Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

CHAPTER XV.

GRAPHIC SURVEYING INSTRUMENTS AND APPLIANCES CONNECTED THEREWITH—PLANE TABLES—ALIDADES—TELESCOPIC ARRANGEMENTS—SUBTENSE MEASUREMENTS—VARIOUS DEVICES FOR HOLDING THE PAPER—CONTINUOUS PAPERS—ADJUSTMENT OF TRIPOD HEADS—METHOD OF USING—EDGEWORTH'S STADIOMETER—SKETCHING PROTRACTOR—SKETCHING CASE—CAMERA LUCIDA, ETC.

697.—Plane Tables.—These instruments have been used for filling in the greater number of topographical surveys in all countries. They possess the merit that any intelligent, untrained person can be readily brought to comprehend their manipulation in the work to be performed, as angles of position of objects are taken directly by drawing lines pointing to them from a point upon a sheet of paper stretched upon a table. In new countries natural objects without very marked outline are conveniently defined for position. The objection to this method, from a point of view of the practical surveyor, is that the work which can be done with equal facility in a comfortable office from the field-book is with this instrument performed in the open air, under risk of rain, dust, and other atmospheric discomforts affecting both the person and the material on which he works. But for countries where the climate can be depended upon, the facility with which surveyors with little experience can map details for filling in superior triangulations made with the theodolite, its use has gained much favour. Natives can be easily taught to use it, and the check on their work through the previous triangulation is perfect. The subject of plane tables will in these pages be considered only in its general aspect, with the examples of a few good instruments, referring the reader who cares to follow the subject further to an excellent paper by Mr. J. Pierce, Jun., read before the Institute of Civil Engineers, February, 1888.[51]

698.—The Plane Table in its simplest form consists of a small drawing-board mounted upon a firm tripod stand, and is shown at Fig. 313.

Fig. 313.—Simple plane table.

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A rule termed an alidade, with sights placed at its ends, gives the direction of any object from a given point on the sheet of paper stretched upon the table, to which a fine line is drawn by an HH pencil to point the direction. The alidade sometimes carries a trough compass fixed upon it, but this is generally a separate instrument which is placed against its fiducial or ruling edge to give a magnetic north to south line, to which all other lines are assumed to take angular direction. A loose spirit level is also provided, by means of which the board may be set level by shifting the legs of the tripod.

699.—Plane Table with Telescope.—Where greater refinement of observation is required than is possible with sights, a telescope is mounted on the alidade, which moves in the vertical plane upon an axis, so that it may be directed in a linear direction with the fiducial edge of the rule to any point in azimuth. The telescope sometimes carries a level, so that the table may be set level by means of the alidade.

Fig. 314.—Plane table.

Fig. 315.—Tripod stand.

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700.—A class of plane table which meets all necessary refinement for ordinary filling in of field work is shown in the illustration, Fig. 314. This nearly resembles those made by the author for filling in details of the great trigonometrical survey of India. The drawing surface of this table consists of a loose panel which stretches the sheet of paper by pressing it into its frame, where it is afterwards held by a pair of ledges which fit at their ends into long slots. The panel of the board, shown in detail Fig. 316, is mounted upon a firmly braced tripod stand. The head of the tripod stand, shown Fig. 315, is secured to the board with a central screw (not shown) which permits the board to be set in any direction, it being the rule that the edge W should always take a north to south direction. Three screws sss at the corners of the triangular head can be raised or lowered by milled heads from the under side. These screws permit about 15° of adjustment to the table without any unsteadiness, as the centre screw clamps it finally hard down upon them when all adjustments are made. A small trough form of magnetic compass a is placed upon the rule to strike the magnetic north to south line, to which all angles are referred in transposing the work of the plane table. The diaphragm of the telescope is provided with a platino-iridium point fixed vertically at the mutual focus of the object-glass and the eye-piece. A pair of points to subtend an angle to measure a staff for distance, Fig. 319, is a convenient addition.

Fig. 316.—Panel board of plane table.

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701.—The Telescopic Arrangement of the alidade is varied in different countries. In some cases it is placed near to one end, which is perhaps better than in the centre of the rule, as it is more easily read. In the modern French military alidade a prismatic eye-piece is used, so that observation is made by looking directly down upon the eye-piece of the telescope. In the Prussian alidade adjustment is made to the standard of the telescope so as to bring the horizontal axis upon which it moves level, that the telescope may move in azimuth, however irregular or uneven the surface of the paper on the board may be. This is necessary for any great degree of refinement in the plane table, as the surface of a piece of wood upon which the paper is stretched will be almost certain to warp if exposed to all weathers, and this, added to the small width of the alidade, can scarcely retain the axis in exact horizontality, placed as it is high above the surface of the table. Some plane tables made by the author for General Robinson for Indian service were of papier-machÉ to remedy the defect of warping, but even this material warps upon exposure. Plane tables have been made in Germany of metal and of glass, but in this case the weight is a great objection. The author has found surfaced slate very good, but it has the same objection of too much weight for a portable instrument.

Fig. 317.—Stanley's plane table.

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Fig. 318.—Alidade to plane table.

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702.—Lateral Adjustment to the Alidade.—The author's plan of obtaining this is to increase the practical width of the rule by giving it an extended point of support on one side so as to set the telescope in azimuth. For this construction the telescope is mounted upon a plate with an arm extending outwards upon the back of the rule. This has a milled-headed screw placed at the near extremity of the arm. The screw is inserted in a deep bush for wear; this attachment is shown in section Fig. 318. The adjusting screw A' has a collar fixed upon its point which is centred upon a tight screw tapped into the milled head. This collar, as it does not turn with the milled head, does not abrade the surface of the paper by contact with it. A small cross level B is put upon the arm between the milled head and the standard of the telescope. The under side of the rule is cut away or placed obliquely to the surface, so that it bears on the outer ruling edge only. The milled-headed screw being at its normal position and the table level, less than half a turn one way or the other will bring the small cross bubble to its centre in a few seconds for any average irregularity of the surface of the table, and by this means cause the telescope to move correctly in azimuth.

Fig. 319.—Subtense points.

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703.—The Telescope Arranged for Subtense Measurements.—Where a stadium, art. 556, is to be used for estimating distances from station to station, when an ordinary telescope is used, the author places two platino-iridium points vertically from top and bottom of the diaphragm, and adjusts these by a screw until a subtense angle upon the stadium of 1 foot cuts the point at a distance of 100 feet, or according to the measurements to which the land is taken. In this case it is necessary to have an altitude arc to the telescope, as shown upon the alidade in Fig. 317. This has a degree scale reading by vernier to about 3 minutes.

704.—Various Devices for Fixing the paper on the Surface of the Table have been made. Many prefer simply pinning it with drawing pins on a quite plain pinewood surface. In this case the table is better slightly sunk round the edges with a rabbet of the depth of the thickness of the head of the pin, so that the alidade may rest firmly even over the pin heads. The French plane tables have very generally rollers at each end of the table, upon which a long slip of paper is rolled, sufficient for twelve or more stations. The rollers for small tables are made of brass tube about 5/8 inch in diameter. They commonly move with a turn-key which is inserted in a square fitting in the end of the roller. The rollers keep the paper tight by means of ratchet wheels and spring pawls at their ends. This plan is very convenient for topographical work, as for instance a river may be followed from station to station right down its course and appear on a single slip, its bearing being indicated by the compass north line. Fig. 320 shows the manner in which the author has made this plane table.

Fig. 320.—Plane table with rollers.

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Fig. 321.—Gurley's plane table adjustment.

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705.—Adjustment of the Plane Table.—There are a great many devices for this. Mr. Pierce, in the admirable paper already mentioned, gives illustrations of the different plans. Some of these have all the complication of the adjustment of the stage of a theodolite, and one has superadded to this a slide-rest motion. These things of course are necessary if the field work is made to take the place of finished office work. One general feature of plane table tripods is some means of adjustment of the table to uneven ground, when the tripod-head cannot be brought nearly level. Gurley's plane table adjustment is perhaps the simplest of any of these devices, and appears to the author to be as good as any other. Fig. 321, D the table top; A a ball fitting turned inside and out, and attached firmly to the table top; C a socket fixed firmly in the head of the tripod; B a bolt with globular head fitting the interior of A, and carried through the head to a winged nut which clamps it firmly. A spring is placed to act against the winged nut, so that when this is slightly loosened the ball fitting A may move between B and C with moderate firmness when the table is being set to an angle.

Fig. 322.—Stanley's high-class plane table.

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To meet all conditions the revisor has designed a high-class plane table shown at Fig. 322. This has a ball and socket rough levelling arrangement and parallel screws for fine levelling, circular motion to table with clamp and tangent, spring rollers for taking any length of paper and instantly clamping it, alidade with extra powerful telescope with vertical circle divided on silver reading by two verniers to minutes with clamp and tangent motion, cross levels, diagonal scale, and adjustment for setting telescope to revolve in vertical plane, circular compass with cross levels and plumbing bar. The board is generally made 30 × 24. The telescope is stadia reading and is made with long sensitive bubble mounted upon it or upon the verniers if preferred as shown at Fig. 323.

Fig. 323.—Stanley's high-class alidade with bubble on verniers.

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706.—Method of Using the Plane Table.—The table is first set and levelled up at a commanding position to observe the extent of country it is intended to plot from observation from a single station. Let Fig. 324 1 be the first station for plotting the enclosure abcdef. Draw lines by the alidade pointing to these angles represented by the letters from a point near 1. Set up a picket or stadium at the station 2 where it is intended that the plane table shall next be set up, and draw the line 1 2 distinctly on the paper. Measure the line 1 2 either by its subtense on the stadium or by direct chain measurement, and plot this from station 1 on the paper according to the scale to be worked to in making the plan. On removing the table set up a picket or distinct land mark vertical with the position of station 1 occupied on the paper. Move the table to station 2 at the measured distance and set the direction of the board by means of the alidade so that the line 2 1 cuts the picket left at station 1. Now draw lines from station 2 to all the points abcde, cutting the former lines as represented by dotted lines in the figure, and the intersections of these lines will give the true positions of abcde, according to the scale selected for the base 1 2, and these may be tied up to represent the boundaries, as shown on the plane table 2. It will be readily seen that the line 1 2 represents a bearing in azimuth; so that if the edge of the table be set, say truly N. to S., in both positions the line on the paper 1 2 will agree in both these positions of the table; but the check by the alidade of this line is valuable to save risk of error.

Fig. 324.—Diagram of plane table work.

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707.—Where an extent of land is to be surveyed by the plane table, longitudinal bands of a mile or so in width are taken. Where the roller plane table with continuous paper, Fig. 320, is used, the forward points of observation are lined in and the backward ones simply tied up, being certain by observations written in pencil upon the paper that identical objects are tied up from the positions of both stations. Where an object cannot be seen from both stations its position may be indicated by the stadium from a single bearing, or it may possibly be tied up from a further advanced station.

708.—Edgeworth's Stadiometer.[52]—The general construction of this instrument is given in the inventor's specification of patent, from which the engraving, Fig. 325 is taken. The vernier plate of an ordinary theodolite is extended to a plate of about 10 inches in diameter. This is adjusted to level by means of parallel plate screws. The plate or plane table is divided on its edge ¼°. The part representing the limb of a theodolite is carried out from its axis by two arms only: upon these the standards RR of the telescope are mounted. These standards leave a striding space near the plate, into which any scale S of equal parts with a zero centre is introduced, which is intended to be used for the plotting, the striding space being so arranged that the fiducial edge of the scale shall pass exactly over the axis of the instrument. The standards unite in the same casting to form the horizontal axis bearing of the telescope. This axis permits the telescope to move in azimuth. The telescope carries a vertical arc divided to degrees, also a scale of centesimal differences of hypotenuse and base, with the ordinary clamp and tangent adjustment of a theodolite. It is also fitted with a level above it which is used in setting up the instrument. Stadia webs are placed in the diaphragm and are made adjustable to subtend upon the stadium a percentage of arc agreeing with the unit to which the land is measured. The inventor does not appear to have known the optical error of the system proposed for measuring distance, art. 558. Neither does this appear to have been recognised by others writing upon the instrument, who have generally followed the late J. F. Heather's description.[53]

Fig. 325.—Edgeworth's stadiometer.

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709.—To Use Edgeworth's Stadiometer.—After it is set up, a circular disc of paper of about an inch less diameter than the table is held down upon it by four spring clips. The telescope is directed consecutively from object to object, the positions of which it is desired to take. It is clamped by the screw below the plate during the observation. The stadium is placed against the object and the distance taken by the subtense of the webs in the diaphragm, which may be exact if a constant be added after proper adjustment, art. 558. If the stadium be above or below the horizontal plane it is inclined by means of a sight-hole through it, as originally proposed by Green, so that the subtense is equal under all conditions. The horizontal distance is taken by the difference of hypotenuse and base, as shown on the vertical arc, so that the record of a complete observation appears for calculation as—

stadium reading + constant - altitude correction.

This distance is at once set off from the centre of the instrument by the scale on a line drawn upon the disc of paper, and observations are written against the line. In making a number of observations from one station two or more discs of paper may be employed to save confusion of lines and interference of descriptions. These papers are separately used in plotting as protractors by pricking holes through the stations defined in the field from the centre of the disc which represents the station of observation.

710.—The Sandhurst Protractor, Fig. 326, is a military protractor adapted especially for topographical delineation, which is commonly used with the plane table. It is different from many instruments of its kind in having only useful matter upon it. It is made of boxwood, upon which the protractor is cut, and has also one scale of 6 inches to a mile in yards, at the lower edge, the tens of which are carried across to make parallels of 90° in the manner of an ordinary military protractor. Over the back of the protractor is a scale which gives a standard for shading slopes of land upon topographical maps, Fig. 327, from 2° to 35°, also lines for contour shades. A small plummet, the cord of which is passed through a hole in the centre, from which the degrees are protracted, is supplied with the instrument. When the protractor is held up, degrees downwards, the cord of the plummet will pass over the degrees and indicate the angle at which it is held. By looking over the edge the angle of inclination of the land may be taken directly, as with a clinometer, or by looking along the edge by a second person reading the plummet the angles of altitude may be taken more exactly.

Fig. 326.—Sandhurst Sketching clinometer protractor.

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Fig. 327.—Example of scale of shades for slopes.

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711.—Military Sketching Board.—This sketching board, Fig. 328, the invention of Mr. Graham F. Hodgson, will be found a very great improvement upon the old pattern boards. It is designed to meet the requirements of military and other officers not conversant with the higher branches of surveying, and it will also be found of great use to surveyors, explorers, and other travellers.

It consists of a mahogany board revolving on a circular metal plate attached to a handle held in the operator's left hand.

Mounted on the metal plate is a compass, which is visible through a glass plate flush with the face of the board and the tracing paper or cloth on which the map is made; the paper being stretched tightly over the board by means of rollers.

A magnetic north line is drawn on the paper over the centre of the compass. The operator invariably has his board in its relatively correct position by always keeping the magnetic north line immediately over the top of and aligned with the compass needle when his sights are taken.

Top view.

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Under view.

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Fig. 328.

The sights are taken by means of an alidade moving along a slotted bar, which itself slides along a bar fixed at the side of the board.

The object aimed at is sighted in a mirror attached to the end of the alidade and aligned with a point at its other end, and the alidade is clamped into position by a thumb-screw on the slotted bar.

The advantages of this instrument are many and will only be fully realised when it is in practical use. No backsights are necessary. Sights can be taken with one hand. The operator is always in a comfortable position and the object aimed at is always immediately in front of him. The alidade remains in position by means of the clamp screw along the ray drawn till the object sighted at is reached and the distance known, which is merely marked off by means of the scale on the alidade. It is invaluable when sketching rivers from launches or canoes when backsights are often impossible. It is light and portable, being easily carried slung over the shoulder in a canvas case.

712.—The method of using is simple and ensures a great degree of accuracy with a minimum amount of time and trouble:—

(1) The tracing cloth is first fixed by means of the rollers over the board. (2) A magnetic north line is then ruled across the paper and passing immediately over the centre of the compass visible under the paper. (3) The operator then, holding the board by the handle underneath, proceeds to make his map and first brings the magnetic north line immediately over the top of and aligned with the compass needle. (4) He then from some point, marked as his starting point on the paper, proceeds to take sights to any objects he may wish to delineate on his map. These sights are taken by means of the alidade fixed above the board. The sighting rule is pushed along a slotted bar, which itself slides along a bar at the edge of a board until the edge of the alidade is against the starting point and is sighted on the object aimed at. The object is sighted in a small mirror fixed at the end of the alidade and aligned with the point at its other end. The alidade is then clamped into position with the thumb-screw on the slotted bar and the operator draws his ray corresponding with the direction of the object aimed at. The distance is then marked off on the scale on the alidade. (5) When the operator moves to the next station no back-sight is necessary. He at once puts the board into its relative correct position by merely revolving it until the magnetic north line is again lying over the top of and aligned with the compass needle and he then proceeds to take all necessary sights at that station.

Fig. 329.—Cavalry sketching case.

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713.—Cavalry Sketching Case.—This forms a very convenient exploring sketching board, permitting sketches to be made on horseback while en route. The pattern shown in Fig. 329 is that of Captain W. Vernier. It consists of a small board 9¼ inches by 7½ inches, at two sides of which there are small rollers to hold paper 7 inches wide and from 3 feet to 6 feet in length, according to its thickness. Two stout indiarubber bands, which hold a small straight-edge to scale in any position on the paper with sufficient firmness to be able to draw a line against it, are passed over the board. A small compass on one side of the board indicates direction. After one sketch is made, a new part of the paper is rolled forward.

714.—Camera Lucida—Optical Compass.—In new countries where landmarks are not clear a sketch of the general aspect of the country will make the points of triangulation more clear. Where the plane table is not used these sketches may be made with accuracy as to positions by the use of the photograph camera, the camera lucida, or points of observation may be taken in correct bearing by the optical compasses. These latter instruments are described in the author's Treatise on Drawing Instruments, seventh edition.

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