATION
Ground navigation is movement between two points in which an individual, using terrain features as guides, knows both his map and ground location throughout the movement. Before you can navigate on the ground you first must determine your location on the map and on the ground by using the following procedure:
1.Orient the map to the north.
2.Inspect the surrounding area or ground to determine all distinct terrain features.
3.On the map, look for an area having the same types of features in the same relative positions as those observed on the ground. Through comparing the map to the ground and using a process of elimination, isolate the terrain feature on which you are located.
4.Confirm this terrain feature by assuring that the direction to, distance from, and difference in elevation from all adjacent terrain features are identical on the map and on the ground.
5.Determine your exact location on the isolated terrain feature by a detailed analysis of all the immediate terrain features.
Ground navigation demands a thorough knowledge of terrain features as they appear on both the map and on the ground. Since terrain features are used as guides during movement, compass use is minimal. Two basic rules must always be applied:
1.Begin from a known location on both the map and the ground.
2.Then orient the map to the ground and keep it oriented throughout the movement.
With the basic rules established, the following steps outline the ground (land) navigation procedure:
Step 1. Through a map study of the terrain, determine the most practical route to your destination and select terrain features along this route to guide your movement.
Step 2. Determine the general direction of movement.
Step 3. Begin movement, considering the horizontal and vertical distances between terrain features along the route.
Step 4. Confirm your location at selected terrain features (checkpoints) along the route.
Step 5. Upon arrival at the final destination, confirm your location by a detailed comparative analysis between the ground position and the plotted map position.
Mounted Navigation
With the addition of more combat vehicles to the Army, your chances of having to navigate while mounted are increasing. The major difference between navigating while mounted and while dismounted is the speed at which you travel. When moving mounted, it is important to designate a navigator who makes sure that the correct distance and direction are followed and recorded, beginning with the leg from the start point to the first prominent feature and then to subsequent easily identifiable features on the ground. The navigator prepares a log to record azimuths and distances for each leg of the movement. During movement, the navigator must face in the direction of travel to keep his map oriented and to identify terrain features.