Manual of Military Training Part 149

The scale of the map enables us to determine the distance; the contours, the height; and the time meridian, the direction.

Thus (see map in pocket at back of book), Pope Hill (sm') is 800 yards from Grant Hill (um') (using graphical scale), and it is 30 feet higher than Grant Hill, since it is on contour 870 and Grant Hill is on contour 840; Pope Hill is also due north of Grant Hill, that is, the north and south line through Grant Hill pa.s.ses through Pope Hill.

Therefore, the position of Pope Hill is fully determined with respect to Grant Hill.

Orientation

=1872.= In order that directions on the map and on the ground shall correspond, it is necessary for the map to be oriented, that is, the true meridian of the map must lie in the same direction as the true meridian through the observer's position on the ground, which is only another way of saying that the lines that run north and south on the map must run in the same direction as the lines north and south on the ground. Every road, stream or other feature on the map will then run in the same direction as the road, stream or other feature itself on the ground, and all the objects shown on the map can be quickly identified and picked out on the ground.

Methods of Orienting a Map

1st. By magnetic needle: If the map has a magnetic meridian marked on it as is on the Leavenworth map (in pocket at back of book), place the sighting line, a-b, of the compa.s.s (Fig. 15) on the magnetic meridian of the map and move the map around horizontally until the north end of the needle points toward the north of its circle, whereupon the map is oriented. If there is a true meridian on the map, but not a magnetic meridian, one may be constructed as follows, if the magnetic declination is known:

(Figure 16): Place the true meridian of the map directly under the magnetic needle of the compa.s.s and then move the compa.s.s box until the needle reads an angle equal to the magnetic declination. A line in extension of the sighting line a'-b' will be the magnetic-meridian. If the magnetic declination of the observer's position is not more than 4 or 5, the orientation will be given closely enough for ordinary purposes by taking the true and magnetic meridians to be identical.

2d. If neither the magnetic nor the true meridian is on the map, but the observer's position on the ground is known: Move the map horizontally until the direction of some definite point on the ground is the same as its direction on the map; the map is then oriented. For example, suppose you are standing on the ground at 8, q k' (Fort Leaven worth Map), and can see the U. S. penitentiary off to the south. Hold the map in front of you and face toward the U. S.

penitentiary, moving the map until the line joining 8 and the U. S.

penitentiary (on the map) lies in the same direction as the line joining those two points on the ground. The map is now oriented.

[Ill.u.s.tration: Fig. 16]

Having learned to orient a map and to locate his position on the map, one should then practice moving over the ground and at the same time keeping his map oriented and noting each ground feature on the map as it is pa.s.sed. This practice is of the greatest value in learning to read a map accurately and to estimate distances, directions and slopes correctly.

True Meridian

=1873.= The position of the true meridian may be found as follows (Fig. 17): Point the hour hand of a watch toward the sun; the line joining the pivot and the point midway between the hour hand and XII on the dial, will point toward the south; that is to say, if the observer stands so as to face the sun and the XII on the dial, he will be looking south. To point the hour hand exactly at the sun, stick a pin as at (a) Fig. 17 and bring the hour hand into the shadow. At night, a line drawn toward the north star from the observer's position is approximately a true meridian.

[Ill.u.s.tration: Fig. 17]

The line joining the "pointers" of the Great Bear or Dipper, prolonged about five times its length pa.s.ses nearly through the North Star, which can be recognized by its brilliancy.

[Ill.u.s.tration: Fig. 18]

=1874. Conventional Signs.= In order that the person using a map may be able to tell what are roads, houses, woods, etc., each of these features are represented by particular signs, called conventional signs. In other words, conventional signs are certain marks or symbols shown on a map to designate physical features of the terrain. (See diagram, Par. 1875 Plate I and II.) On the Elementary Map the conventional signs are all labeled with the name of what they represent. By examining this map the student can quickly learn to distinguish the conventional signs of most of the ordinary features shown on maps. These conventional signs are usually graphical representations of the ground features they represent, and, therefore, can usually be recognized without explanation.

For example, the roads on the Elementary Map can be easily distinguished. They are represented by parallel lines (======). The student should be able to trace out the route of the Valley Pike, the Chester Pike, the County Road, and the direct road from Salem to Boling.

Private or farm lanes, and unimproved roads are represented by broken lines (= = = =). Such a road or lane can be seen running from the Barton farm to the Chester Pike. Another lane runs from the Mills farm to the same Pike. The small crossmarks on the road lines indicate barbed wire fences; the round circles indicate smooth wire; the small, connected ovals (as shown around the cemetery) indicate stone walls, and the zigzag lines (as shown one mile south of Boling) represent wooden fences.

Near the center of the map, by the Chester Pike, is an orchard. The small circles, regularly placed, give the idea of trees planted in regular rows. Each circle does not indicate a tree, but the area covered by the small circles does indicate accurately the area covered by the orchard on the ground.

Just southwest of Boling a large woods (Boling Woods) is shown. Other clumps of woods, of varying extent, are indicated on the map.

The course of Sandy Creek can be readily traced, and the arrows placed along it, indicate the direction in which it flows. Its steep banks are indicated by successive dashes, termed _hachures_. A few trees are shown strung along its banks. Baker's Pond receives its water from the little creek which rises in the small clump of timber just south of the pond, and the hachures along the northern end represent the steep banks of a dam. Meadow Creek flows northeast from the dam and then northwest toward Oxford, joining Woods Creek just south of that town.

York Creek rises in the woods 1-1/4 miles north of York, and flows south through York. It has a west branch which rises in the valleys south of Twin Hills.

A railroad is shown running southeast from Oxford to Salem. The hachures, unconnected at their outer extremities, indicate the fills or embankments over which the track runs. Notice the fills or embankments on which the railroad runs just northwest of Salem; near the crossing of Sandy Creek; north of Baker's Pond; and where it approaches the outskirts of Oxford. The hachures, connected along their outer extremities, represent the cut through which the railroad pa.s.ses. There is only one railroad cut shown on the Elementary Map--about one-quarter of a mile northeast of Baker's Pond--where it cuts through the northern extremity of the long range of hills, starting just east of York. The wagon roads pa.s.s through numerous cuts--west of Twin Hills, northern end of Sandy Ridge, southeastern end of Long Ridge, and so on. The small T's along the railroad and some of the wagon roads, indicate telegraph or telephone lines.

The conventional sign for a bridge is shown where the railroad crosses Sandy Creek on a trestle. Other bridges are shown at the points the wagon roads cross this creek. Houses or buildings are shown in Oxford, Salem, York and Boling. They are also shown in the case of a number of farms represented--Barton farm, Wells farm, Mason's, Brown's, Baker's and others. The houses shown in solid black are substantial structures of brick or stone; the buildings indicated by rectangular outlines are "out buildings," barns, sheds, etc.

Plates I and II give the Conventional Signs used on military maps and they should be thoroughly learned.

[Ill.u.s.tration: Plate I]

[Ill.u.s.tration: Plate II]

In hasty sketching, in order to save time, instead of using the regulation Conventional Signs, very often simply the outline of the object, such as a wood, a vineyard, a lake, etc., is indicated, with the name of the object written within the outline, thus:

[Ill.u.s.tration: Fig. 19]

Such means are used very frequently in rapid sketching, on account of the time that they save.

By reference to the map of Fort Leavenworth, the meaning of all its symbols is at once evident from the names printed thereon; for example, that of a city, woods, roads, streams, railroad, etc.; where no Conventional Sign is used on any area, it is to be understood that any growths thereon are not high enough to furnish any cover. As an exercise, pick out from the map the following conventional signs: Unimproved road, cemetery, railroad track, hedge, wire fence, orchard, streams, lake. The numbers on the various road crossings have no equivalent on the ground, but are placed on the maps to facilitate description of routes, etc. Often the numbers at road crossings on other maps denote the elevation of these points.

Visibility

=1875.= The problem of visibility is based on the relations of contours and map distances previously discussed, and includes such matters as the determination of whether a point can or can not be seen from another; whether a certain line of march is concealed from the enemy; whether a particular area is seen from a given point.

On account of the necessary inaccuracy of all maps it is impossible to determine exactly how much ground is visible from any given point--that is, if a correct reading of the map shows a certain point to be just barely visible, then it would be unsafe to say positively that on the ground this point could be seen or could not be seen. It is, however, of great importance for one to be able to determine at a glance, within about one contour interval, whether or not such and such a point is visible; or whether a given road is generally visible to a certain scout, etc. For this reason no effort is made to give an exact mathematical solution of problems in visibility further than would be useful in practical work with a map in the solution of map problems in patrolling.

In the solution of visibility problems, it is necessary that one should thoroughly understand the meaning of profiles and their construction. A profile is the line supposed to be cut from the surface of the earth by an imaginary vertical (up and down) plane.

(See Fig. 21.) The representation of this line to scale on a sheet of paper is also called a profile. Figure 21 shows a profile on the line D--y (Figure 20) in which the horizontal scale is the same as that of the map (Figure 20) and the vertical scale is 1 inch = 40 feet. It is customary to draw a profile with a greater vertical than horizontal scale in order to make the slopes on the profile appear to the eye as they exist on the ground. Consequently, always note especially the vertical scale in examining any profile; the horizontal scale is usually that of the map from which the profile is taken.

[Ill.u.s.tration: Fig. 20]

[Ill.u.s.tration: Fig. 21]

A profile is constructed as follows: (Fig. 21): Draw a line D'--y'

equal in length to D--y on the map. Lay off on this line from D'

distances equal to the distances of the successive contours from D on the map. At each of these contour points erect a perpendicular equal to the elevation of this particular contour, as shown by the vertical scale (960, 940, 920, etc.) on the left. Join successively these verticals by a smooth curve, which is the required profile. Cross section paper with lines printed 1/10 inch apart horizontally and vertically simplifies the work of construction, by avoiding the necessity of laying off each individual distance.

=1876. Visibility Problem.= To determine whether an observer with his eye at D can see the bridge at XX (Figure 20). By examining the profile it is seen that an observer, with his eye at D, looking along the line D--XX, can see the ground as far as (a) from (a) to (b), is hidden from view by the ridge at (a); (b) to (c) is visible; (c) to (d) is hidden by the ridge at (c). By thus drawing the profiles, the visibility of any point from a given point may be determined. The work may be much shortened by drawing the profile of only the observer's position (D) of the point in question, and of the probable obstructing points (a) and (c). It is evidently unnecessary to construct the profile from D to x, because the slope being concave shows that it does not form an obstruction.

The above method of determining visibility by means of a profile is valuable practice for learning slopes of ground, and the forms of the ground corresponding to different contour s.p.a.cings.

Visibility of Areas

=1877.= To determine the area visible from a given point the same method is used. First mark off as invisible all areas hidden by woods, buildings, high hills, and then test the doubtful points along lines such as D--XX, Figure 20. With practice the noncommissioned officer can soon decide by inspection all except the very close cases.

This method is a rapid approximation of the solution shown in the profile. In general it will not be practicable to determine the visibility of a point by this method closer than to say the line of sight pierces the ground between two adjoining contours.

CHAPTER II