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: 1/2 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 s.p.a.cing 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 a.s.sume 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.

[Ill.u.s.tration: Plate #10]

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 pa.s.ses 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 pa.s.ses 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 pa.s.s 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-s.p.a.ced 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. (CLa.s.sROOM--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.

1. Location of the ground shown.

2. Line of magnetic north shown by an arrow, and if declination is known, the true north also.

3. Graphic scale and representative fraction--R.F.

4. Vertical interval--V.I.

5. Sketcher's name and organization to which he belongs.

6. 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 ma.s.s 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 20 inches a half-inch interval subtends 25 mils.

[Ill.u.s.tration: Plate #11]

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 1/2 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.

1. If possible select a base line.

2. Locate as many points by intersection as possible.

3. Make traverse by road, check locations by resection.

4. At good observation points observe and complete the sketch as far as possible.

At each station keep the following points in view:

1. Back sight on previous station.

2. Select new sighting point ahead.

3. Determine elevation by slope board.

4. Put in contours where possible noting the drainage and critical points of the general slope and the terrain.

5. Put in details along traverse just made of all topographical features of military importance.

6. Determine your present elevation.

7. Make as many shots for intersection as you can and mark them.

8. 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:

1. Start carefully and give attention to every part of the map.

2. Keep the board properly oriented.

3. Watch the water drainage systems.