  LEVELLING STAVES—CONSTRUCTION—VARIOUS READINGS DISCUSSED—SOPWITH'S—FIELD'S—STRANGE'S—STANLEY'S NEW—METRICAL—SIMPLE CONSTRUCTION MINING STAFF—PAPERED LEVELLING STAFF—PRESERVATION—PACKING PADS—STAFF PLATE—STAFF LEVEL—PRACTICE OF LEVELLING—INDEX OF BUBBLE—LAMP—CURVATURE CORRECTIONS—STATION PEGS—REFINEMENT OF LEVELLING—LEVELLING BOOKS—INK BOTTLE, ETC. 258.—Levelling Staves.—Since great improvements have been made in the telescopes used as part of all modern surveyors' levels, particularly by increasing their light-receiving capacity, all systems of vanes which were formerly made to be seen distinctly at a distance have disappeared from use by British surveyors; it is now found that the plain reading of a divided staff can be taken by means of the telescope at a sufficient distance from the observer for all practical purposes. In this country one construction of staff is now generally adopted; and the only variations that are made in this are found occasionally in the readings. The construction of the level staff in common use is that invented by the late Thomas Sopwith, Figs. 93, 94.—Section of Sopwith's staff. Larger image 259.—The ordinary construction of Sopwith's staff and the best mode of manufacture is shown, with the joints grooved together, in section Fig. 93. The outer tube or case A, which in the 14-feet staff is 5 feet in length, is made of mahogany 5/16 inch thick, the front being ¼ inch. The outer dimensions of the section are 3-1/8 inches by 2 inches. The second tube B is 5 feet 1 inch long, of outer dimensions 2-3/8 inches by 1¼ inches. The inner slide C is solid, 5 feet 2 inches long, 1¾ inches by ¾ inch. All the slides are sunk on the face about 1/16 inch to prevent the divisions being rubbed by exposure in sliding together. The slides have each a brass shoe and cap. They are held when extended by 260.—The most important consideration in the manufacture is that the telescopic work should fit well, and that the boxes should be glued up quite square and out of winding. The boxes should, after the glue is quite set, be screwed with brass screws at distances of about 6 inches apart, to secure the joints which may afterwards in use be exposed to long-continued rain. The fittings should be carefully made, so that when the staff is extended there should be no shakiness sufficient to cause serious vibration when it is used in windy weather. The interior of the slides when finished should be thoroughly oiled with raw linseed oil, and the outer surfaces be well soaked in shellac dissolved in spirit, and then French polished over this. The brass work should be well lacquered. Fig. 95.—Section of semi-cylindrical staff. Larger image 261.—Semi-circular Staff.—This is another kind of telescopic staff, with Sopwith sliding arrangement, which possesses a certain merit, but is more expensive to make. It is semi-cylindrical, the cylindrical part being made without any joint. This is shown in the section Fig. 95. The general dimensions of the face of the staff are the same as those Fig. 96.—Sopwith pattern staff. Fig. 97.—Field's pattern. Fig. 98.—Stanley's old pattern. Larger image 262.—What was originally considered as the defect of the Fig. 99.—Sopwith's staff. Fig. 100.—Rogers Field's staff. Fig. 101.—Col. Strange's staff. Larger image 263.—The original form of reading designed by Sopwith is still much more in use than any other. It is similar in pattern to Fig. 96, shown in detail for 1 foot Fig. 99. The dots at the end of the lines shown in the figure were 264.—Sopwith's pattern is sometimes printed on paper for pasting on the staff, and in this manner the staff comes out much cheaper than by drawing the readings in solid paint. Paint, however, is strongly recommended, not only because it wears much better and keeps cleaner, but that the painting and varnishing add very much to the durability of the staff, exposed as it must necessarily be to rainy weather; further, the paper, however well it is fixed at first, is liable to creep away from the edges of the staff, and leave a space into which rain enters very freely by capillary attraction; but it does not again freely evaporate, so that it rots the staff and makes the paper reading after a time mouldy. It is, nevertheless, convenient to take a set of first length papers if a surveyor is going abroad, as from accidents—grazing by carrying the staff with the tripod of the level, etc.—the first length of surface is very liable to become too much injured and effaced for fair reading. A description of fixing the paper will be given further on. 265.—For Reading the Sopwith Staff, the foot readings are taken from the tops of the red figures. The ·1 foot figures are in black, and are all odd numbers, 1, 3, 5, 7, 9. These read also from the top. The height of the figure is exactly ·1 foot, so that the bottom of each figure reads the lower even number—thus the bottom of 3 reads 2, of 5 reads 4, of 7 reads 6, and of 9 reads 8. The 6 and 9 foot figures if made alike, from effect of telescopic inversion, may cause error. The author has for many years made the head of the 9 a solid black block to avoid this. 266.—Various Readings.—A very large number of surveyors design their own staff readings. This was formerly very much the fashion, consequently a great number of patterns come before the manufacturer. The author for about twelve years kept a copy of what he considered the most 267.—Rogers Field's and Colonel Strange's Readings.—The author made some experiments to obtain a clear staff, readable beyond the ordinary range of staves with a 14-inch level; but much more complete experiments were made with the author's set of patterns by Mr. Rogers Field, C.E., whose ingenuity is well known. This gentleman finally designed a staff which in the author's opinion is still one of the best, but it has not generally pleased the profession: this is illustrated, Figs. 97, 100. The author has tried it at all distances: at 20 chains he has found a reading of ·01 foot could be taken approximately with a good 14-inch level with his point index-stop level, Fig. 75. The late Colonel Strange made a series of experiments with the author's patterns placed at 10 and 20 chains distance. He also had for these experiments one of Mr. Rogers Field's staves. He arrived at the conclusion, for distant reading particularly, that the black markings on all the twenty staff patterns he had were excessively heavy, so that the lightest and most open readings were the clearest. This led him to design a staff, a part of which is shown in Fig. 101, which has been since generally used on the great India survey. This staff somewhat resembles the English ordnance pattern. The fault found with these patterns is that they do not read the ·01 foot, which is necessary for close reading in hilly districts, otherwise they may be read very clearly at a distance of 20 chains, where the Sopwith becomes a blur. We may take it that the surveyor, if he be a fairly good draughtsman, would subdivide the ·05 block to the ·01 foot; but it is argued that his assistant, who might be a fair leveller, might not. Another objection is that the reading is on one side and Fig. 102.—Details of Stanley staff; A bottom length, B middle, C top with dot figures. Larger image 268.—The author designed another staff especially for his point index. This is shown above, Fig. 102. It has had a very fair popularity, being good both for distant and near sighting. In this staff for the close figures 11, 12, 13, on 269.—Our space will not permit the discussion of the various staff readings that have been designed, many of which are, in the author's opinion, superior to the Sopwith; but some variations are necessary occasionally for personal reasons. Some surveyors, from imperfect colour vision perhaps, strongly object to the red foot figure as being indistinct at a distance, hence in many patterns a clear black figure is employed. Some get confused with the number of equal lines of ·01 foot in the Sopwith, what is sometimes termed Sopwith's ladder. In this case these lines may be made unequal in different ways: several patterns have this peculiarity. Some persons cannot get over the inverted figure as seen in the telescope. In this case it would be much better, perhaps, to read with an erecting eye-piece to the level; but practically the manufacturer has to invert the figures. Other less important variations are common. 270.—Metrical Staves.—These are in this country generally made 14 feet, to keep the length the same as the tripod. The most approved patterns are shown Figs. 103 and 103A. In using the metre pattern at short distances often a complete metre cannot be taken in the field of view, so that there is a little difficulty in being certain to what metre interspace the subdivisions belong. To avoid this the author places a dot or dots after the decimetre figures that follow the metre—one dot for 1 metre, two dots for 2 metres, three dots for 3 metres. Fig. 103.—Metres and Half Centimetres. Fig. 103A.—Centimetres. Stanley's metre levelling staves. Larger image 271.—Feet and Inches Staff.—For building works, drainage, and some other cases, the staff is divided into feet and inches, and subdivided again into eighths or tenths of inches. This is most convenient when the work has to be carried out with 5 or 10 feet rods and the 2-feet rule. The intermediate inches between the feet are better marked 3, 6, 9 only than fully figured. For rough usage the author has made a solid 10-feet pine staff, well painted. This has a strong hinge in the centre, and is kept stiff when open by a strong open hook. It closes face to face in two parts, which keeps the face clean. Fig. 104.—Stanley's rough levelling staff. Larger image 272.—Mining Staves.—For levelling in mines, large sewers, and other cases were there is no height for the ordinary staff, the Sopwith staff is made in its closed form commonly 2 feet 3 inches and 3 feet 3 inches only in length, to open out respectively 5 feet and 8 feet, or in some few instances even shorter than these dimensions. The mine staff is in every way, except its length, similar to the ordinary Sopwith, art. 259. Fig. 105.—Stanley's patented mine staff. Larger image 273.—Stanley's Portable Staff.—The writer has made a portable staff in lengths of 18 inches, somewhat like a French folding rule. It may be formed of three, four, five, or six lengths, opening out respectively 4 feet 6 inches, 6 feet, 7 feet 6 inches, and 9 feet. The separate pieces are flat boards, slightly sunk on the face to prevent the divisions being scratched in opening and closing, but left solid at the 274.—A portable mine staff designed by Mr. G. J. Jee, 275—Papering or Repapering a Sopwith Staff.—The staff, if new, is painted with three coats of rather flat, thin white-lead paint on the face, and left to season till the paint is quite hard. It is then washed thoroughly with a sponge dipped in stout, until this adheres without beading, and is again left to dry. For repapering an old staff, this is soaked with hot water in which there is some washing soda, and rubbed until the old paper is brought off. After the staff is in either of the states described above, it has to be made warm and coated with one or two coats of size. The size may be made of a piece of glue left in water for a night, and then melted in a jam-pot placed in a saucepan To avoid the trouble of marking off and stretching each foot, the author has introduced jointless levelling staff papers, so that the entire length of each section may be put on in one piece. These are of special paper, and it is only necessary to paste the face of the staff well and smoothly, and lay the paper unpasted down in position upon it. After the papers are thoroughly dry they require two coats of thin isinglass size, and then a coat or two of varnish. Paper varnish can be bought; but in defect a varnish may be made of Canada balsam dissolved in oil of turpentine. This should 276.—Preservation of the Levelling Staff in Use.—Where two staves are used they may be placed face to face for carrying, and be strapped together, and will take little harm with moderate care. Where one only is used it is generally strapped to the tripod. A strip of wood is sometimes used to protect the face of the staff. Fig. 106.—Pad for holding a staff and tripod. Larger image 277.—For carrying the staff with the tripod, a convenient plan is to have two pads formed of stout ox-hide butt, each pierced with two slots near their ends at the exact distance apart of the width of the staff, Fig. 106. The strap of calf leather is passed from one slot round the staff into the other slot, and then passed round the tripod and pulled up tightly and buckled. The pad of course protects the front of the staff from grazing by the friction of the tripod against it. There is a certain amount of risk, under any circumstances, of the cylindrical tripod pressing against the front of the staff and splitting it. To avoid this the author has lately made the pads with a mahogany bridge piece, so that the pressure is distributed, coming upon the edges of the front where the staff is strongest to resist it. This is shown, Fig. 107. 278.—For the entire protection of the staff a leather-bound sailcloth case is very generally used. This may be divided into two compartments for the staff and the tripod, with pads between. The whole case has a neat appearance, and forms a protection from slight bruises and dirt, either in travelling or when set up in an office corner for future use. Fig. 107.—Improved pad for staff and tripod. Larger image 279—Repairing Figures and Divisions.—Surveyors going abroad will find it very convenient to have a few tubes of artist's oil colours—white, black, and vermilion, with one or two sable brushes to touch up any divisions or figures upon the staves that have become accidentally injured or worn off by friction. A tube of medium is also useful, which will cause the colour to dry quickly and leave it bright. The tubes of colour will keep any number of years if the caps are carefully replaced. The brushes after use should be well washed with soap and hot water, rubbing the soap in quite thickly till they are quite clean, and then well rinsed before putting them by. Fig. 108.—Iron triangle to support a staff. Larger image 280.—Iron Triangle.—For use of the staff in the field, particularly in open grass or moist lands, a triangular plate of iron, as represented Fig. 108, is very useful. This is trodden down firmly by the staff holder before he places the staff upon it. In use it gives a certain base to turn the staff upon from fore to back sight. It is very inexpensive. 281.—Staff Level.—This is a small circular level, shown at Fig. 109, the upper surface of which is formed of a glass worked slightly concave and fixed into a short cylindrical box. The glass is hermetically sealed after being nearly filled with spirit. The circular level is mounted on a plate with a dovetail fitting which fits in a slot in the holding plate attached to the back of the staff. In use the staff holder has to observe when the bubble under the concave glass is in its centre. A very little practice is required to hold the staff vertically by means of this little contrivance, which only weighs, about 2 oz. Fig. 109.—Staff level, ½ scale. Fig. 110.—Staff-holder, 1/10 scale. Larger image 282.—Staff-holder.—This implement, shown Fig. 110, striding a staff, is very generally used in Germany and other parts of the Continent. The staff is sunk into one side of a hardwood block. The block is turned at one end to form a handle. A second similar handle is cut with a strong screw and screwed into the end of the block. This screw handle by turning brings up a following piece, shown inside next the staff, which is covered with leather. When it is screwed up, the staff may be held firmly by the handles only, without the risk of the fingers coming in front. With this accessory it is also held more easily and truly vertical. It is a comfort in use in cold weather. 283.—Practice of Levelling with the Staff.—This subject can be followed here only so far as to exemplify the uses of the instruments and of accessories connected with such 284.—For Holding the Staff, Mr. Holloway, in the work referred to in the last note, gives instructions in such concise form that they may be quoted with advantage. He says:—"I generally enter into confidential chat with my staff holder, in which I explain to him the vast importance of his duties, i.e., I endeavour to make him a man of importance in his way, and I never fail to get those duties properly performed. My instructions to him are seven in number:— "1. Draw out the slides of the staff, and be sure the joints are properly locked. Draw out one length only unless signalled to do otherwise. "2. When the staff is once on a point never move it unless signalled to do so. "3. Examine the staff regularly before setting it down to see that no dirt is sticking to the bottom of it. "4. Always stand erect behind the staff, so that the figures face the level. "5. Do not let any part of the hand come before the face of the staff. "6. In no case put a downward pressure on the staff. "7. If the grass be long, mossy, or spongy, tread it down, so that the staff shall have a firm footing—select a firm spot if the selection is left to yourself." 285.—The manner of setting up a level has been already described in the previous chapter. The leveller generally Fig. 111.—Level height tape. Larger image 286.—Occasionally in town surveys the height of the level has to be taken. For this a small steel spring pocket tape is used to take the height of the axis of the telescope, Fig. 111. The tape may be adjusted by taking a piece off the first end, and allowing for the width of the tape case, so that by placing the ring of the tape upon the hook under the instrument and bringing the case just to the ground, the height of the axis of the telescope above the ground may be read off at the point where the tape leaves its case. 287.—The Reading of the Staff.—The first position, which is afterwards termed the back reading, is taken at a distance behind the first forward position of the level. This is recorded exactly as it appears in the telescope, the height Fig. 112.—Practice of levelling. Larger image 288.—The staff reading, as already described, is divided into feet, with two places of decimals. The safest method of taking this reading is to take the second decimal place first and then record it, then the first decimal, and finally the foot. In this manner no effort of memory is required, and the staff being sighted three times assures the certainty of the reading. The telescope should not be touched during the operation, so that the reading in this manner is only a cautious transfer. 289.—If two staves are used on fairly level ground, the second staff is now advanced 5 chains from the level to a measured station, the staff holder here sighting the line through the level to the back staff, and firmly treading down the staff plate if the land is soft or grass, or otherwise requires it, or an iron triangle is used. When time is given to hold the staff vertically by means of the staff level, the reading is taken in this position by the leveller as before, and this is recorded in the levelling book. The level is now moved 290.—The equal back and fore sights as far as practicable are insisted upon by all levellers, as by this means any inaccuracy in the level, if the run of the bubble is kept constantly true, is thereby compensated; but it is not always convenient, and when it is not the accuracy of the work must depend largely upon the qualities of the level. It is not necessary or convenient at all times to take the back and fore sight in a line—obstructions of woods, rivers, etc., may occur. In these cases very often what is quite equivalent may be done by taking equal angular back and fore sights from the apex of an equilateral triangle thus:—Say an obstruction occurs for the chain by a pond or wood, but that both points to which the levels are to be taken are visible at some lateral position. Levels may be taken from this place, and if the intermediate point of distance is equal from both stations there will be no instrumental error. Thus, suppose the direct level line east (90°), and that the two stations can be seen and the staves read at 150° and 210°; here, evidently, this is equivalent to a direct back and fore sight, the right angle to the level course being 180°—the one station is 150° = 180° - 30°, and the other 210° = 180° + 30°. If these equal angles can be even approximated with a fairly good level the error will be small. In this manner intermediate and extended points may often be 291.—In levelling hilly ground great loss of time would sometimes be incurred from taking equal back and fore sights; the best plan in this case is to make as much use as possible of the length of the staff in use. It is in hilly districts only that a staff longer than 14 feet is advantageous. With any staff in descending a hill only 5 feet of the staff can be used for the back sight, that is, a part of it equal to the height of the level, and sometimes 4 feet or less if there is grass, brambles, or other obstruction. Whereas for the foresight all the staff upwards of the height of the level, that is, about 9 feet in a 14-feet staff, can be used with certainty. The distance of setting up of the levels and staves must in this case entirely depend upon the length of the staff and other conditions present. 292.—For near reading of the staff on sharp inclines, reading to two places of decimals is not near enough, as errors may accumulate rapidly. It is in such cases that a fully divided staff is best. The divisions upon a near staff appear in the telescope much magnified; and three places of decimals may easily be taken by anyone used to reading a chain scale, particularly if a point diaphragm be used. Through valleys the level may be often checked at some point from hill to hill by a back sight: the contour must nevertheless be followed for the section. It is in these shorter unequal ranges and in distant sights that accuracy in the level is demanded; and it becomes interesting to know how nearly this may be depended upon for such readings. 293.—As already mentioned, a sensitive 14-inch level of Y construction, or a dumpy in perfect adjustment supported on the tribrach system, will work with a level tube divided to read 5 seconds in divisions 1/20 inch apart. There will be a little personal error in reading the bubble from difference of reflection, Fig. 113.—Calder stove used as a lamp. Larger image 294.—Lamp.—At heights between hills in wide valleys check levels may be taken from five to ten miles very well with a good 14-inch level in still clear weather in dark nights by the use of an oil lamp. Coincident points above datum being selected, the lamp is set upon the ground, or at a measured height at a calculated point, or raised or lowered to lantern signals, allowance being made for curvature and refraction. The wide band of light is read very easily by shifting the observer's position and raising or lowering his tripod. The "Calder" lamp stove answers very well as a The heliostat is sometimes used for check levelling in sunlight. This will be described further on with the theodolite. 295.—Curvature Corrections of the earth and of Refraction to be made use of occasionally for check levelling. The rule for finding curvature is "That the difference between true and apparent level is equal to the square of the distance between two places or stations in miles—divided by the earth's mean diameter, 7916 miles"; consequently, by this rule the correction is always proportional to the squares of the distances. By proportioning the excesses of height to the squares of the distances, we may obtain a curvature table for corrections. This is, however, always in excess of the true curvature by the refraction caused by the increase of density of the air towards the earth's surface, which bends the visual ray. The curvature of the earth may be corrected for refraction one-fifth to one-sixth, 296.—The following table, which takes curvature minus refraction, will be found useful to have at hand: it may be written out and pasted inside the lid of the level case:—
Where great precision in levelling is required, as for important trigonometrical surveys, many precautions are required to be taken which would be quite superfluous, for instance, in railway work. Thus much greater exactness and freedom from personal error is secured by having two levellers to go over the same ground simultaneously. Errors by two persons in the same part of the track are very unlikely to occur, and by comparing books every part may be checked. 297.—Pegs.—Where the work is to be entirely pegged for chain measurements, the pegs may be made of natural sticks sawn off and pointed up with a bill hook. If they are sawn from timber they are generally made about 9 inches long and sawn to a point, the head being full 2 inches by 2 inches. Where great precision is required a cast-brass or iron nail is driven into the head after the peg itself is driven down. This is used to turn the staff upon, Fig. 114. A the peg shown with a nail in its head, 1/8 size. B nail about full size. Fig. 114.—A, staff pegs of sawn timber, 1/8 scale; B, nail, full size. Larger image 298.—It is considered a precaution with an ordinary level to mark one leg of the tripod and always place this in the same position to the staff. Thus, if the marked leg is placed to the forward staff at first, it is put at the next station backward to the back staff. This corrects any general error from defective work in the instrument and want of adjustment; and if the staves are placed at equal stations any instrumental defect whatever, to act cumulatively upon a distant station, is 299.—Differences of true level have been found between working southward towards the sun from working northward from it, which are caused by the expansion of the instrument and bubble tube upon the side heated by his rays. These matters of higher refinement may be followed in some of our best works on levelling. Most excellent instructions in this matter will be found in the appendix of A Manual of Surveying for India, 300.—Levelling Books which record the levels as they are taken are considerably varied in form, much influenced, no doubt, by the method pursued by the civil engineer for the execution of his work. The illustration, Fig. 115, shows the most general forms, but there are many others. 301.—Entries are very generally made in levelling books in black lead. Faber's artists' pencils, which require no cutting, are very generally used, No. 2 being black and moderately hard. It is very convenient to carry a small file for sharpening the lead frequently. In the author's surveyor's knife, described further on, a file forms one of the blades. 302.—Where it is desirable to make the original levelling book readings permanent for reference or otherwise, they are very commonly written in ink, Morrell's registration ink being very generally used, or the author's drawing ink answers; this being permanent is not liable to corrode the pen, nor permit the writing to be effaced in any degree by moisture. Fig. 115.—Specimens of levelling books, 1/3 scale. Ordinary Level Book with columns for No., Back Sight, Intermediate, Rise, Fall, Reduced Level, Distance, Remarks. Larger image Collimation Level Book, with columns for Back Sight, Intermediate, Fore Sight, Height of Collimation, Reduced Level, Distance, Remarks. Larger image Railway Engineers' Level Book, ruled for Back, Intermediate, Fore Sight, Rise, Fall, Distance, Reduced Level, Formation Levels, Cutting, Embankment, Remarks. Larger image Tacheometer Survey Book, ruled as above illustration. Larger image Traverse Survey Book, ruled as above illustration. Larger image 303.—The Ink Bottle mostly used is that known as the Fig. 116.—Excise ink bottle. Larger image |
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