Top 4 Instruments Used in Land Surveying |Land Surveying | Surveying

This article throws light upon the four main instruments that are used in land surveying. The instruments are: 1. Hand Level 2. Clinometers 3. Enlarging and Reducing Plans 4. Box-Sextant.

Instrument # 1. Hand Level: (Fig. 13.1):

This is a simple, compact hand instrument used for locating contours, taking cross-sections etc. in preliminary reconnaissance surveys where great accuracy is not required. It consists of a square or circular tube 10 to 15cm long with a bubble mounted on the top. It has a pin-hole at one end and a cross-wire at the other.

Immediately below the bubble tube is provided an opening in the top of the tube through which the bubble is seen reflected in the mirror fixed inside the tube inclined at 45 to its axis and immediately under the bubble tube. The mirror extends half way across the tube, and the objects are sighted through the other half. The line joining the pin-hole and the intersection of the cross-wire is the line of sight. It is ‘horizontal when the cross-wire bisects image of the bubble seen in the mirror.

To use the instrument:

(i) Hold it in hand or against a ranging rod at a known height say 1.5 m above the ground, and sight the staff.

(ii) Raise or lower the forward end of the tube until the image of the bubble is bisected by the cross-wire.

(iii) Observe the reading at which the cross-wire appears to cut the staff.

Instrument # 2. Clinometers:

The clinometers are light, compact, hand instruments used for measuring vertical angles, finding out the slope of the ground and for locating points on a given grade. They are used for rough and rapid work.

The various forms of the clinometer are:

a. The Abney’s level or abney clinometer.

b. The Indian pattern clinometer or the tangent clinometer.

c. The Ceylon ghat tracer.

The first three forms are more commonly used and are explained in this chapter:

a. Abney’s Level (Fig. 13.2):

It is the most commonly used type of clinometer.

It is very useful for rapid work and is chiefly used for:

(a) Measuring vertical angles,

(b) Measuring the slope of the ground,

(c) Tracing a grade contour, and

(d) Taking cross-sections in hilly areas.

It may also be used as a hand level by setting the vernier to the zero of the scale.

The Abney’s level consists of:

(i) A square sighting tube with an eye-piece or a small telescope at one end and a cross-wire at the other. A mirror is placed inside the tube behind the cross-wire inclined at an angle of 45° to the axis and occupying half the width of the tube.

(ii) A semicircular graduated are, the middle point of which is marked with zero, and graduation upto 90° on both the side of zero, The slopes are marked at the inner [rim of the semi-circle which are read by the outer edge of the vernier plate.

(iii) A small bubble tube attached to the vernier, arm, which can be rotated by a worm wheel and milled-head screw. The vernier is of extended type and can read angles up to 5 or 10 minutes.

(a) To measure a vertical angle:

(i) Hold the instrument in hand and direct it towards the object.

(ii) Bisect the object with the cross-wire and at the same time turn the milled wheel until the cross-wire bisects the reflection of the bubble as seen in the mirror.

(iii) Take the vernier reading on the arc, which is the required angle in elevation or depression according as the object is above or below the horizontal line or sight.

Note:

For any inclination of the tube, the bubble tube remains horizontal and the vernier arm vertical.

(b) To measure the slope of the ground:

(i) Make a mark on a ranging rod at the height of your eye, say, by tying a red rag round it. Keep the ranging rod at one end of the slope and hold the instrument at the other end.

(ii) Direct the instrument on to the mark on the ranging rod and turn the milled wheel until the reflected image of the bubble is bisected by the cross-wire. In this position, the bubble tube is horizontal while the telescope is parallel to the slope of the ground.

(iii) Take reading on the arc, which is the required slope of the ground.

(c) To trace a grade contour:

(i) Calculate, the angle corresponding to the gradient and set it on the arc by means of vernier and make a mark on him ranging rod at the height of your eye.

(ii) Hold the instrument at the starting point and direct it towards the mark on the ranging rod held at a probable point on slope.

(iii) Direct the assistant to move the ranging rod up or down-hill until the mark on the rod and the reflected image of the bubble in the mirror are bisected simultaneously by the cross-wire.

The line joining the instrument-station to the foot of the ranging rod is on the given grade, mark the foot of the ranging rod by a peg.

(iv) Move to this peg and hold the instrument over it. Repeat the process to establish the next point. Continue the process until the last point is established.

Testing the Abney’s Level:

First Method:

(i) Sight a well-defined distant object and observe the angle of elevation or depression (say α₁).

(ii) If these two values (α₁ and α₂) do not agree, the instrument is out of adjustment.

Second Method:

This method gives good results and is preferred.

(i) Take two ranging rods and make marks upon them at the same height, say, 1.5 m.

(ii) Hold one ranging rod at the top of a slope and the other at the foot of the slope. Standing at the foot of the slope and holding the instrument against the mark on the ranging rod, sight the mark on the ranging rod held at the top of the slope and observe the angle of elevation (α₁).

(iii) Then hold the instrument against the mark on the ranging rod at the top of the slope and sight to the mark on the ranging rod held at the foot of the slope. Observe the angle of depression (α₂).

(iv) If these two values agree, the adjustment is correct.

Adjusting the Abney’s Level:

(i) If these two value do not agree, obtain the corrects value of the angle of inclination by taking the mean of the two readings

(ii) Set the vernier to read and sight the object in test and bring the bubble in the center by turning the adjusting screws.

Note:

If the instrument is out of adjustment, the index error=

 

b. Indian Pattern Clinometer or the Tangent Clinometer (Fig. 13.3):

This is a very handy and useful instrument used mostly along with a plane table. It measures angle of elevation or depression and their tangents.

It consists of:

(i) A brass or gunmetal base plate mounted on three ivory buttons so that the instrument may be moved over the paper without spoiling its surface.

(ii) A brass bar with two vertical folding vanes about 20 cm apart at ends. The eye-vane is about 9 cm high and the object-vane about 18 cm high above the bar. The eye-vane has a pin-hole while the object- vane has a long vertical slit.

The left side of the vertical slit is graduated in degrees from the centre upwards for angles of elevation, and downwards for angles of depression, and the right side of the slit is graduated in natural tangents of the corresponding angles on the left. The zero of the scale is opposite to the eye hole.

(iii) The object-vane is provided with a frame carrying a horizontal hair, which is moved along the scales by a rack and pinion arrangement. The line joining the eye-hole and horizontal wire furnishes the line of sight.

(iv) A spirit level is attached to the bar on its upper side. The bubble can be centered by means of milled-head screw provided at one end of the base plate. When the bubble is central, the line joining the eye­hole and the zero of the scale is horizontal.

The angles of elevation and depression and the reduced levels of points are determined as follows:

(i) Set up and level the plane table over a given station.

(ii) Place the instrument on the board and make the bubble central by means of the milled-head screw.

(iii) Measure accurately the height of the eye-hole above the ground.

(iv) Look through the eye-hole and slide the frame along the scale until the horizontal wire bisects the object. Observe the graduations opposite to the horizontal wire. The reading on the left side of the slit gives the vertical angle (angle of elevation or depression) in degrees and that on the right side of the slit gives the value of the tangent of the corresponding angle.

Let α = angle of elevation or depression to the point sighted.

h = height of eye-hole above the ground.

d = horizontal distance between the plane table station and the point sighted as scaled from the plan.

Then the difference in height between the eye-hole and the point sighted = d tan a.

When d is large, correction for curvature and refraction shall have to be applied.

The reduced level of the line of sight = the reduced level of the station occupied by plane table + the height of the eye-hole above the ground.

Hence the reduced level of the object = the reduced level of the station occupied + h ± d tan α + correction for curvature and refraction.

For d tan α, use + sign if α is the angle of elevation, and – sign, if α is the angle of depression.

Test and Adjustment:

The instrument is in correct adjustment if the line joining the eye-hole and the zero of the scale is horizontal when the bubble is central.

To test it establish a distant point at the same level as the eye-hole with a level or theodolite, and find whether the line of sight passing through the zero of the scales strikes it.

If not, the instrument is out of adjustment. Move the milled head screw and make the line of sight to strike the established point. Now the line of sight is horizontal but the bubble is out of the centre. Then bring the bubble in its central position by means of the adjusting screw at the end of the bubble tube.

c. Ceylon Ghat Tracer (Fig. 13.4.):

The Ceylon ghat tracer is extensively used for locating points on a given gradient in the preliminary survey of a hill road and also for measuring the angles of slope. This is very cheap and the work can be conducted quickly.

It consists of:

(i) A hollow brass tube having an eye-hole at one end and a large opening with cross-wire at the other. The tube is attached to a bracket and can be held suspended from a wooden staff shown separately.

(ii) A horizontal racked bar which is attached rigidly to the tube. The bar is parallel to the tube and is at a distance of 2 to 3 cm from it.

(iii) A weight, which can be moved along the rack by means of a milled-head screw. The upper part of the weight has a pointed edge which forms the reading index.

(iv) On the side of the tube at which the pointed edge of the weight is in contact with it, are marked the gradients from 1 in 120 to 1 in 6 on both sides of the centre of the scale. This facilitates reading of the falling and rising gradients.

The line joining the eye-hole and the intersection of the cross-wires furnishes the line of sight. When the sliding weight is kept at centre so that the index touches the zero of the scale, the line of sight is horizontal. The line of sight can be titled to ant desired gradient by sliding the weight along the rack to the required reading on the scale.

To trace the grade contour, proceed as follows:

Suppose it is required to lay out a gradient of 1 in 50 along a hill slope:

I. Suspend the instrument from the pin inserted in the wooden staff and hold the staff at the given station.

II. Slide the weight along the rack by means of the milled-head screw until the index reads 50 on the scale.

III. Send an assistant with a sight vane to a probable point on the hill slope.

Note:

Sight vane is a T-shaped staff on which is marked the height of the axis of the suspending tube above the foot of the suspending staff. If this is not available, take a ranging rod and make a mark at the same height as that of the object-vane.

(iv) Look through the eye-hole and direct the assistant to move up or down hill until the cross-wire bisects the centre of the sight vane. Mark the foot of the sight vane with a peg. This is the required point, and the line from the instrument station to this point lies on gradient of 1 in 50.

(v) Proceed to the point so established and repeat the process to locate the next point. Continue the process until the last point is established.

To measure a slope:

(i) Hold the instrument at one end of the slope and sight vane at the other.

(ii) Slide the weight along the rack by means of milled-head screw until the centre of the sight vane is bisected by the cross-wire.

(iii) Note the reading at the index edge of the weight, which gives the value of the slope.

Instrument # 3. Enlarging and Reducing Plans:

The most accurate method for enlarging or reducing plans is to replot them from the field-notes to the required scale.

But for ordinary purposes, plans may be enlarged or reduced by:

(1) The graphical method,

(2) The photographic method, and

(3) The instrumental method.

1. Graphical Method by Squares (Fig. 13.5 a & b):

(i) Draw horizontal and vertical faint lines at right angles to each other in pencil on the plan so as to form a network of squares. Number and letter each line both ways as shown in fig. 10.5 (a).

 

(ii) Then draw the same number of squares, but of a size larger or smaller according to the given scale on the new sheet of drawing paper.

Number and letter each line both ways as before as a fig. 13.5 (b). Fig. 13.5 (b) is reduced to 1/4th the original size as the side of each square is halved.

(iii) Note the points of intersection of the sides of squares and the boundary of figure and measure their distance with reference to the sides of the squares. Mark these points on the corresponding square lines on the new sheet by transferring the distances by scaling or by judging by eye. The work of transferring the points is much facilitated by the use of proportional compass.

2. Photographic Method:

This is also a graphical method.

Special Cameras:

(Block maker’s Cameras) are used on which a graduated scale for reduction or enlargement is provided. The plan to be reduced or enlarged is placed in front of the camera, and after setting the desired scale on the camera, the negative sensitised plate is exposed to light.

This is the most accurate method.

3. Instrumental Method:

There are ordinarily three instruments in use:

(a) Proportional compass,

(b) Pantograph,

(a) Proportional Compass (Fig. 13.6):

It consists of two similar brass arms with sharp points at ends and longitudinal slots in the middle.

These are held together by a milled-head screw passing through a slider which moves in the slots. The slider has index marks on both faces.

On one face of the instrument are marked, a scale for lines on side of the slot and a scale for rations of circles on the other. On the face of the instrument are marked, a scale for the ratios of plans on one side of the slot and a scale for the ratios of solids on the other.

To use the instrument, first fold the legs one upon the other, loosen the milled-head screw. Move the slider until the index line coincides exactly with the division marked with a number representing the given ratio, say 3 on the scale of lines. Then tighten the screw and pull the legs apart.

The distance between the points at long end will be exactly thrice that between the points at the short end for enlarging a plan, measure the distance from the plan with the points at the short end, and transfer them on the new sheet with the points at the long end and vice-versa.

(b) Pantograph. (Fig. 13.7):

It consists of four tubular brass arms, each square in section. The two long arms AB and AC are hinged together at one end A and two short arms DE and DF are hinged together at D, and also attached to the longer arms at E and F.

The four arms are so arranged as to form a parallelogram having all sides equal. The instrument is supported on several small rollers so that it remains parallel to the surface of the paper and can move freely in all directions.

The arms AC and DF are graduated and marked with division like 1/2, 1/3, 1/4 etc on their upper surface.

These are also provided with sliding frames, which can be clamped at any of the divisions on the arms. The frame on the longer arm is attached to a weight w known as fulcrum, which keeps the instruments in stable position where as the frame on the short arm carries a pencil point with it.

The other long arm AB carries a tracing point at BH which is moved over the lines of the original plan, while the pencil fitted to the pencil point produces new plan. As shown in the figure the instrument is set for reduction.

If it is to be used for enlarging a plan interchange the tracing point and the pencil. When the instrument is correctly set, the tracing point, the pencil point and the fulcrum will be in one straight line as shown by dotted line in the figure, and will remain so far all positions of the instrument. The pencil can be lifted off the paper by means of a lever attached to the cord when tracing point runs over blank spaces.

For reducing a plan, first set the indices of the sliding frames at the graduations representing the required reduction and clamp them. Place the tracing point on the original plan and the pencil point on the drawing sheet on which a new plan is to be made. Then move the tracing point along the lines on the original plan, when a reduced plan will be drawn by the pencil on the sheet beneath it.

For enlarging a plan, interchange the tracing point and the pencil. The original plan is placed below the tracing point and a drawing sheet on which new plan is to be obtained under the pencil.

The instrument is commonly used for reducing plans and does not give satisfactory results when used for enlarging plans due to unsteadiness in the joints and supports.

When the weight is kept on the long arm AC, erect copies are produced, while reverse copies are produced, if the weight is kept on the short arm DF and the pencil on the long arm AC, the divisions to the left being used during the operation.

Planimeter (Fig. 13 8):

Instrument # 4. Box-Sextant:

The sextant, of which the box sextant is the most compact form, is a reflecting instrument capable of measuring angles upto about 120° and normally to an accuracy of one minute. It is one of the most precise hand instruments yet devised for measuring angles.

It is called a ‘sextant’ because its limb (the graduated are on which the reading is taken) includes one sixth of a circle. Although the arc is limited to 60°, yet the instrument will measure angles upto 120°.

The theory of sextant is based upon the optical principle that if a ray of light undergoes two successive reflections in the same plane by two plane mirrors, the angle between the first and the last direction of the ray is twice the angle between the mirrors. Referring to fig. 13.9, let L and M be the two objects being viewed from E so that angle θ is file required angle between the two objects.

Let I be the index glass fixed to the index arm (IA) which terminates into a vernier (V) capable of moving along the graduated arc (A-B). The index glass along with the index arm can be turned and fixed in any suitable direction. A second mirror, called the horizon glass (H), having the lower half unsilvered and upper half silvered is rigidly attached to the frame. (E) is the position of the eye where a telescope can also be fixed for taking long sights.

The ray from signal M passes through the unsilvered portion of the horizon glass and through the telescope to the eye (E) .The light from the signal L strikes the index mirror at X and is reflected to Y at the horizon glass and then through the telescope to the eye. Each set of rays forms its own images .By moving the index arm, two images can be gives the required angle θ.

To prove that the angle between the mirrors i.e θ = 2α, draw XP and YO normal to index and horizon glass respectively.

Now in order to directly read the angle between the objects, the graduation on are A-B arc plotted on twice the natural scale.

The principal parts of the box-sextant as indicated in fig. 13.10 are:

Now in order to directly read the angle between the objects, the graduation on are A-B arc plotted on twice the natural scale.

The principal parts of the box-sextant as indicated in fig. 13.10 are:

(i) A-B:

the graduated arc.

(ii) AK:

adjusting key.

(iii) E:

eye-hole where a telescope may be fitted for taking long sights.

(iv) H:

The horizon glass, the lower half is unsilvered and upper half is silvered. One of the two objects can be seen directly through the lower half while the upper half reflects the ray reflected by index glass.

(v) I:

The index glasses which is fully silvered and reflects the ray from one this two objects

(vi) IA:

The index arm fixed to index glass.

(vii) J&K:

The key- holes for adjusting the horizon glass, screw at key hole J tilts for horizon glass, screw at key hole tilts the horizon glass to place it at right angle to the sextant plane. The horizon glass should be at right angles to the sextant plane.

The screw at key hole K moves the glass laterally and affects parallelism to the index glass. It is essential that index arm of should read zero on the graduated arc when the plane of the horizon glass is parallel to the plan of the index glass.

(viii) MG:

Adjustable magnifying glass for magnifying and facilitating the reading of graduated arc.

(ix) S:

The screw by which the index glass may be moved simultaneously.

(x) SG:

Sun glass for intercepting the sun’s rays.

(xi) V:

The vernier at the end of the index arm.

Measurement of Angles:

To measure the horizontal angle, hold the sextant horizontally in the left hand and look through the eye-hole towards the left hand object seen through the unsilvered portion of the horizon glass. With right hand, turn screw S until the other object reflected from the index glass (I) appears upon the upper silvered portion of the horizon glass. When the left hand signal and the image of right hand signal in the horizon glass coincide, take reading on graduated arc to get the required angle.

If the vertical angle between two objects is to be measured, hold the sextant vertically viewing the lower object directly. Turn the screw (S) till the image of the higher object gets coincident with that of the lower object. The reading at the arc gives the required angle.

To get better results, it is beneficial to view directly the nearer objects and view the distant object by reflection if the two objects are at unequal distances. Also we may bisect the brighter object out of the two by reflection.

Advantages of Box-Sextant:

1. It is portable requiring no support other than the hand.

2. It is more useful than compass because:

(i) the angles can be read to one minute with the help of vernier and the graduated arc.

(ii) it being a reflecting instrument, is not affected by local attraction or magnetic variation.

3. It can be used on horseback and in a boat.

4. It can be used as an optical square in chain surveying by adjusting the vernier at 90°.