Manual of Military Training Part 151

+-------------------------+-----+-----+-----+-----+-----+-----+-----+ |Slopes | 0| 5| 10| 15| 20| 25| 30| +-------------------------+-----+-----+-----+-----+-----+-----+-----+ |Length of step ascending | 30.4| 27.6| 24.4| 22.1| 19.7| 17.8| 15.0| |Length of step descending| 30.4| 29.2| 28.3| 27.6| 26.4| 23.6| 19.7| +-------------------------+-----+-----+-----+-----+-----+-----+-----+

For the same person, the length of step decreases as he becomes tired.

To overcome this, ascertain the length of pace when fresh and when tired and use the first scale in the morning and the latter in the afternoon.

The result of the shortening of the pace due to fatigue or going over a slope, is to make the map larger than it should be for a given scale. This is apparent when we consider that we take more paces in covering a given distance than we would were it on a horizontal plane and we were taking our normal pace.

In going up or down a slope of 3 or 4 we actually walk 5 units, but cover only 4 in a horizontal direction. Therefore, we must make allowance when pacing slopes.

In counting paces count each foot as it strikes. In counting strides count only 1 foot as it strikes. A stride is two paces.

In practice it has been found that the scale of strides is far more satisfactory than a scale of paces.

=1888. How to make a scale of paces.= Having determined the length of our pace, any one of the following three methods may be used in making a working scale:

_1st method._ The so-called "One thousand unit rule" method is as follows:

Multiply the R. F. (representative fraction) by the number of inches in the unit of measure multiplied by 1000; the result will be the length of line in inches necessary to show 1000 units.

For example, let us suppose that we desire a graphic scale showing 1000 yards, the scale of the map being 3 inches equal 1 mile:

Multiply 1/21120 (R. F.) by 36 (36 inches in 1 yard, the unit of measure) by 1000,--that is,

(1/21120) 36 1000 = 36000/21120 = 1.7046 inches.

Therefore, a line or graphic scale 1.7 inches in length will represent 1000 yards.

If we desire a working scale of paces at 3 inches to the mile, and we have determined that our pace is 31 inches long, we would have (1/21120) 31 1000 = 31000/21120 = 1.467 inches.

We can now lay off this distance and divide it into ten equal parts, and each will give us a 100-pace division.

_2nd method._ Lay off 100 yards; ascertain how many of your paces are necessary to cover this distance; multiply R. F. by 7,200,000, and divide by the number of paces you take in going 100 yards. The result will be the length of line in inches which will show 2000 of your paces.

_3rd method._ Construct a scale of convenient length, about 6 inches, as described in Par. 1863, to read in the units you intend to measure your distance with (your stride, pace, stride of a horse, etc.), to the scale on which you intend to make your sketch.

For example, suppose your stride is 66 inches long (33 inch pace) and you wish to make a sketch on a scale of 3 inches = 1 mile. The R. F.

of this scale is 3 inches/1 mile = 3 inches/63360 inches = 1/21120.

That is 1 inch on your sketch is to represent 21120 inches on the ground. As you intend to measure your ground distances by counting your strides of 66 inches length, 1 inch on the sketch will represent as many of your strides on the ground as 66 is contained into 21120 = 320 strides. For convenience in sketching you wish to make your scale about 6 inches long. Since 1 inch represents 320 strides, 6 inches will represent 6 320 = 1,920 strides. As this is an odd number, difficult to divide into convenient subdivisions of hundreds, fifties, etc., construct your scale to represent 2,000 strides, which will give it a length slightly in excess of 6 inches--6.25. Lay off this length and divide it into ten main divisions of 200 strides each, and subdivide these into 50 stride divisions as explained in Par. 1862.

=1889. Position sketching.= The following are the instruments used in position sketching:

1. Drawing board with attached compa.s.s (Fig. 2);

2. Loose ruler, on board (Fig. 2);

3. Rough tripod or camera tripod;

4. Scale of M. D.'s (shown on ruler, Fig. 2);

5. Scale of sketchers, strides or paces (at six inches to one mile), on ruler;

6. Clinometer (not necessary if board has slope board, Fig. 6);

7. Scale of hundreds of yards shown on ruler;

8. Scale of paces.

Methods to be used

(1) Select a base line,--that is, a central line 1/4 to 1/2 mile long in the area to be sketched. It should have at its ends some plainly marked objects, such as telegraph poles, trees, corners of buildings, etc., and from its ends, and intermediate points, a good view of the area should be possible. The base line selected should be capable of being measured.

(2) Set up, level and orient the drawing board at one end of the base (A), Fig. 1, Y, for example. Draw a meridian on the sheet parallel to the position of the magnetic needle. a.s.sume a point (A), Fig. 1, Y, corresponding to the ground point (A), 890, on the sheet, in such a position that the area to be sketched will lie on the sheet.

(3) Sight at hilltops, stream junctures, stream heads, etc., to begin the locations of these points by intersection, labelling each ray so as to be able to identify it later.

(4) Traverse to (b) and complete the locations by intersection as previously explained. If the base line is not accurately measured, the map will be correct within itself in all of its proportions, but its scale will not necessarily be the scale desired.

(5) Draw the details of the country between A and B and in the vicinity of this line, using the conventional signs for roads, houses, etc.

(6) The lines from station (b), Fig. 1, X, to any of the other located points may now be used as a new base line to carry the work over additional area.

(7) In case parts of the area are not visible from a base line, these parts are located by traversing as before explained.

(8) Having learned by several repet.i.tions the above steps, the sketcher will then combine contouring with his horizontal locations.

=1890. Outpost sketching.= The same instruments are used as in position sketching, and so are the methods the same, except that the sketcher cannot advance beyond the outpost line, toward the supposed position of the enemy. It is often possible to select a measurable base line well in rear of the line of observation,--for instance, along the line of resistance. Secondary base lines may then be taken on or near the line of observation, from the extremities of which additional base lines may be selected, if necessary, and points toward the enemy's position located by intersection. Details are sketched in as in position sketching. For obvious reasons, no traversing should be done along the line of observation.

=1891. Road sketching.= The following are the instruments used in road sketching:

1. Drawing board or sketching case;

2. Loose ruler;

3. Scale of strides, or paces, if made dismounted; scale of time trotting or walking, if mounted;

4. Scale of hundreds of yards, at three inches to 1 mile;

5. Scale of M. D.'s;

6. Slope board (if clinometer is not available).

Methods to be used

(1) At station 1, Fig. 10, orient the board as described in par. 1872, holding the board in the hands, in front of the body of the sketcher, who faces toward station 2.

[Ill.u.s.tration: Fig. 10]

(2) Important points in the vicinity, such as the railroad bridge, the stream juncture, hilltops, are sighted for intersections, lines drawn as shown and the sketcher traverses to station 2.