A map is a representation of a portion of land drawn to scale that use labels, symbols, and colors to identify key features on the ground. You will see many maps in the military but the map that is commonly used in land nav is a topographic map. A topographic map is a map that portrays land and its terrain features using contour lines. Contour lines are lines that represent terrain and its vertical and horizontal positions.
Figure 3-1 is a topographic map reduced to show margin information. The numbers with a circle around them is marginal information that is relevant for the map user.
The circled numbers correspond to the following marginal information:
To simplify the lesson, we have expanded on the terms that are most relevant for basic navigation.
Scale- Found in the upper left margin after the series name and in the center of the lower margin. The scale of a map is a representative fraction which is a ratio of map distance to ground distance. The most common scale of land navigation maps is 1:50,o00. The unit of measure for this scale can be anything: centimeters, inches, meters, feet, etc. For example, if you measure 1 cm of distance on the map, that will be representative of approximately 50,000 cm on the ground.
Index to Boundaries
Adjoining Sheet Diagram
Declination Diagram- Found in the lower margin of large-scale maps (land nav maps are large-scale) and indicates the relationship of true north, grid north, and magnetic north. There is usually a note indicating how to convert from a grid azimuth to a magnetic azimuth and from a magnetic azimuth to a grid azimuth next to the diagram. Note: Magnetic azimuths, those read when using a compass, can differ slightly from grid azimuths, those read when measuring with a protractor on a map, due to earths magnetic fields; therefore, a conversion (addition or subtraction of a defined angle) must be done before applying a compass azimuth to the map or a map azimuth to the compass.
Bar Scales- Found in the center of the lower margin. They are rulers used to convert map distance to ground distance. Maps have three or more bar scales, each in a different unit of measure. Care should be exercised when using the scales, especially in the selection of the unit of measure that is needed.
Contour Interval Note- Found in the center of the lower margin normally below the bar scales. It states the vertical distance between adjacent contour lines of the map. When supplementary contours are used, the interval is indicated. In recent edition maps, the contour interval is given in meters instead of feet.
Vertical Datum Note
Horizontal Datum Note
Grid Reference Box
Unit Imprint and Symbol
Legend- The legend is located in the lower left margin. It illustrates and identifies the topographic symbols used to depict some of the more prominent features on the map. The symbols are not always the same on every map. Always refer to the legend to avoid errors when reading a map.
Stock number identification
COLORS The MAP
To help identify map features, topographic maps are usually printed in color. The following colors indicate the the following:
Black- Man-made features such as buildings, roads, surveyed spot elevations, and all labels.
Red-Brown- Used in contour lines that show terrain and elevation
Blue- Water features such as lakes, swamps, rivers, and drainage.
Green- Identifies vegetation with military significance such as woods, orchards, and vineyards.
Brown- Used in contour lines that show terrain and elevation on older edition maps.
Red- Classifies cultural features such as populated areas, main roads, and boundaries on older maps.
Other- Occasionally, other colors may be used to show special information. As a rule, these are indicated in the marginal information.
Grid squares- Grid squares divide the map by lines of measurement running north to south and east to west that intersect at 90 degrees. Each grid square typically represents 1000 m (1 km) of distance on large-scale maps.
Grid Coordinate Scales
Grid Coordinate Scales are found on military protractors. This tool divides grid squares accurately and is the primary tool for plotting grid coordinates. (See Figure 4-16.)
The grid coordinate scales divide the grid squares for more accurate measurements. We have drawn a circle and arrow to the grid coordinate scale typically used for land navigation (1:50,000 large-scale maps)
Contour lines represent elevation or the vertical distance above or below sea level. There are 3 types of contour lines: Index, Intermediate, and Supplementary
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 listed. There are normally four intermediate contour lines between index contour lines.
Supplementary- These contour lines resemble dashes. When drawn, they show changes in elevation of at least one-half the contour interval. Supplementary lines are normally found where there is very little change in elevation, such as on fairly level terrain.
Contour Intervals- Contour intervals are given in the marginal information.
Procedure for finding elevation on a map:
Determine the contour interval and unit of measure (usually meters)
Find the numbered index contour line being determined for elevation
Determine if the elevation is going higher or lower (reference index numbers to see if they are increasing or decreasing in the direction being measured)
Contour Elevation Example:
In Figure 9-2, point (a) is between the index contour lines. The lower index contour line is numbered 500 (meaning 500 m above mean sea level). The upper index number is 600 m indicating that each intermediate contour line is increasing the elevation by 20m as indicated by the contour interval note. With this information we can determine that point (a) is 540 m. With this information we can also determine that point (b) is 580 m.
Elevation on Hilltops
In this example, you may ask: What about point (c)? For the approximate elevation at hilltops, the contour interval is split by half and added to the last contour line. In this example half of the contour interval of 20 m would be 10 m. And the last contour line represents 600 m of elevation before the hilltop is reached. Therefore, the elevation at the hilltop would be 610 m.
There are different types of slopes when navigating terrain, understanding the height and degree of a slope can be useful when determining routes (such as where to navigate and what to avoid). Types of slopes: Uniform (Gentle or Steep), Concave, and Convex
Contour lines showing a uniform, gentle slope are evenly spaced and wide apart. (See Figure 9-5.)
Contour lines showing a uniform, steep slope on a map are evenly spaced but closer together. The closer the contour lines, the steeper the slope. (See Figure 9-6.)
Contour lines showing a concave slope on a map are closely spaced at the top of the terrain feature and widely spaced at the bottom. (See Figure 9-7.)
Contour lines showing a convex slope on a map are widely spaced at the top and closely spaced at the bottom. (See Figure 9-8.)
All terrain features are derived from a complex landmass known as a mountain or ridgeline. (See Figure 9-15.) The term ridgeline is not interchangeable with the term ridge. A ridgeline is a line of changes in low to high ground. These changes can encompass a total of 10 natural or man-made terrain features.
Major Terrain Features
Major terrain features are hills, valleys, ridges, saddles, and depressions. A good mnemonic used to remember the major terrain features is hidden valley ranch salad dressing
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. (See Figure 9-16.)
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. (See Figure 9-17.)
A valley is a stretched-out groove in the land, usually formed by streams or rivers. It 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 upon 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. (See Figure 9-18.)
A ridge is a sloping line of high ground. The centerline of a ridge normally has low ground in three directions and high ground in one direction, with varying degrees of slope. If a ridge is crossed at right angles, a Soldier climbs steeply to the crest and then descends steeply to the base. When moving along the path of the ridge, depending on the geographic location, there may be either an almost unnoticeable slope or a very obvious incline. Contour lines forming a ridge tend to be U-shaped or V-shaped. The closed end of the contour line points away from high ground. (See Figure 9-19.)
A depression is a low point in the ground or a sinkhole. It could be described as an area of low ground surrounded by higher ground in all directions, or simply a hole in the ground. Usually, only depressions that are equal to or greater than the contour interval is shown. On maps, depressions are represented by closed contour lines that have tick marks pointing toward low ground. (See Figure 9-20.)
Minor Terrain Features
Minor terrain features include draws, spurs, and cliffs
A draw is a stream course that is less developed than a valley. In a draw, there is essentially no level ground and little or no maneuver room within its confines. 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. (See Figure 9-21.)
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 roughly parallel streams cutting draws down the side of a ridge. The ground slopes down in three directions and up in one. Contour lines on a map depict a spur with the U or V pointing away from high ground. (See Figure 9-22.)
A cliff is a vertical or near-vertical feature that 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. (See Figure 9-23, A.) Cliffs are also shown by contour lines very close together and, in some instances, touching each other. (See Figure 9-23, B.)
Supplementary Terrain Features
Supplementary terrain features include cuts and fills.
A cut is a man-made feature resulting from cutting through raised ground, usually to form a level bed for a road or railroad track. Cuts are shown on a map when they are at least 10 feet high, and they are drawn with a contour line along the cut line. This contour line extends the length of the cut and has tick marks that extend from the cut line to the roadbed, if the map scale permits this level of detail. (See Figure 9-24.)
A fill is a man-made feature resulting from filling a low area, usually to form a level bed for a road or railroad track. Fills are shown on a map when they are at least 10 feet high, and they are drawn with a contour line along the fill line. This contour line extends the length of the filled area and has tick marks that point toward lower ground. If the map scale permits, the length of the fill tick marks are drawn to scale and extend from the base line of the fill symbol. (See Figure 9-24.)
Terrain Features by the Numbers
Before referencing the answer key below, try to identify all 10 terrain features on the map and recall if they are major, minor, or supplementary terrain features:
Major Terrain Features: 1-5; Hill, Valley, Ridge, Saddle, Depression
Minor Terrain Features: 6-8; Draw, Spur, Cliff
Supplementary Terrain Features: 9-10; Cut, Fill
Headquarters, Department of the Army. (2013). Map Reading and Land Navigation (FM 3-25.26).