U G ,!if,SrNnLF 557 t ements of Military Sketching Digitized by the Internet Archive in 2007 with funding from IVIicrosoft Corporation http://www.archive.org/details/elementsofmilitaOObarnrich ELEMENTS of MILITARY SKETCHING ARRANGED FOR USE OF THE ORGANIZED MILITIA OF VERMONT By 1st Lieutenant John B. Barnes Fifth U. S. Infantry Copies may^be obtained through The U. S. Infantry Association, Washington, D. C, at fifty cents per copy. Special price if ordered in quantities. Copyrighted 1913 by JOHN B. BARNES -3i PREFACE. The diagrams herein were executed by Sergeant W. Weiden- bach, Company "K", 5th Infantry, to whom credit is also given for valuable assistance in originating and preparing the text. The publication of this pamphlet was undertaken with a view of supplying a simple and short treatise devoted exclusively to the important subject of Military Sketching. While intended, pri- marily, for the use of the Organized Militia of Vermont, it is thought that it may be of value to the militia of other states. 281998 Elements of Military Sketching. MAP SCALES. 1. A map is a graphical representation of a portion of the surface of the earth. A 7nap is made with the proper survey equipment and is supposed to be accurate. A sketch is as nearly an accurate map as can be made by field drafting with the time and instruments available, and frequently with restricted recon- naissance. Distances on a map have a fixed proportion to distances on the ground represented. In order to read or make a map it is, therefore, necessary to know the proportion, or ratio, existing between any given map distance and the actual ground distance represented by it. This ratio is expressed by means of map scales. The principle of scaling is that one unit of any length on the map represents a certain number of like units on the ground. 2. This ratio existing between map distance and ground distance may be expressed in three ways : ( 1 ) In words and figures, as : 3 inches = i miles. (2) By what is known as ''representative Fraction" — a fraction, the numerator of which shows units of length on the map, while the denominator shows the corresponding distance on the ground, thus: Representative Fraction, (R. F.) , means that one 21120 inch, or one unit of any measure on the map, represents a distance of 21 120 inches, or units of that measure, on the ground. If the scale of a map were 4 inches to the mile, then 4 inches (map) would represent 63360 inches (mile) on the ground, 4 (map-dist.) or 63360 (ground-dist.) As the R. F. is usually written with numerator unity, we would have : R. F. -^ — = R. F. 63360 • 15840 Emphasis is laid upon the fact that R. F. is internation- ally used; this circumstance enables us to readily determine the ElvKMENTS OF MlUTARY SkKTCHING. scale of foreign maps in inches to the mile. (3) The scale may be represented graphically. A graphical scale is a plotted line of certain length to represent a given ground distance. Instead of writing 3 inches = i mile, we may draw a line three inches in length and mark it i mile. Subdivisions of this line represent shorter distances and are marked with their respective values. The scales of American (military) maps are: I inch (approximately) to the mile, or R. F. 62250 (Geological Survey Maps) : 3 inches to the mile, or R. F. (Scale generally used 21120 for road sketches and area maps). 6 inches to the mile, R. F. (Scale used for posi- 10560 tion sketches.) 12 inches to the mile, R. F. (Scale used for forti- 5280 fication-plans, war game maps, etc.) Less frequently used are the scales 2 inches to the mile, I I (R. F. ) and 4 inches to the mile, (R. F. -). 31680^ 15840^ Construction of Graphicai. Scai.es: A first requirement in map-making or map-reading is a knowledge of scales. Assume I that a map has R. F. (3 in. = i mile) but no graphical 21 120 scale. In order to measure off any definite distance on the map it is necessary to have a graphical scale of 3 in. to the mile. This scale may be constructed as follows : Draw a line 3 inches long ; this line, representing one mile, should be divided into 1760 equal parts — each part representing i yard — but as such division can Elements of Military Sketching. scarcely be made without special instruments, a division into 17 6 parts is sufficient. Each subdivision then represents 100 yards ground-distance. In constructing this, or any scale, a simple method, based upon a principle of plane-geometry can be advantageously follow- ed, viz: "If an angle is cut by a series of parallels, the correspond- ing segments of the legs of the angle are proportional." The angle B-A-C is cut by parallels; thus proportional di- visions are produced on both legs of the angle. (Diagram i). In Diagram la, A-B is three inches, the length of the de- sired scale. To divide A-B, a fixed line, into 17 ^ parts, we simply divide A-C, a line of indefinite length, into 17 ^ parts, and draw parallels through the division-marks on A-C to A-B. The advantage of this method is that the divisions on A-C can be of any convenient length. EXAMPLES. Let us assume the scale of a map to be 4 inches to the mile, I (R. F. ). A graphical scale is desired. 15840 We know that four inches represent 1760 yards. Select a more convenient number of yards as 1000, 1600, 2000, etc. We can solve the problem by proportion; if 4 inches represent 1760 yards, how many inches represent 1000 yards? 4 : 1760 : : X : 1000 . x = 2.27. A line 2.27 inches in length is drawn and divided into equal parts, using parallels as in Diagrams i and 2. In this case A-C may be 2y2^^ long. Divide A-C into ^/^ inch lengths from your ruler and draw parallels from these points through A-B. A-B is then divided into 200 yard lengths. By estimation, or parallels, subdivide the left extension to read small- er distances, and erase line A-C and parallels. Similiar proportions can be formed with any convenient number of yards as given ground-distances. The scale can also be computed as follows : Elements of Military Sketching. R. F. that is, 3 inches = i mile (1760 yards). 21120 1760 I inch = = 586 + yards. 3 A graphical scale 3 or 4 inches long is desired. Select a convenient number, as 2000 yards. If 586 yards are represented by i inch, how many inches represent 2000 yards ? 2000 — —= 34 inches. 586 4. The scale reading yards, meters, miles, etc., usually found on a completed map is called "reading-scale." When making a map, however, a so-called "working-scale" is necessary. In mak- ing military maps, particularly road or position sketches, the distances on the ground may be measured by pacing, taking the time of a horse's trot, counting telegj-aph-poles with reference to their interval, counting revolutions of a wheel, by speedometer, odometer, etc. In any case it will be necessary to have a "work- ing-scale" — ^that is, a scale of the units employed in measuring the distances. The units of the ''reading scale" have a fixed and standard value such as yards, meters, miles, etc., while the units of any "working-scale" used in making a map depend upon the individual and the means at hand for measuring distances. The working scale does not appear on the completed map. It is desired to make a road sketch 3 inches to the mile, distance measured by pacing. Assume the stride of the mapper to be 60 inches. He will cover 100 yards with 60 strides To construct the working-scale, we use the same method em- ployed in constructing a reading-scale, but changing the value of the scale-divisions as expressed in yards to the corresponding number as expressed in strides or paces, thus ; 100 yards = 60 strides. 120 strides = 200 yards, etc. (It is advantageous to combine the working-scale with reading scale, as in Diagram 3.) This scale would usually be computed as follows : I stride = 60 inches ; 1 = 1056 strides to 60 the mile, hence 3 inches represent 1056 strides. fQ "'v ■s •-, S \ \ \ / \ \ \ /v \ 'V /V < n^oal ^^ oox ooSl- 7 00 5- oo*r' *7 / 7 oot -f ^/ GO'S ool o '< C9/I c9^l 9^0. - co/l oroi — ~ ce9l oc9l 0.^ - -r>oS( 00 coir — -oo»rJ — 00^1 oj,?r - -T?c9< - OC^I oU " -opTI - co7l c?^ - -ooH — OOll o9^ - —vooi d cool (/3 OCl'^' - coh — ooB d Q -00/ ~ OOi. o7h - -oo<) - 009 o^C " — t>oi — oc^ ooC~ -ooir ~ oo-h O-t.7 ■ -oof ooZ 08I - - ooZ - 0015 oti • - 00! — ool 09 - » . " ,0 . o 1 Z3.0 Tto i 660 1 YARDS 1 MOO 1 132.0 176« o 1 z 1 3 J — MINUTE5 1... . J i 7 J a DiAG. 4 ISOLATED. FOREST TREES. PIME. ORCHARD. PALMS. €RA5S CORjsI r^ CULTr/ATED. PLOUGHED BOADS IMPROVED:: TSATLRQADS. SINGI£ TRACK: -^ UNIMPROVED: ------zz:z- lX)UBLETRAai. • ■ I 1 TRAIL- ELECTRIC ' ' ■ i ■ FILL: cut: VINE-X^D RICE. IT' FENCES. — HEDGE . s?2E5?.Ts--.2Er5 !=« STONE ,3^^z=x=^ =1=1 worm: TELEGRAPH' t T T 7 T WIBE, BARBED. STEEAMS UNDER I5'VVIDE : FOlRIi^LE UNFORDABLE ■ CAiMjrr. [:il ^-miLLEKf: iji iji i] SEOTRY. INFty- ti SENTKf.O^'.: 5 RAVINE- 'rZTU:^?^^^ hospital: "^"'^'''-^'■'" " trench; DEF!RESS10N . cAMP: AAA WIRE.smooth: — ^-* — — ABMTTIs: ^r V^rv VVIREEN- U TANSLEMENI:!^ PALISADE DEMOLITION. 1^;^ aiFT CHURCH i post orncE PO. VIOTER-WDKK WW. HOUSE TOWN CEMETERT. [173 DiAQ. 5. Elements of Military Sketching. 3 : 1056 : : X : 1000 . x = 2.84". Suppose by actual experiment over a measured course, a horse has been found to trot a mile in 7 min. 40 sec. Then 7 % min. = 1760 yards. We desire to make a working scale 3" to the mile. Take 10 min. as a convenient number. Then S ' 7 % • * X : 10. X ^ S-Q"- Construct a scale 3.9 inches long divided to read the fraction of a minute desired. As these data vary with the individual and the means used it is, therefore, necessary to determine carefully the length of the measuring unit before making the working scale. When preparing a working-scale for pacing, much atten- tion should be given to determining correctly the length of the pace in inches. In testing the number of paces over a measured course, the gait and length of step should be natural. A mea- sured course of 100 yards may suffice, but a mile course over an undulating road will give more accurate results. The mean of several trials will give still greater accuracy. The same applies to a scale of minutes. By careful tests it must be ascertained how many minutes a horse requires to cover a given distance at a trot or walk. The average horse covers one mile in 8 minutes at a trot^ and in 16 minutes at a walk. (Diagram 4). (Note) — Prepared Working Scales of any length of pace, or minutes of a horse trotting can be obtained at a nominal cost from the Book Department, Service Schools, Ft. Leavenworth, Kans. CONVENTIONAL SIGNS. 5. The purpose of a military map is the graphical represent- ation of ground with constant reference to probable military use. It should, therefore, not only be true in regard to distances and directions, but should also give a comprehensive picture of the condition of the ground as to cultivation, geological formation, and other features of military interest, as railroads, telegraph lines, bridges, etc. Consequently it has been necessary to invent characteristic graphic signs, (called Conventional Signs), to re- present all objects of military importance, and the form of the ground itself. The more common conventional signs used in military topo- graphy are given in Diagram 5. 10 Elements oe Military Sketching. Many of these conventional signs are used only in elaborate topographical work. For hasty sketching, (road or position- sketches), the simplified conventional signs as given on pages 209 and 210, Appendix C, Field-Service Regulations 1910, an- swer all purposes. Grass, trees, cultivated land, woods, etc., are therein more simply represented by drawing the outline of the particular feature and placing inside the appropriate symbol or letter, (Diagram 5a). This method is recommended in all re- connaissance or hasty sketching. Diagram 6 indicates the character of stream, span and con- struction of the bridge. (Stream 20 feet wide, 3 feet deep and fordable. Wooden queenpost post bridge 35 feet long, 14 feet wide and 10 feet above the water.) "The following abbreviations are authorized for use on field maps and sketches. When these words are used they must be written in full or abbreviated as shown. The abbreviations must not be used for words other than those in the table. Words not in the table are not as a rule abbreviated". (F. S. R. 1910). abut. abutment P.O. postoffice B.S. blacksmith shop Pt. point bot. bottom Q. quarry Cr. creek q.p. queen post cul. culvert R. river cult. cultivated R.H. round house d. deep R.R. railroad E. east S. south f. fordable s. steel gir. girder S.H. school house G.M. gristmill S.M. saw mill i. iron Sta. station I. island St. stone jc. junction str. stream kp. • kingpost tres. trestle L. lake tr. truss Mt. mountain W.T. water tank N. north W.W. water works n.f. not fordable W. west p. pier wd. wide pk. plank w. wood CO o o (/D g uu i «/0 o DiAG. 5a. DiAG. 6. E.X/\MRLE or f^aCONN/KiSZ^NCe ^/ Elements of Military Sketching. ii ORIENTATION. 6. Orientation is placing the map in its true relation to the ground represented ; in general, the location of any point in relation to the north and south of the map as determined by the magnetic needle. As a rule maps have a north and south line. It should be known, however, that the compass-needle does not in all localities point true north, but usually varies several degrees East or West. This is called ''magnetic variation" (deviation, declination). Consequently there may be two direction-lines on a map, the "true" north and the ''magnetic" north. (Diagram 8.) In this instance the magnetic needle has a deviation of 14' 26' 30" East from the true north or true meridian. The true meridian lies in the direction of the North Pole. It is a datum-line, a constant, whereas the N. & S. Line as indi- cated by the compass-needle is variable because of magnetic influences, depending on the locality and the time. The magnetic meridian is usually represented by a spear, one side of the point of which is missing, while the true meridian is represented by the spear with point completed. To orient oneself it is only necessary to bring the magnetic • North of the map to point in the same direction as the north end of the compass needle and then the bearings of lines on the map and corresponding lines on the ground will be the same. That is, lines on the map will be parallel to the lines on the ground represented. In Diagram 9, a road A-B is plotted on the map ; the magnetic N. and S. line is indicated. You are standing on the road and desire to orient the map. Place the compass on the map so that the line marked N. and S. on the dial is parallel to and agrees with the direction line on the map. Turn the map (without disturbing the position, on the map, of the compass box) until the north end of the needle points towards N. The map is now oriented, and A-B on the map has the same direction as the road on the ground. Diagram 10 illustrates, that, if the compass-ntedle and the direction line on the map do not coincide — no coincidence in the direction of the roads is obtained, and the map is not oriented. 12 ^ Elements of Military Sketching. The true meridian can be determined by means of a watch, as follows : Place the watch face up with the hour hand pointed towards the sun. A line midway between the hour hand and the number XII will lie in the direction of the true meridian. The north and south ends of this line are evident from the hour of the day and the position of the sun. (To bring the hour hand in the direction of the sun, hold a straw perpendicularly between the sun and the watch, and bring the hour hand in its shadow.) (Diagram ii.) With the true meridian on the map the magnetic declina- tion of the compass-needle can be ascertained as follows : Bring the N. & S. line as indicated on the compass-face to coin- cide with the true meridian, map oriented. The compass-needle will then be found to vary several degrees to the East or West of the true meridian line. This deviation varies from o° to 25° in the United States. RESECTION AND INTERSECTION. 7. Resection is a method of locating unknown points by taking the bearings of two or more given points. (Diagram 12). We think we are on some point of the road A-B, and we want to locate our exact position. Facing B, we have two distinct landmarks on our left, the top of hill (X) and the cottage in front of the pine-wood, both of which we can locate on our map and identify on the ground. Keeping the map oriented, we sight towards the hill until a point (X on the map) and the actual top of the hill are in the same line of sight. We plot the line of sight by drawing a back line through X across the road. This line, obtained through connecting three points (top of hill, X, ourselves), would sufficiently locate our place on the map if we were sure to be on the road A-B. In order to ascertain this, a second similar sight is taken on point Y (cottage) and the line of sight plotted in the same manner. Both lines of sight will cross at a certain point. The point of intersection locates our exact place on the map, as we are on the line S-X, and also on the line S-cottage. 8. Intersection is the same principle applied to locate xyiiii y^ ... "11 ^ ^l\ ■■;,- V. ~s.j^ f/ % ^^1))" • J ^ - / ft \'Y (o^^:iU/o/ ) . \\ /o /T- ^ /'• / / ^ :■. \\ , 1 V X^^y.-''o)K'> '" -// ""^ / C^^"^ ' ^- V V: ^^ '^^^XT'^y^ ^^\\ •• V .0^ /^ \tj ■ "vL --J i^^\hy/r ■•- •;- " \ y ,^^S0^M^ V GliLt, A'\ /^ < i^^^^^yVr" A-^../\S DiAG. 12. ■E. ^^:^ //////////////// i iiinwwww v DiAG. 15. /NTePiSE C T/ON / \. DiAG. 13. -fe / / / ® (^ DiAG. 14. A lilil.Milil.lil.l.l.liMil lil.l.l.Mil.l.lil.l.liMiiil. r wreREES DiAG. 17. Elements of Miutary Sketching. 13 and plot points situated off the course of the topographer. (Dia- grams 13 and 14). Intersection is used in both map reading and map making. You are on the road B-A, (Diagram 14), and desire to plot an object several hundred yards off the road. Stop at any point of the road (as i), orient the sketch and sight towards this objeci, plotting the line of sight by drawing an indefinite straight line from your position towards the object. After having march- ed a certain distance (as at 2) a second sight is taken and the line of sight plotted in the same manner. This will intersect the first line. The point of intersection locates the object. This method is very accurate provided the base-line is correct; care should, therefore, be taken to measure and plot correctly the distance between the first point of sighting and the second. The smaller the angle at X the greater will be the chance of error. Therefore, this angle should approach a right angle when prac- ticable. SKETCHING IMPLEMENTS. 9. Cavalry A drawing board with a compass set in. The Sketching Case paper is tightly rolled over two metallic rollers (Diag. 15.) on opposite ends of the board. An arm with a brass scale (3 inches to the mile) is fastened to it, freely moving around a pivot. This arm with the graduations on the bottom of the board is also used for measuring slopes in the manner described below, the movable ruler taking the place of the pendulum. The grad- uations are for slopes of from 1° to 20°. Plane- Table A good device for mapping, is a simple plane- (Diag. 16.) /a&/^, (a smooth board about 16 inches square) with a compass set in. Slopes are measured with some form of hand level or slope board, and in taking bearings the board is kept orient- ed by means of the compass or by "backsight- ing." The paper is held in place by thumb tacks. The board is fastened to a tripod. A modifi- cation of this device, always available, is a smooth board or stiff card board about 14 Elements of Military Sketching. I2"xi2'', without tripod, and a loose box com- pass. A ruler, paper, thumb tacks, pencil and scales complete the equipment. Under service conditions, hasty sketching will usually be done with such an improvised plane table. Any paper that will stand erasing is suitable; tracing paper is very good. HH to HHHH pencils are generally used, except with tracing paper when B or HB pencils are preferable. When sketching by pacing, a pace tally for registering paces or strides is convenient. When sketching mounted, a stop watch should be used. Slope-Board. The slope-board is a simple device for mea- (Diag. 17. suring degrees of slope. The essential part 18 and i8a.) of the slope-board is a pendulum registering every change of level. It is constructed as follows : Suspend a small weight on a string, thus forming a crude plumb line and bob. Attach the free end of the string to the middle point of one edge of a rectangular board (the drawing board.) If this edge of the board be kept uppermost and level the plumb line, or pendulum, will hang perpendicularly, and will thus bisect the board. Mark a point O on this bisecting line near the bottom. If the level of the upper edge of the board is disturbed by sighting, the pendulum will appear to move forward or back of O. This apparent course of the pendulum (A-B, Dia- grams 17 and 18) is a sector that can be divided into degrees. In the diagrams, from C to the line A-B is assumed to be 5, 7 inches, and the divisions on A-B are i-io inch apart. Each divi- sion on a slope board so constructed will read an angle of 1°. To use, sight at the object along the edge (top) of the board, at an elevation on the object of about 5 ft. (height of the eye) and read the degrees registered by the pendulum. (Diagram i8a.) Forward is a minus and backward a plus elevation. It is usually more convenient to use both hands in sighting and to hold the plumb line against the board with the left thumb when it is turned to be read. The slope-board should be constructed on the reverse side of the drawing board. Elements of Military Sketching. The Locaters Hand Level is a practical and handy slope measuring instrument especially designed for use of sketchers. (Diagram i8b). It can be used as a hand level and clinometer or gradienter at a single observation. When the tube is level an ob- ject on a line with the base of the locaters' level object glass is — 5°. Thus, in the figure, the observer sights the top of the barn, as 0° (level) and at the same time can see that the eaves of the barn are — 1°, and the base of the barn is — 3°, being on a line with the third graduation below the center, observing the bridge — 5°, which is the limit of the field. But an object outside the neld can still be measured in vertical angle. To measure the vertical angle to the foot of the tree : — disregard the bubble, hold hand level so that an object which is — 5° (as the bridge) is seen at the top of the object glass, and observe the number of degrees between the bridge and the foot of the tree; in this case it is the whole field of the tube or 10°, hence the base of the tree is — 15°. The locaters' level is made with three separate graduations : degrees, grade in per cent, and in mils (for artillery.) (For sale by the U. S. Cavalry Association, Fort Leavenworth, Kans). Scale of the units of measure : — This scale if not prepared by the individual, can usually be purchased at a trifling cost. (If pacing a scale of strides should be used.) Scale of M. D. A Triangular Ruler, with the scales pasted on it, facili- tates plotting and sighting. Box-CoMPASS. (Diagram 19) — A simple box-compass is preferable for the beginner. The graduations may be clockwise from 0° to 360°, contra-clockwise from 360" to o", or in quadrants from 0° to 90°. A compass graduated contra-clockwise (i. e. in reverse order of the graduations on the face of a clock) is the better as it reduces chances of error in plotting with a protractor. Attention is invited to the arrangement of the cardinal- points on the dial of the box-compass in Diagram 20. Note that E is to the left of N. This reversion of East and West has been made purposely. A compass with a dial so lettered is more con- venient for reading directions. As an example, let us assume the course of a road to be North and then turn to North- West. In the first instance the position of the magnetic needle will be as i6 Elements oe Military Sketching. in Diagram 21. The road then turns to N-W (Diagram 22.) We sight along the edge of the compass-box in the new direction. The needle leaves N and moves toward W (on the dial). We take the reading from N to the needle, degrees N-W. The change of direction has really been N - W. If a reversion of the cardinal-points E and W had not been ;made, the reading would obviously be more difficult, and it is very pro- bable that the sketcher, while working rapidly, would have read — degrees N-E if this reversion of E and W were not shown. ROAD sketching — WITH A DRAWING BOARD ( PLANE TABLE) OR SKETCHING CASE. (DIAGRAM 23.) 10. The starting point is marked (i), [station one]. With the beginner all changes of direction should be similarly marked ; with more practice all such data can be omitted. The needle of the compass is allowed to settle over the North and South line indicated by letters N and S on the compass-face. (If a drawing board with no compass set in is being used, lay a box-compass on the board) ; hold the board steadily so as to allow the needle the least possible swing. (To be exact, the needle should be absolutely immobile ; this, however, is hardly to be accomplished without a tripod, unless the board is laid on the ground). The needle being settled, draw a line parallel to it. Mark the north end. This line is the N. & S. line (meridian) of the map, and with reference to this line all further directions are determined. Plot the starting point on the paper. Keeping the needle pa- rallel with the plotted map-meridian (N & S. Line), sight along the ruler in the direction of the road by moving the ruler, [using station ( i ) as a pivot] , until it and the road are in the same line of sight. Draw an indefinite line forward along the ruler while holding it firmly in place with the other hand. Step off with the right foot, counting the strides, and plotting the distances to scale. When a change of direction occurs, stop, orient the board, and plot the new direction line. Observe carefully the "lay" of the ground. Plot houses, woods, telegraph-lines, etc., etc. LOCATERS HAND LEVEL ^ Brid9fc This 15° reading is takan disregarding the bobble DiAO 18b. DiAG. 19. so o DiAG. 20. DiAG. 24. Elements of Military Sketching. 17 It is customary to take in all military details within three hun- dred yards of the road traversed, and conspicuous landmarks, high hills, etc., by intersection or estimation to greater distances, depending on the object of the sketch. Cuts, fills, railroad embankments, etc., should never be overlooked as they may have tactical value. In traversing it is not necessary to halt and plot each feature as it is reached. A note can be made of the distances and the features "plotted in" when it becomes necessary to halt on account of a distinct change of direction. For example, starting from (i), carefully orient your board, draw an indefinite straight line toward (2) and then pace (or otherwise measure) the distance without halting to plot, hav- ing made notes en route of the location of the woods to the right and left; at (2) lay off the distances by scale and complete the sketching to that point. Then stand in the middle of the road (board oriented) and sight towards (4) ; at (3) it will be necessary to halt to get the direction of the side-road. Now pace to (4) and in laying off this distance, lay off the whole number of strides from (2). If, for example, you have had 260 strides from (2) to (3) and 230 strides from (3) to (4), in plotting (4) lay off the whole distance 490 strides from (2). This method lessens the accumulation of errors arising in taking distances from your scale. At (4) sight towards (6). Halt at the stream to get its direction, and the data for the bridge. Continue in this manner. [Contouring methods will be explained later.] If taking strides, count every time the left foot strikes the ground, and be careful that hundreds are not gained or lost. During each sight the board must be kept oriented. This may be done by means of the compass, or by backsighting on the last station. Telephone — or telegraph — wires or other metal objects, de- flect the compass needle, and this cause of inaccuracy must be guarded against when sighting or orienting the board. The hill 320 is located by intersection. Sight on the hill from station (6), plotting the line of sight; take a second sight from (7) or (8) or (9) and plot the line of sight. The i8^ Elements of Military Sketching. point of intersection of the two lines of sight locates the object on your paper. II. In road sketching, it often happens that a change of direction causes the plotting to run off the paper. (Diagram 25.) Draw a cross-line at right angles to the edge of the paper, cutting the point where the plotting runs off (A-B, Diagram 26.) By plotting the angle X' on B-A, we would have the direction of the road, because X' and X are opposite angles and equal. The usual way, however, is to select the point A' (which is the same point on the road as A) at any convenient place on the paper with a view of continuing the sketch; then plot a new north and south line and begin a new sketch. If the general direction of the route is known, the starting point on the paper should be selected with a view of getting as much of the sketch as possible on that sheet. To this end the board should be so oriented that the longer axes of the paper will be in the general direction of the route to be sketched. We may thus have a series of sections of the same route. Number these sections in sequence. They may be put together by pasting the several sections together, or on a new sheet in their proper order. In diagram 26, the sketch (2) is a continuation of sketch (i). When putting the sections together point A' is laid on point A, care being taken to keep the meridians of all sections parallel. After completing the sketch a simple "legend" or title, is added to the plotting, describing route, scale, vertical interval, etc., etc. (Diagram 24). Written titles will answer all purposes, though printing is generally used. The Reinhardt system of printing, as in the diagram, is most easily acquired. All officers and non-commissioned officers should be capable of readily making a hasty reconnaissance or outpost sketch, as such sketch accompanied by a written report will generally give more complete and clear information than can be conveyed by any written report alone. As the proximity of the enemy may pre- vent traversing, it will sometimes be necessary to make such a sketch from one station and with only a note book and pencil. The report to accompany the sketch (Diagram 7) might be about as follows: < DiAG. 28. DiAG. 29. DiAG. 31. DiAG. 32. ■o P r- Elements of Military Sketching. 19 "Station (i) and (2) are joined by an improved road; the bridge is guarded by a detachment of hostile troops; hostile sentries are posted along the edge of the wood on the north side of the river. The wood is thick and reaches as far as the village of Boxford. A body of our 'troops could easily win the fire-protection of the houses, which are generally of brick. The cultivated ground to the front affords no cover. Natives state that the river is fordable just west of the bridge." CONTOURS AND CONTOURING. 12. Contours are imaginary horizontal lines circumscrib- ing elevations (or depressions) on the Earth's surface at equal vertical distances. The theory and application of contouring methods thus offer a means of representing, graphically, the forms of the ground. The map-scale deals with horizontal distances (areas), while the contour deals with vertical distances, i. e., height or depth. Before the principles of contouring were applied, ground forms were represented by means of "Hachures" — parallel dashes in the direction of the slope, shading-work with an effort to give plastic impression. (Diagram 27). This method is not accurate and is rendered obsolete through the evident superiority of contouring. How can the hill X be represented on a map? (Diagram 28) In Diagram 29, a series of horizontal planes (with equal vertical distances) are passed through the hill X. (See diagram 32.) It is apparent that the edge (a Hne) of each plane follows the form of the ground. (Diagram 30, upper half.) These lines are called "CONTOURS", and the sum of their vertical distances is the total height of the elevation. The lines are plotted by "horizontal projection;" that is, if perpendiculars are dropped from points along the edges of planes to the (lowest) base-plane (water level) and the projected points on the base-plane are then joined, a true picture of the correspond- ing plane is obtained. (Diagram 30 lower half). 13. By comparing the lower with the upper half (diagram 30) it is seen that the plotted contours are far apart along the 20 Ei.e:mi:nts of Military Sketching. gradual slope D-E, while they close along the steeper slope F-E. The location of these contours is determined from the relation of ground distance to degree of slope, and involves a consideration of the meaning of the terms Vertical Interval, and Map Distance. Vertical Interval (V. I.) is the vertical distance (i. e. the difference in elevation) between adjacent contour planes. In the slope represented in diagram 35, the contour planes passing through the points A, R, C, are 20 feet, vertical distance, apart, (R-Q), and are said to have a V. I. of 20 feet. The actual ground distance between A, and the first contour point is represented by A-R. A-Q is the horizontal projection of this ground distance and is called Map Distance (M. D.) when plotted to scale on the map. Map Distance is, therefore, the plotted horizontal or level distance between adjacent contours. In diagram 35a, A-D and A-C, representing slopes of different degrees, have the same horizontal distance, A-S. Con- tours for the slope A-D would be plotted on the map at W, W,' W," W,'" and at S, while contours with the same V. I. for the slope A-C would be plotted at Q, and S only. The total horizontal distance is the same for each slope, but the Map Distances be- tween contours on each slope are in reverse proportion to the degrees of slope : and, AW : AQ : : angle SAC : Angle SAD. It is sufficient to know that on a 1° slope we will get an elevation of one foot in a horizontal distance of 57.3 feet. Therefore, we would get an elevation of 20 feet in a horizontal distance of 1146 feet (20x57.3), and on a 5° slope we would rise 20 feet in a horizontal distance of 229.2 feet (1146-^ 5), etc. The M. D. for any degree of slope may be determined by the following formula: V.I.X57.3 M. D. = plotted to the scale of the map. Degree of slope. In sketching, where time and instruments for measuring horizontal distances are usually not available, the actual ground distances are plotted instead of the horizontal distances, as would obtain in a survey. DiAG. 34. DiAG. 34a. JPJ^^^rz'j^ I'LATC.e^, ^^ iiiiiimiiiiiiiiji^- Si. khl «K.^ - - - - - - - £51 ys:^<^ J % 16 ^ QD Fl ^ "-0 ^ ^^ ^ ^ N ^ ^ ^C^ CO ^ ^ CV ^^ ^ c^ ^< ■^ ^ <^ ■^ ■^ 5^ '^^ l*^ "N 1^ *R (§ ^ *© ^ ^ «D ^ 1 ^ ^ ^ S ! 1 IF p 1 r = ~ ~ — E E E ~" ^c n ^ 1 1 \f ^^ .L ^L . '9 ll