Map Sketching.

Map sketching is an important factor in trench warfare to-day as it is in a war of movement. A fairly accurate map will indicate more than many words and in much less time. Time is the great factor in war. Instruction must also be rapid. Here are ten lessons which would occupy a week if taken morning and afternoon. The aim of the instruction as in company rifle shooting is to train many men to do a satisfactory job, not to make a few finished topographers. Neatness, accuracy and initiative are cardinal points.

For the instructor, reference should be made to Grieves' "Military Sketching and Map Reading", 2nd edition, if he desires to supplement any points given here.

LESSON 1. (CLASS ROOM—FOUR HOURS.)

Problem—Map Reading.

Study the conventional signs found in the "Manual for Non-commissioned Officers and Privates of Infantry of the Army of the U.S.," 1917, page 273, or in Grieves, pages 28-35. These conventional signs are not universal and must be used only as indications of the general practice.

In map sketching in the field few conventional signs are used, and the items of importance are written on the map, such as WOODS, CULTIVATED, HEDGE, SWAMP, etc.

TAKING UP MAP SCALES.—There are three ways of indicating the relation between the actual distance on the ground and the space the same distance occupies on the map:

1. The graphic scale is a straight line divided into units, as miles, yards, feet or meters, which represents the actual ground distance. Thus if 6" = 1 mile the line would be six inches long and marked at one end and 1 mile at the other, three inches being marked ½ mile, etc. It is important to always have this graphic scale on a map so that if the paper gets wet or is stretched from its original size the scale will change in the same proportion.

2. A Statement in words or figures, e.g., 3 inches equal one mile, meaning that 3 inches measured anywhere on the map represent 1 mile on the actual ground.

3. The Representative Fraction (generally known abbreviated as R.F.) having a number above the line that shows the unit length on the map and below the line the number of units which are in the corresponding actual ground distance. For example, if 1" = 1 mile, then the R.F. is:

1" (map distance) / 63,360" (1 mile—ground distance)

if 3" = 1 mile the R.F. is:

3" (map distance) / 63,360" (1 mile—ground distance) or 1 / 21120

if 6" = 1 mile:

6" / 63360" or 1 / 10560

if 12" = 1 mile:

12" / 63360" or 1 / 5280

In reading a map one must know the scale and also where the North is. This is always indicated by an arrow pointing either to the magnetic North or the true North. If to the magnetic North the needle will have but one barb away from the true North. The angle between the magnetic and the true North is the declination.

Placing the map in proper relation to the ground so that points of the compass coincide on map and ground is called orienting the map.

In map work there is one vital point to remember; practically all the ground surface is in its present form as a result of water action

1. Look for the water courses, that is the drainage system. It will give the general slope of the land.

2. Look for the high points between the water courses, remembering that there is always a valley then a hill then a valley again continued in succession.

3. Finally locate towns, railroads, main highways and work down to the minor details.

In measuring a map to get the actual distances on the ground, copy the graphic scale on any piece of paper and apply this directly or if your distances exceed your scale use the edge of a piece of paper and then apply it to the graphical scale on the map.

Lesson 2. (Field Work—five Hours.)

Problem—Stride Scale Map Making.

Producing a map from the actual ground requires certain instruments. The second lesson takes up the preparation of the stride scale on the alidade and the different kinds of maps, made in military sketching.

The alidade is a triangular ruler with one or more working scales on it beside other measurements. The working scale is, for infantry, the stride or the space of ground covered from left foot to left foot again in walking, reduced to the proper map distance. This varies with individuals of course. Any scale of units, however, can be used as, horse trot, telegraph poles, etc.

The working scale for each man is made by having him step off a measured course, say 440 yards. The ground should not be too even as a general average is needed, moreover the pace must be the natural gait of the individual under ordinary circumstances. Let him count the course three times then average the three results for the final estimate.

Now to convert this into a working scale for the alidade made on a scale of six inches to the mile; take the case of a man who takes 220 strides in 440 yards:

440 yards = 15,840 inches

15,840 ÷ 220 = 72, or his stride in inches

then 880 strides = 1 mile or 6" on the scale.

It is better to have a scale of 1,000 strides which is easily done by the proportion:

1,000 sts. : 880 sts. :: x : 6

x = 6.8

now draw a line 6.8 inches long and a diagonal line from it; divide this diagonal line into 10 equal parts for each 100 paces at any convenient scale and draw a line from the end of the tenth part to the end of the 6.8 inches line; draw lines parallel to this line from each of the divisions. The 6.8" line is then divided into 10 equal parts; each of these parts may be divided in the same manner into tenths.

Your scale is ready to be pasted or transferred to the alidade and each 6.8. inches on the map will equal 1,000 of your strides on the ground, or about 1-1/12 miles (2,000 yards).

There are two general classes of sketches:

1. Road Sketch.—A traverse (passing over) made along a definite rout showing all features of military importance for a distance of 200 or 300 yards on each side of the road. A road sketch is always made on a scale of 3 inches to 1 mile.

2. Area Sketch.—A map of a definite locality. There are 3 kinds of area sketches according to opportunity for observation:

Position Sketch—when access may be had to the whole area.
Outpost Sketch—where part of the ground must be mapped without passing over it. This form is applicable particularly to trench warfare. Intersection and resection are used to locate points within the enemy's lines.
Place Sketch—when sketch must be made from one point, as when the proximity of the enemy would prevent any movement; as from trench observation stations, etc.; also an elaboration of the landscape or horizon sketch which is used everywhere in the trenches today. From one point an actual outline of the opposite trench and background is made in perspective, reference points on the horizon being marked on the edge of a pad at arm's length. These marks are then prolonged on the paper and the horizon is sketched. In like manner the middle distance and the foreground come under observation and are put on in one below the other.

Time must be allowed the men to make their stride scales and to paste or transfer them to their alidades.

Lesson 3. (Field Work—five Hours.)

The problem is to make a Position Sketch about one mile square closing the traverse. First considering the sketch board, compass, pencils, etc.; next the orienting, sighting and pacing; finally the uses of intersection and resection and in making allowance for error.

The sketch board should be about 12 to 18 inches square, being used with or without a tripod. A cheap camera tripod is excellent. The board should have a compass attached so that it will remain in the same relative position on the board. If iron thumb tacks are used avoid getting them too near the compass. A hard pencil must be used to obtain good results. The paper must be smooth and where possible covered with another sheet fastened on but one side which will readily fold back when one desires to work on the sketch.

By always placing the board so that the compass reads North it will be oriented correctly. Care must be used when near electric wires or masses of metal as automobiles, railroad tracks, etc., which will attract the needle from its true azimuth (N. and S. direction) and thus throw off the whole map. In such cases it is far better to back sight and use the compass only at intervals to verify the sights.

This brings up the matter of sighting. It is important to make long shots thus reducing the amount of individual error. In taking a sighting point make sure it can be recognized when reached and make sure to look at the reverse side in order to recognize it in case of back sighting if necessary. Always carry several large-headed pins using one at your present station and resting the side of the alidade against it, swinging the other end for sighting.

After sighting and lining the sight on your sketch, step off evenly to pace the distance. Time is always a factor in military mapping and where possible make mental notes as you go along as to where roads or other important features are located, so that you can place them in their proper place on the map when you have reached the next station. It is well always to set a good pace for here time can be readily saved.

Making an intersection is very simple. For as the sketcher moves along he ties his map together by sighting at any prominent object near his area, running these lines very lightly and only where he assumes the points to lie on his map. An abbreviation on the line or a number referring to a list off to one side will answer to recall the object. At any other station where the same point can be seen a similar line is drawn and where the two lines cross will be the location of the object. In the case of three lines not crossing at the same point take the middle of the triangle so formed.

Resection is just the reverse of this process. The mapper wants to know where he is located on the map. If he is properly oriented and can aim at two points on the ground which he has located on the map, he places a pin at one of these locations on the map and aims with the alidade at the object on the ground drawing a line towards himself; this is repeated with the other known point and where the two lines cross on the map will be the point he is standing at.

In intersection the greatest accuracy is obtained by running the rays so as to meet as nearly as possible at right angles.

In running a traverse the sketcher must expect to find some error at his closing point. This error must be distributed over the whole traverse so as not to have all the error concentrated at one point.

Lesson 4. (Field Work—four Hours.)

Problem.—Make a simple sketch, containing topographical details using the traverse made during the preceding lesson. Use of conventional signs should be emphasized and the appreciation of features of military importance impressed. A tendency is to put in details to a point of confusion. Judgment must be developed to choose telling points.

A sharp pencil is always needed in sketching; in putting in the topographical details special attention must be given to the pencil. Keep the point sharp and make clear, distinct signs.

Lesson 5. (Class Room—four Hours.)

Problem.—Contours, the Vertical Interval, Use of the Slope Board, Map Distance, Visibility and Profiles.

A contour is an imaginary line on the surface of the earth all points of which have the same elevation from a base or datum level, sea level usually being this base. Slice an apple into pieces ½-inch thick; where the cuts come may represent the contour lines. Take these individual slices, beginning at the bottom and outline them on a sheet of paper with a pencil (having run a nail through the apple first to keep each piece in place). The resulting circles will represent the apple's outline at ½-inch intervals.

Contours are always at equal elevations from each other, and the Vertical Interval (known by the abbreviation V.I.) is the measure between successive contour lines. In military maps the V.I. is always the same for each map scale:

1 inch to the mile, the V.I. is 60 feet.
3 inch to the mile, the V.I. is 20 feet.
6 inch to the mile, the V.I. is 10 feet.
12 inch to the mile, the V.I. is 5 feet.

Note that the V.I. changes in proportion to the scale, a map on a 3 inch to the mile scale is 3 times as large as one on a scale of 1 inch to the mile, while the V.I. is 1/3 as great, hence the former shows 3 times as many contours as the latter.

Map Distance means the horizontal distance between two contour lines on a map and indicates a certain degree of slope. As the scale increases the V.I. decreases in proportion and the M.D. therefore remains the same for the same degree of slope whatever the scale of the map. By computation we find that a one degree slope rises one foot for every 57.3 feet horizontal distance, so a one degree slope would have a 20 foot rise in 1,146 feet horizontal distance, this distance equals .65 of an inch on the map if the scale is 3" to 1 mile.

The term "Map Distance" is also loosely used to denote distance between points as measured on the map. Care should be taken to distinguish between these two meanings.

Distances between contours, scale 3" to 1 mile: ½° slope = 1.3", 1° slope =.65", 2° slope =.32", 3° slope =.22". These distances are already on the alidade and if you get a slope of 2° with the slope board and have the distance from your station on the map to the point of aim either by pacing, intersection or resection, apply the M.D. scale as many times as it will go. This will give the number of contour lines crossing the traverse and the difference in elevation. The spacing of the contours may not be even between your station and the point of aim in which case the position of the contours must be estimated by eye.

If your elevation above the datum or sea level is unknown at the start assume any elevation which is great enough to put the datum lower than the lowest spot of the area to be sketched.

The sketching board is easily made to serve as a slope board in this manner. Hang a plumb bob about an inch below the center of a straight edge of the board while pointing at the horizon, using the back of the board. Mark a point 5.7" directly below and draw a semicircle through it with the same radius. Now mark the point below the center zero and from it divide the arc using chords one tenth of an inch long. This will give a scale reading in degrees. By sighting along the top of the board at some object at the height of the eye from the ground the degree of slope is shown by the plumb bob on the scale below. Care must be exercised to prevent the wind from disturbing the reading. A protractor may be used in the same manner by sighting along the top and using a plumb bob to record the angle.

In reading maps it is important to know whether points are visible from each other due to intervening ridges or other topographical features. This can be told by laying off accurately the distance on the map between the points in question and using as datum the lowest of the 3 points, then draw vertical lines, from the 2 higher points, making them in proportion to their elevation with any convenient scale. Draw a line between the first and last points and, if the intervening vertical cuts this line the second point is not visible from the first. Take for example, two points A and B, 1,760 yards apart, by the map, A 500 feet and B 450 feet above sea level, the intervening point C is 475 feet above sea level and 500 yards from B. As B is the lowest we will call its elevation zero or at datum, then elevation of A is 50 feet and C 25 feet.

Another method of deciding visibility is by proportion. Measure the distance between the three points A, B, and C, and obtain their elevations above the datum (lowest of the 3) and using similar triangles. Take the same case as above, letting X represent the point above which the view is clear at 1,260 yards from point A, the line of sight passes through this point.

1760 (A—B) : 500 (B—C) :: 50 (elev. A) : X
solving, X = 14.2

Now, since the ground at point C is 25 feet above the base and the line of sight passes within 14.2 feet of the base at this place, an observer at A is unable to see B.

The matter of profiling is very simple. Merely mark where the contours cut the edge of a piece of co-ordinate paper and extend the proper elevations, then pass a line through these points, remembering that the surface of the ground has a natural curve.

Lesson 6. (Field Work—five Hours.)

Problem.—By use of the slope scale on the sketch board and the contour interval scale on the alidade, each man will secure vertical data on the flat sketch made in the fourth lesson. Certain critical elevations will be determined and marked with red flags before hand. The elevations of two points on the ground will be furnished, one as the datum and the other as a check. Draw in contours of this sketch with the help of drainage lines and elevations already secured.

The chief points to be considered are to take slopes from points established on the sketch; to take several sights and average the angle of slope; to properly lay off the elevation by using the slope scale on the alidade; and finally to put in the contours along these lines of sight on the spot thus allowing for difference in topography between the point of sight and the station from which the elevation is taken. Careful note must be made of the drainage systems as these are the keynotes to the sketch and finally the contours are connected together, keeping in mind always that no contour stops unless it makes a closed curve or goes off the map. Remember also that contours make fingers pointing up stream and are blunt around hill sides. Contours cross streams to opposite points and break at roads, continuing on the other side. Uniform slopes have equally-spaced contours. Do not try to measure every slope, two intersecting elevation sights on a hill will check the height. Put the intervening contours in by eye.

Lesson 7. (Classroom—four Hours.)

Problem.—Completing the map sketch previously made and making a landscape sketch.

It is important to complete a map and no matter how good it is, if certain points are omitted, the value of the work is very much decreased. The sketcher must clear the sketch of all unnecessary lines and notes and make his lettering clear on the map. Be sure that the following items are on the sketch before it is turned in.

Location of the ground shown.
Line of magnetic north shown by an arrow, and if declination is known, the true north also.
Graphic scale and representative fraction—R.F.
Vertical interval—V.I.
Sketcher's name and organization to which he belongs.
Date.

A landscape sketch is a place sketched with details shown in perspective. The horizon is always of military importance and should be shown as well as intervening crests, woods, houses, etc. Landscape sketching in trench warfare is a necessary accomplishment of the observer. The beginner will at first be confused by a mass of details, but he must note only the outline of the features sketched. First draw the sky line and crests, then fill in the other details with fewest lines possible. Unnecessary shading tends to detract from the clearness of the sketch. There will be great difficulty in getting the perspective, note the size of objects, the further away they are the smaller they seem. Make them so. In making the sketch, hold the pad in front with one eye closed, the upper edge of the pad horizontal; a string 20 inches long is tied to the pad and held between the teeth to insure the same distance from the eye each time. Moreover, if it is desired to locate objects by deflection of an angle from a reference point, this can be done by using mils. One mil is 1-6400 of a circle. At 2 inches a half-inch interval subtends 25 mils.

The paper is oriented by bringing the sector desired along the upper edge of the pad. The points desired are then in proper positions, both horizontally and vertically.

Place a mark at the upper edge for points desired. The sky line should be located first. Now carry these lines down, having drawn three horizontal lines about ½ inch apart, beginning with the highest point on the top line. Marks locating the other features are likewise transposed in vertical and horizontal portions.

Now draw sky line connecting transposed marks, then such other points as crests, trenches, houses, etc. After practice most other features can be drawn in without reorienting, the sky line having been drawn. The vertical elevation should be slightly exaggerated. Objects in the background should be drawn in lightly while nearby features are indicated by heavy lines. Avoid details, draw only silhouette, shade only in showing woods.

Lesson 8. (Field Work—five Hours.)

Problem.—Make complete area sketch including contours, with no data furnished other than the initial elevation.

Before commencing the work summarize the important points involved.

If possible select a base line.
Locate as many points by intersection as possible.
Make traverse by road, check locations by resection.
At good observation points observe and complete the sketch as far as possible.

At each station keep the following points in view:

Back sight on previous station.
Select new sighting point ahead.
Determine elevation by slope board.
Put in contours where possible noting the drainage and critical points of the general slope and the terrain.
Put in details along traverse just made of all topographical features of military importance.
Determine your present elevation.
Make as many shots for intersection as you can and mark them.
Look for possible resection shots.

Lessons 9 And 10. (Field Work—nine Hours.)

Problem.—Make a road sketch of about 12 miles with scale of 3 inches to the mile, V.I. 20 feet. This should include details of military importance to a distance of 300 yards on either side of the road.

Keep in mind these points:

Start carefully and give attention to every part of the map.
Keep the board properly oriented.
Watch the water drainage systems.
Put down all necessary details at each setup.
Note high hills and towns not on immediate route, condition of roads, fences, cultivation, hedges, cuts and fills, bridges (kind and length), railroads, telegraph and telephone lines, schools, churches, etc., notice particularly woods and points of concealment for hostile troops.

Do the work at each station for elevation, contours and the noting of necessary details so that the sketch will be complete as you go along. Make certain that the title of the sketch, scale, orientation, etc., are all clearly indicated, for a road map may have to be completed by another or may be called for suddenly when it will be useless without these details.

Remember there are but two things absolutely essential to a good road sketch; a good traverse and the location of the drainage system in its relation to this traverse. With this control approximate contours can be drawn by anyone having a knowledge of the principles of topography. Never plot unimportant details. Prominent buildings and farm houses are of value for locating oneself. Woods and orchards are shown for tactical reasons but no one can expect to show every fence, ditch or bit of cover that might hide a patrol.

Map Reading.

(Gettysburg 3" Map—hunterstown Sheet.)

Plattsburg Barracks, N.Y., Sept. 17, 1917:

What is the shortest distance by road from Biglersville to Texas?
Describe the road between Texas and Table Rock.
Is it a cut or a fill along the railroad about ½ mile east of Granite Hill Station?
What is meant by 931 on Chestnut Hill?
Can a man on the summit of hill 712 (about one mile southwest of Plainview) be seen from the town of Plainview?
Point out two fords on the Conewago River.
Where is the highest point on the road from Plainview to Heidlersburg?
Describe the fences along the road from Texas to Table Rock Station.
Is Hill 566 S.W. from D. Wert visible from Henderson Meeting House?
Of what material is the bridge at Bridge School House constructed?

Harvard College:

Can a sentinel standing at 707 see road fork 535 (about 1,500 yards south)?
An enemy patrol is marching north on the 544-616 road, and has crossed the stream (750 yards north of 544.) Can this patrol see the Red outguard at 707 from any point between stream and cross roads 616?
Can the sentinel at 712 see the road fork 518 (1,850 yards southwest from 712)?
Can the sentinel at 712 see the cross roads 561 (about 1,200 yards southeast)?

Assuming the height of a man as 5' 0" above the ground and trees and buildings as 30' 0".

Is the ground at road fork 552 near D. Wirt visible to a patrol on Hill 712? If not what is the obstructing point? Turn in profile, using cross section paper.
Disregarding trees, is a man standing on Bridge 523 near Bridge S.H. visible from Hill 712?

Solve by any method desired indicating the method.

Make a profile from location of the letter "U" of Chestnut Hill near Center Mills to Hill 712, 2-½ miles to the south.
Is the location of the letter "B" of Beatrich visible from "U" of Chestnut Hill? If not what obstructs?

Can a man on Hill 712 see a man at cross roads 554 in Hunterstown (disregard trees)?
To a man standing at the point where contour 680 crosses the road just south of 707, where does the roadbed first become invisible?

When the point arrives at Hill 647 can it see the road fork 610 to the northwest?
When the flank patrol reaches Benders Church cross roads can it see an enemy patrol at the house midway on the road 534-554 one mile to the northeast?
Looking north along the Center Mills road from Hill 647, where does the road first become invisible?

What does 1/21120 mean?
What direction is the general drainage system on this sheet?

CHAPTER 6.

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