MINOR INSTRUMENTS –
CLINOMETER, CEYLON GHAT TRACER, HAND LEVEL, BOX SEXTANT, PLANIMETER AND PANTAGRAPH
1) Indian pattern clinometer (Tangent Clinometer):
Indian pattern clinometer is used for determining difference in elevation between points and is specially adopted to plane tabling. The clinometer is placed on the plane table which is leveled by estimation. The clinometer consists of the following:
- A base plate carrying a small bubble tube and a leveling screw.Thus,the clinometer can be accurately leveled.
- The eye vane carrying a peep hole. The eye vane is hinged at its lower end to the base plate.
- The object vane having graduations in degrees at one side and tangent of the angles to the other side of the central opening.
The object vane is also hinged at its lower end to the base plate. A slide, provided with a small window and horizontal wire in its middle, can be moved up and down the object vane by a rack and pinion fitted with a milled head.
When the instrument is not in use, the vanes fold down over the base.
Procedure for using the Indian pattern clinometer with Plane Table:
- Set the plane table over the station and keep the Indian pattern clinometer on it.
- Level the clinometer with the help of the leveling screw.
- Looking through the peep hole, move the slide of the object vane till it bisects the signal at the other point to be sighted. It is preferable to use a signal of the same height as that of the peep hole above the level of the plane table station.
- Note the reading ,i.e,tangent of the angle, against the wire.Thus,the difference in elevation between the eye and the object =distance x tangent of vertical angle=
d tan α.
The distance d between the plane table station and the object can be found from the plan. The reduced level of the object can thus be calculated if the reduced level of the plane table station is known.
Other types of clinometers are:
1. Burel Hand level:
This consists of a simple frame carrying a mirror and a plain glass. The mirror extends half-way across the frame. The plain glass extends to the other half. The frame can be suspended vertically in gimbles.The edge of the mirror forms vertical reference line. The instrument is based on the principle that a ray of light after reflected back from a vertical mirror along the path of incidence is horizontal. When the instrument is suspended at eye level, the image of the eye is visible at the edge of the irror,while the objects appearing through the plain glass opposite the image of the eye are at the level of observer’s eye.
2. De Lisle’s clinometer:
This is another form of clinometer, similar to that of Burel Hand level, used for measuring the vertical angles, determining the slope of the ground ,and for setting out gradients. This consists of the following:
- A simple frame, similar to that of a Burel level, carrying a mirror extending half-way across the frame, the objects being sighted through the other half which is open. The frame can be suspended in gimbles.The edge of the mirror forms a vertical reference line.
- A heavy semicircular arc, is attached to the lower end of the frame. The arc is graduated in gradients or slopes from 1 in 5 to 1 in 50.The arc is attached to the vertical axis so that it may be resolved to bring the arc towards the observer(i.e. forward) to measure the rising gradients or away from the observer to measure the falling gradients.
- A radial arm is fitted to the centre of the arc. The arm consists of a beveled edge which acts as index. By moving the arm along the arc, the mirror can be inclined to the vertical. The inclination to the horizontal of the line from the eye to the point at which it appear in the mirror equals the inclination of the mirror to the vertical. The arm also carries a sliding weight. When the weight is moved to the outer stop(at the end of the arm),it counter balances the weight of the arc in horizontal position and makes the mirror vertical. To make the line of the sight horizontal, the weight is slided to the outer slope and the radial arm is turned back to its fullest extent.
Procedure to measure a gradient:
- Slide the weight to the inner stop of the arm. The arc should be turned forward for rising gradients and backward for falling gradients.
- Suspend the instrument from the thumb and hold it at arm’s length in such a position that the observer sees the reflected image of his eye at the edge of the mirror.
- Move the radial arm till the object sighted through the open half of the frame is coincident with the reflection of the eye. Note the reading on the arc against the beveled edge of the arm. The reading obtained will be in the form of gradient which can be converted into degrees if so required.
3. Foot Rule Clinometer:
A foot rule clinometer consists of a box wood rule having two arms hinged to each other at one end, with a small bubble tube on each arm. The upper arm or part also carries a pair of sights through which the object can be sighted. A graduated arc is also attached to the hinge, and angles of elevations and depression can be measured on it. A small compass is also recessed in the lower arm for taking bearings.
To sight an object, the instrument is held firmly against a rod, with the bubble central in the lower arm. The upper arm is then raised till the line of sight passes through the object. The reading is then taken on the arc.
4. Abney Clinometer (Abney Level):
Abney level is one of the various forms of clinometers used for the measurement of slopes ,taking cross sections, tracing contours, setting grades and all other rough leveling operations. It is a light, compact and hand instrument with low precision as compared to engineer’s level. The abney level consists of the following.
- A square sighting tube having peep hole or eye piece at one end and a cross wire at the other end.
- A small bubble tube, placed immediately above the openings attached to a vernier arm.
- A semi-circular graduated arc is fixed in position.
The Abney level can be used for
- measuring vertical angles
- measuring slope of the ground, and
- tracing grade contour.
Measuring vertical angles:
- Keep the instrument at eye level and direct it to the object till the line of sight passes through it.
- Since the line of sight is inclined, the bubble will go out of centre .Bring the bubble to the centre of its run by the milled screw. When the bubble is central, the line of sight must pass through the object.
- Read the angle on the arc by means of the vernier.
Measuring slope of the ground:
- Take the target, having cross-marks,at observer’s eye height and keep it at the other end of the line.
- Hold the instrument at one end and direct the instrument towards the target till the horizontal wire coincides with the horizontal line of the target..
- Bring the bubble in the centre of its run..
- Read the angle on the arc by means of the vernier..
Tracing grade contour:
- To locate the contour gradient in the field, a clinometer,a theodolite or a level may be used.
- Let it be required to trace a contour gradient of inclination 1 in 100 ,starting from a point A ,with the help of a clinometer.The clinometer is held at A and its line of sight is clamped at an inclination of 1 in 100.
- Another person having a target at a height equal to the height of the observer’s eye is directed by the observer to move up or down the slope till the target is bisected by the line of sight.
- The point is then pegged on the ground.
- The clinometer is then moved to the point so obtained and another point is obtained in a similar manner.
- The line between any two pegs will be parallel to the line of sight.
CEYLON GHAT TRACER:
It is a very useful instrument for getting out gradients. It essentially consists of a long circular tube having a peep hole at one end and cross-wires at the other ends. The tube is supported by a A-frame having a hole at its top to fix the instrument to a straight rod or stand. The tube also engraved to give readings of gradients. A heavy weight slides along the tube by a suitable rack and pinion arrangement. The weight, at its top, contains one beveled edge which slides along the graduations of the bar, and serves as an index. The line of sight is defined by the line joining the hole to the intersection of the cross wires and its prolongation.
a) To measure a slope:
- Fix the instrument on to the stand and hold it to one end of the line.Keep the target at the other end.
- Looking through the eye hole, move the sliding weight till the line of sight passes through the cross mark of the sight vane.
- The readings against the beveled edge of the weight will give the gradient of the line.
b) To set out a gradient:
- Hold the instrument at one end
- Send the assistant at the other end with the target
- Slide the weight to set it to the given gradient, say 1 in n
- Direct the assistant to raise or lower the target till it is bisected.
Derive a peg at the other end so that the top of the peg is at the same level as that the bottom of the target.
A hand level is a simple, compact instrument used fro reconnaissance and preliminary survey, for locating contours on the ground and for taking short cross-sections. It consists of a rectangular or circular tube,10 to 15 cm long,provided with a small bubble at the top.
To use the instrument:
- Hold the instrument in hand(preferably against a rod or staff) at the eye level and sight the staff kept at the point to be observed
- Raise or lower the object end of the tube till the image of the bubble seen in the reflector is bisected by the cross-wire.
- Take the staff reading against the cross-wire.
In some of the hand levels, telescope line of the sight may also be provided.
Adjustment of the hand level
To make the line of sight horizontal when the bubble is centered.
- Select two rigid supports P and Q at about 20 to 30 meters apart.
- Hold the level at a point A on the support at P and mark a point D on the other support Q, when the bubble is central.
- Shift the instrument to Q, hold it at the point D, centre the bubble, and mark the point B where the line of sight strikes the first support. If A and B do not coincide, the instrument requires adjustment.
- Select a point C midway between A and B.With the adjustment screws, raise or lower the cross-wire till the line of sight bisects C.
The box sextant is small pocket instrument used for measuring horizontal and vertical angles, measuring chain angles and locating inaccessible points. By setting the vernier to 90º,it may be used as an optical square.
A box sextant consists of the following parts.
- A circular box about 8 cm in diameter and 4 cm high.
- A fixed horizon glass, silvered at lower half and plain at upper half.
- A movable index glass fully silvered
- An index arm pivoted at the index glass and carrying a vernier at the other end.
- An adjustable magnifying glass, to read
- A milled-headed screw to rotate the index glass and the index arm.
- An eye hole or peep hole or a telescope for long distance sighting.
- A pair of colored glasses for use in bright sun.
- A slot in the side of the box for the object to be sighted.
A planimeter is a measuring instrument used to determine the area of an arbitrary two-dimensional shape. There are several kinds of planimeters, but all operate in a similar way. The precise way in which they are constructed varies, with the main types of mechanical planimeter being polar, linear and Prytz or “hatchet” planimeters.
They consist of a linkage with a pointer on one end, used to trace around the boundary of the shape. The other end of the linkage is fixed for a polar planimeter and restricted to a line for a linear planimeter. Tracing around the perimeter of a surface induces a movement in another part of the instrument and a reading of this is used to establish the area of the shape. The planimeter contains a measuring wheel that rolls along the drawing as the operator traces the contour. When the planimeter’s measuring wheel moves perpendicular to its axis, it rolls, and this movement is recorded. When the measuring wheel moves parallel to its axis, the wheel skids without rolling, so this movement is ignored. That means the planimeter measures the distance that it’s measuring wheel travels, projected perpendicularly to the measuring wheel’s axis of rotation. The area of the shape is proportional to the number of turns through which the measuring wheel rotates when the planimeter is traced along the complete perimeter of the shape.
Developments of the planimeter can establish the position of the first moment of area (center of mass), and even the second moment of area. The pictures show a linear and a polar planimeter.
The pointer M at one end of the planimeter follows the contour C of the surface S to be measured. For the linear planimeter the movement of the “elbow” E is restricted to the y-axis. For the polar planimeter the “elbow” is connected to an arm with fixed other endpoint O. Connected to the arm ME is the measuring wheel with its axis of rotation parallel to ME. A movement of the arm ME can be decomposed into a movement perpendicular to ME, causing the wheel to rotate, and a movement parallel to ME, causing the wheel to skid, with no contribution to its reading.
A pantograph is an instrument used for reproducing, enlarging or reducing the maps. It is based on the principles of similar triangles. It consists of two long bars AB and AD hinged together at A and supported on castors or rollers at B and D.Two short arms EF and GF are hinged together at F and are connected to AD and AB at E and G respectively. Thus AGFE is a parallelogram of equal sides for all positions of the instrument. The long bar AD carries a movable tubular frame which can be slided along it. The sliding frame carries an index and also a heavy weight Q which forms the vertical axis of the instrument; the whole instrument moves about the point Q,The bar EF carries a pencil point P attached to a carrier which can also be set to a desired reading on the bar EF.
The larger arm AB carries tracing point at the end B.For any setting of the instrument, the point B,P and Q are in a straight line. The original map is kept at Band is traced .Correspondingly, the pencil point P also moves, but the point Q remains fixed in position.Thus,if B is moved straight by an amount BB’ ,the point P moves to P’ the ratio between BB’ and PP’being equal to the ratio of reduction. For any position of the tracing point, the points B’,P’ and Q are always in a straight line.
If it is desired to enlarge the map, the pencil point is kept at B,the tracing point at P and the map under the point P.The moving frames at Q and P are set to the same reading equal to the ratio of enlargement. The pencil can be raised off the paper, by means of a cord passing from the pencil round the instrument to the tracing point, if so required.