TERRAIN
Terrain is another name for a piece of ground. For our purposes, it is a region or territory viewed for its suitability as a battleground. How you make use of the terrain-its texture, even its color at different times of the day-affects everything you or the enemy do or can do. Both you and the enemy have the terrain of the battlefield in common. More often than not, victory goes to whoever understands and uses the terrain best. Usually, the terrain dictates troop movements and formations, positions to be defended, and locations of weapons. You cannot memorize definite rules to cover every situation. However, there are certain principles discussed in this chapter that, when applied intelligently, will result in sound solutions or decisions. Knowledge of these principles is not enough to give you the advantage. You must know the terrain intimately to use it properly. Other than making a personal reconnaissance of the terrain, you must be thoroughly familiar with the use of commercially prepared maps as well as maps drawn hastily in the field. You must also interpret signs and symbols used on maps, prepare field sketches and overlays, and use the lensatic compass properly.
TERRAIN APPRECIATION
Terrain appreciation is the analysis of an area to determine the effect that the terrain features have on probable military operations by either of the opposing forces. Terrain can be viewed with either offensive or defensive intentions in mind. However, regardless of the type of mission, each Seabee leader must evaluate the terrain for both offense and defense. Then the Seabee leader can anticipate the enemy’s analysis of the situation as well as his own. Information on the terrain may be acquired through various sources, but a physical reconnaissance of the area is the most important and reliable method of obtaining accurate information. When a physical reconnaissance is not possible or additional information is desired, it may be provided by one or more of the following sources:
1. Aerial reconnaissance and photographs
2. Maps of the area
3. Terrain models provided by higher authority
4. Intelligence reports
5. Patrolling
6. Friendly natives, undercover agents, or captured enemy prisoners
COMPONENTS OF TERRAIN
In military terminology, terrain is simply the ground over which we intend or propose to. fight. To a military man, the word terrain
is an all-inclusive term, referring not only to the ground itself, but to all other conditions that influence the ability of the combatant to carry out an assigned mission. For the sake of simplicity, terrain has two major aspects. The first is weather, climate, and season; the second is topography.
WEATHER CLIMATE, AND SEASON
WEATHER is the day-to-day changes in atmospheric conditions. CLIMATE is the average weather over an extended period of time. SEASONS are characterized by particular conditions of weather, such as summer and winter in the United States or the rainy and dry seasons in Southeast Asia. Of these three elements, weather is the most important consideration from a tactical viewpoint. For long-range planning or in the absence of weather information, climatological and seasonal data may be used to estimate weather conditions.
The Elements of Weather
Weather consists of several atmospheric elements, each affecting tactics in its own way. These elements are as follows:
1. TEMPERATURE. The degree of hotness or coldness in a geographic area.
2. HUMIDITY. The percentage of water vapor in the air.
3. VISIBILITY. The ability to see both horizontally and vertically. It is influenced by fog, haze,
heat refraction, clouds, or precipitation.
4. PRECIPITATION. The depositing of moisture (rain, mist, snow, sleet, hail) from the atmosphere upon the surface of the earth, expressed in kind and amount.
5. WIND. The movement of air within the atmosphere. It is expressed as strength (velocity) and
direction.
6. PHASES OF THE MOON. Usually, phases of the moon are expressed in quarters. The first quarter is between the new moon and the full moon; the second, or last quarter, between the full moon and the new moon. The two phases have a direct bearing on night visibility and the amount of rise and frill of the tides.
The Effect of Weather on Tactics
Weather has a direct effect on the visibility, the movement, and the use and effect of weapons. Horizontal visibility may be materially reduced (resulting in reducing the observation of the enemy or the effect of your fire on them) by fog, haze, heat refraction, or precipitation. Vertical visibility may be restricted by fog, precipitation, or a large mass of low-lying clouds, thus reducing the effectiveness of air support or aerial reconnaissance. Ease of movement, both logistical and tactical, on roads or cross-country may vary drastically from day to day because of precipitation and temperature changes. A heavy rain may change a passable area into an impassable quagmire; but a severe temperature drop may cause the same quagmire to frieeze, thus aiding movement. Weather affects weapons, both in employment and in the effectiveness of the weapon itself. The trajectory of artillery and mortar rounds is greatly influenced by temperature and humidity. Extreme cold and hot weather require special treatment and handling of gasoline engines, thus affecting the use of equipment and vehicles. The effects of weather are particularly noticeable in air and naval weapon systems support. Air support may be restricted or prevented entirely by clouds, fog, or heavy precipitation. Fog, snow, or heavy rainfall reduce visibility; therefore, naval gunfire support cannot be delivered as effectively, and new targets cannot be rapidly located and engaged.
TOPOGRAPHY
Topography consists of the physical aspects of the surface of the earth and includes such features as relief and drainage, vegetation, surface materials, and cultural features.
Relief and Drainage
RELIEF is the term given to the differing areas of elevation and depression on the surface of the earth.
DRAINAGE refers to those areas of surface depression that serve as water runoffs or collection points, such as marshes, swamps, streams, rivers, ponds, and lakes. Knowledge of the general shape of the land is gained through a detailed study of the relief and drainage features. The steepness of slopes; the height and size of hill masses; the depth, the width, and the length of
drainage features; and the sizes of valleys and draws are major features to consider when studying the terrain of a given area. These irregularities in the surface of the earth influence tactics by the degree of observation they provide the opposing forces, the ease or difficulty of movement, and the degree of protection provided against enemy fire. Flat ground provides equal observation for the opposing forces; normally, high ground in rolling or mountainous terrain provides for better observation. Any advances made parallel to a series of ridges or to a river or stream are mechanically easier than movement perpendicular to them. The steepness of a slope may limit movement; tanks, for instance, cannot climb slopes greater than 30 degrees. Flat ground offers little protection against enemy fire, but rolling ground will, particularly against flat-trajectory weapons.
Vegetation
Vegetation is classified for practical purposes as either NATURAL or CULTIVATED. Natural plant life includes all types of grasses, bushes, and trees growing without the assistance of man; cultivated vegetation includes all crops and orchards tended by man. Density, height, and types of growth, as well as the diameter of tree trunks, are significant features when you are studying vegetation. Although vegetation may restrict vision, it offers concealment and limited cover. Of course, the thicker the growth, the harder it is for the forces to move about.
Surface Materials
Surface materials are studied to determine the trafficability of an area. “Trafficability” is defined as the ability of a soil in its normal state to support vehicular traffic moving cross-country or on unimproved roads and trails. In general, all types of soil, except very loose sand, afford good trafficability when dry. However, soils are seldom completely dry. Water may change soil from a hard, baked clay to slippery, impassable mud within a matter of minutes, especially in tropical areas. Another aspect to consider, along with the types and condition of the soils, is the slope of the ground, the type of vegetation, and the roughness of the surface.
Cultural Features
CULTURAL FEATURES include all the works of man, such as towns, airfields, roads, railroads, and bridges. For military purposes, man-made features are considered an integral part of terrain. Cities and towns are frequently the objectives of an attacking force. For tactical purposes, cultural features may be centers of resistance as well as physical obstacles in your path. Roads, railroads, and bridges are vulnerable links in logistics and communication networks.
MILITARY ASPECTS OF TERRAIN
Various combinations of weather and topography give certain qualities to an area. These qualities, known as the MILITARY ASPECTS OF TERRAIN, must be closely evaluated by each unit leader. These qualities determine to a large degree how he employs his forces and weapons. You can remember these military aspects of terrain by using the acronym KOCOA.
K – Key terrain features
O – Observation and fields of fire
C – Cover and concealment
O – Obstacles
A – Avenues of approach
KEY TERRAIN FEATURES
Key terrain is any locality or area that provides the possessor a marked advantage over the enemy. Usually, the factors that make a feature or an area key terrain are superior observation and fields of fire. Obstacles may be considered key terrain when their possession by one force prevents the movement of the opposing force. In some areas, such as mountains and jungles, where movement depends on established roads and paths, routes of communications could be key terrain. A bridge over an unfoldable river could be key terrain, particularly when its seizure eliminates the need for an assault crossing. An airfield could be key terrain when its seizure facilitates the success of local operations or serves as a base to support future operations.
In selecting key terrain, the unit leader is beginning to tie his mission to the ground. Inasmuch as key terrain features offer an advantage to one or both combatants, it is apparent that the defender will strive to retain them while the attacker tries to seize them. For this reason, key terrain is often assigned as the objectives of attacking units; conversely, key terrain aids the defender in disposing his forces to best maintain his battle position. Selection of key terrain features varies according to
the mission. In the attack, the unit leader selects key terrain features forward of the line of departure. In the defense, the terrain that must be held to maintain the integrity of the battle position is designated as key terrain. Selection of key terrain also varies at the different levels of command. For example, at force level, a large city may offer a marked advantage as a communications center or as a base for supply and maintenance facilities. At division or regiment level, high ground dominating the city may be important for observation and fields of fire. At battalion, company, and lower echelons, key terrain might be hills and valleys within the general high ground around the city.
OBSERVATION AND FIELDS OF FIRE
OBSERVATION of the battlefield is essential to bring effective fire on the enemy, to control maneuvering of your troops, and to prevent beingsurprised by the enemy. Observation is classified aseither long or short range. Long-range observation is that which provides observation in excess of the effective range of small-arms fire (usually over 400 meters). Short-range observation covers the immediate foreground and extends to the effective range of small-arms fire. Observation is limited or denied by such factors as fog, heavy precipitation, heat refraction, darkness, vegetation, cultural features, and relief.
FIELDS OF FIRE are areas into which your weapons can be fired effectively. An ideal field of fire for the defense would be gentle sloping ground, fitted to the trajectory of your weapons, and on which the enemy can be seen with no protection from your fire. This situation is rarely encountered. However, you can improve the natural fields of fire by cutting or burning weeds, grass, and crops; by clearing brush and trees; by demolishing buildings; and by cutting lanes through woods. The commander must exercise caution in ordering such work, since obviously constructed fire lanes can disclose the location of your positions to an observant enemy. Observation and fields of fire are so closely related that they are considered together. They are not synonymous, but fields of fire are based on observation, since the enemy must be seen to bring effective fire on him. These aspects are particularly important to the defender. The primary considerations for choosing a defensive position are maximum observation and long fields of fire.
COVER AND CONCEALMENT
COVER is shelter protection from enemy fire, either natural or artificial. Geographical relief features, drainage areas, cultural features, and other artificial shelters provide cover. Cover from flat trajectory fire is best exemplified by the concept of reverse slope; that is, when there is a projection of relief, such as a hill, between you and the enemy. Cover must be considered in relation to the types of fire encountered. For example, a trench offers excellent protection against rifle fire, but only limited protection against mortar or artillery fire.
CONCEALMENT is protection from observation. Vegetation, cultural features, geographical relief features, drainage areas, weather conditions, and darkness can provide protection from observation. Frequently, you can obtain concealment by properly evaluating and using just the terrain. At other times, you may need artificial means (camouflage) in addition to natural, available concealment. Concealment is the reverse concept of observation. Since the defender usually has the opportunity to choose the ground he wishes to defend, he selects positions that take maximum advantage of natural cover and concealment, adding field fortifications and natural concealment with camouflage to improve the position. It is important that you judge your own cover and concealment by looking at it from the point of view of a potential attacker.
OBSTACLES
Obstacles are obstructions that stop or divert troop movement. Common natural obstacles of military value include mountains, rivers, streams, lakes, marshes, gullies, steep inclines, and heavily wooded areas. Common artificial obstacles include minefield, cut and falls, trenches, antitank ditches, roadblocks, barbed wire, blown bridges, and road craters. The proper evaluation of natural obstacles permits the most effective use of artificial obstacles. Obstacles perpendicular to the advance route of the enemy generally favor the defending force. Obstacles parallel to the advance of the enemy may favor the enemy by protecting his flanks, although the obstacles may also limit his lateral movement. The effectiveness of an obstacle must be carefully examined. An obstacle by itself is rarely an absolute block to military movement by a determined enemy. A defender who puts full faith in an obstacle by itself stands the risk of being surprised by enemy movement over or through that obstacle. Maximum effectiveness is gained from an obstacle kept under observation and fire.
AVENUES OF APPROACH
An avenue of approach is a terrain feature or combination of features that offer a maneuvering unit a suitable route of movement to their objective. The desirable characteristics of an avenue of approach are listed below.
EASE OF MOVEMENT toward the objective.
COVER and CONCEALMENT from the fire and observation of the defender.
FAVORABLE OBSERVATION and fields of fire for the attacker.
Adequate ROOM FOR MANEUVER and dispersal by the attacking unit.
You, as a defender, must pay particular attention to all avenues of approach. These approaches into your Seabee position represent potential weak spots in the defense, and Seabees must be positioned to block and cover them effectively. As a defender, you must also consider the use of these avenues of approach by your own forces should you wish to launch a counterattack.
MAPS
A map is a small-scale, flat-surfaced representation of a part of the surface of the earth. Man-made and natural features are shown by the use of symbols, lines, colors, and forms. Maps show the location and distances between ground features, such as towns, populated areas, roads, airfields, streams, and other lines of communication. They also indicate variations in the landform and the height of natural features.
Grids
Military maps are divided into grids to provide a uniform system for referencing and making measurements. Military grids consist of two sets of equally spaced parallel, straight lines intersecting at right angles and forming a series of squares. Each grid line is an even interval of the selected measurement unit, such as yards or meters. The regularly spaced lines that make up the grids on any large-scale map are divisions of the 100 000-meters square; the lines are spaced at 10 000- or 1000-meter intervals. Each of these lines is labeled at both ends with a number showing its relation to the origin of the zone. For the 1000-meter grid, except for the numbers labeling the first grid line in each direction from the southwest corner of the sheet, the last three digits (000) of the number are omitted. Two digits of the numbers are printed in large type, and the same two digits appear at intervals along the grid line on the face of the map. They are called the PRINCIPAL DIGITS and represent the 10,000 and 1,000 digits of the grid number; they are of major importance to the map reader because they are numbers he uses most often for referencing points. The smaller digits complete the COORDINATES of the grid lines, but they are rarely used for point designation. On sheets with grid line spacing at 10000 meters, only one principal digit is shown, representing the 10,000 digit of the grid number.
The designation of a point is based on the military principle of “Read RIGHT then UP.” The precision desired determines the number of digits to be read beyond the principal digits.
Elevation and Relief
A knowledge of map symbols, grids, scale, and distance provides enough information to identify two points. You locate them, measure between them, and determine the length of time required to travel between them. But what happens if there is a 300-foot cliff between the two points? The map user must also become proficient in recognizing the various landforms and irregularities of the surface of the earth. Then he is able to determine the elevation and differences in the height of all terrain features.
1. DATUM PLANE. This is a reference from which vertical measurements are taken. The datum
plane for most maps is mean, or average, sea level.
2. ELEVATION. This is defined as the height (vertical distance) of an object above or below a datum plane.
3. RELIEF. Relief is the representation of the shape and height of landforms and the characterization of the surface of the earth.
The elevation of points and the relief of an area affect the movement and deployment of units by limiting the route of travel, their speed of movement, and the ease or difficulty of attacking or defending an area. Also relief affects observation, fields of fire, cover, concealment, and the selection of key terrain features.
Contour Lines
There are several ways of indicating elevation and relief on maps. The most common way is by contour lines. A CONTOUR LINE is a line representing an imaginary line on the ground along which all points are at the same elevation. Contour lines indicate a vertical distance above or below a datum plane.
1. HILL. This is a point or small area of high ground. When you are located on a hilltop,
the ground slopes down in all directions.
2. VALLEY. A valley is a course of a stream that has at least a limited extent of reasonably level ground bordered on the sides by higher ground. The valley generally has maneuvering room within its confines. Contours indicating a valley are U-shaped and tend to parallel a major stream before crossing it. The more gradual the fall of a stream, the farther each contour parallels it. The curve of the contour crossing always points upstream.
3. DRAW. A draw is a less developed course of a stream in which there is essentially no level ground and, therefore, little or no maneuvering room within its confines. The ground slopes upward on each side and toward the head of the draw. Draws occur frequently along the sides of ridges at right angles to the valleys between them. Contours indicating a draw are V-shaped, with the point of the V toward the head of the draw.
4. RIDGE. Normally, a ridge is a line of high ground with minor variations along its crest. The ridge is not simply a line of hills; all points of the ridge crest are appreciabley higher than the ground on both sides of the ridge.
5. SPUR. A spur is a short, continuously sloping line of higher ground normally jutting out from the side of a ridge. A spur is often formed by two roughly parrallel streams cutting draws down the side of a ridge.
6. SADDLE. A saddle is a dip or low point along the crest of a ridge. A saddle is not necessarily the lower ground between two hilltops; it maybe simply a dip or break along an otherwise level ridge crest.
7. CLIFF. A cliff is a vertical or near vertical slope. When a slope is so steep that it cannot be shown at the contour interval without the contours coming together, it is shown by a ticked “carrying” contour or contours. The ticks always point toward lower ground.
8. CUTS and FILLS. Cuts and falls are man-made features caused when the bed of a road or railroad is graded or leveled by cutting through high areas and filling in low areas along the right-of-way.
9. DEPRESSION. A depression is a low point or a sinkhole, surrounded on all sides by higher ground.
Slope
The rate of rise or fall of a landform is known as its slope and may be described as being gentle or steep.
Scale and Distance
A map is a graphic representation of an area and, therefore, is not made to full scale (actual size). Since it is not full size, some means of measuring the distance from one point on the map to another is necessary. This is done with the aid of a scale. There are two types of scales in general use on military maps. The first is called a graphic scale and is indicated by a special scale legend that is printed on the map. The second type of scale is the ratio between the horizontal distance on the map and the corresponding distance on the ground.
Direction
Directions are expressed as units of angular measure, and there are several systems used.
1. The most commonly used unit of angular measure is the degree with its subdivisions of minutes and seconds.
2. Another unit, less frequently used, is the mil (abbreviated m). For the U.S. military Purposes, a complete circle is divided into 6,400 mils. The mil is commonly used in artillery, tank, and mortar gunnery.
Base Line
In order to measure anything, there must always be a starting point, or zero measurement. There are three base lines–true north, magnetic north, and grid north. Those most commonly used are magnetic and grid; the magnetic when working with a compass, and the grid when working with a military map.
TRUE NORTH. This is a line from any position on the surface of the earth to the North Pole. All lines of longitude are true north lines. True north is usually symbolized by a star.
MAGNETIC NORTH. The direction to the magnetic North Pole is indicated by the north-seeking needle of a magnetic instrument. Magnetic north is usually symbolized by a half arrowhead.
GRID NORTH. This is the north established by the vertical grid lines on the map. Grid north may be symbolized by the letters GN.
Topographic Map Symbols
The purpose of a topographic map is to permit you to visualize an area of the surface of the earth with pertinent features properly positioned.
Topographic symbols are usually printed in a number of standardized colors. This is done so the features on the map are easier to identify and to give them a more natural appearance and contrast.
Black–the majority of cultural or man-made features
Blue–water features, such as lakes, rivers, and swamps
Green–vegetation, such as woods, orchards, or vineyards
Brown–all relief features, such as contours
Red–main roads, built up areas, and special features
Others–occasionally used to show special information; generally, explained in the marginal notes
Military Symbols
In addition to topographic symbols used to represent natural and man-made features of the earth, the military establishment requires some method for showing the identity, strength, locations, and movements of its troops, activities, and installations. The symbols used to represent these features are known as military symbols. As these features are constantly changing and moving, they are not normally printed on the maps.
LENSATIC COMPASS
The lensatic compass is the most commonly used and simplest instrument for measuring directions and angles in the field. In order to use a map effectively in the field for purposes of identification, location, or reporting, you must orient, or align, the map with the ground. A map is oriented when, in a horizontal position, its north points to the north and all map lines are parallel to their corresponding lines on the ground.
Azimuth and Back Azimuth
The most common military method of expressing a direction is by using azimuths. An “azimuth” is defined as a horizontal angle, measured in a clockwise reamer from a north base line. When the azimuth between two points on a map is desired, the points are joined by a straight line. Then a protractor is used to measure the angle between grid north and the drawn line. This measured angle is the grid azimuth of the drawn line. When using an azimuth, you imagine the point from which the azimuth originates as the center of the azimuth circle. Azimuths take their name from the base line from which they have been measured; true azimuths from true north, magnetic azimuths from magnetic north, and grid azimuths from grid north. Therefore, any one given direction can be expressed in three different ways: a grid azimuth, when measured on a military map; a magnetic azimuth, when measured by a compass; or a true azimuth, when measured from a meridian of longitude.
The BACK AZIMUTH of a line is its forward azimuth plus 180°; or if this sum is greater than 360°, the back azimuth is the forward azimuth minus 180°. For example, if the forward azimuth of a line is 112°, the back azimuth is as follows:
112° + 180° = 292°
When the forward azimuth of a line is 310°, the back azimuth is as follows:
310° - 180° = 130°
Protractor
Protractors come in several forms–full circle, half circle, square, and rectangle. All of them divide a circle into units of angular measure; and regardless of their shape, they consist of a scale around the outer edge and an index mark. The INDEX MARK is the center of the protractor circle from which all the direction lines radiate.
Intersection
Locating an unknown point by successively occupying at least two, but preferably three, known positions and sighting on the unknown point is called intersection. It is used to locate features that are not defined on the map or which are not readily identifiable. The two methods of intersection are the map and compass method and the straightedge method.
Resection
Locating the unknown position of the user by sighting on two or three known features is called resection.
Land Navigation is a perishable skill. This section is not a complete guide for Navigation. It is intended to aid in teaching and learning.