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Found 20 results

  1. Types of Meridian and Bearings

    Meridian Meridian is a reference direction with respect to which the direction of lines are mentioned. There are three types of meridian - True Meridian, Magnetic Meridian & Arbitrary Meridian 1 - True Meridian It is the reference direction of north pole of earth from a given station point. It is also called geographic meridian. 2 - Magnetic Meridian It is the direction of north pole indicated by magnetic needle. 3 - Arbitrary Meridian This is any assume direction to a well defined object. It may be useful for small areas. e.g A mosque is taken as reference and location of road will be mentioned with respect to this mosque. Direction of magnetic north with respect to true north is called magnetic direction. Bearings Bearing is the angle which a certain line make with a certain a certain meridian. Bearing with respect to true meridian is called true bearings while magnetic bearing is the angle which a line makes with respect to magnetic meridian. There are two ways to represent the bearings, Whole circle bearing (W.C.B) Reduced Bearing (R.B) 1) Whole Circle Bearing (W.C.B) It can be taken 0° to 360°. Quadrants are taken clock-wisely and angles are also determined in clockwise direction. 2) Reduced Bearing Reduced bearing or Quadrantal bearing is the angle which a line makes from North or South Pole whichever may be near. It is value is from 0° to 90°. Using the above figures you can easily convert the Whole Circle Bearing into Reduced Bearing. Some Examples are given are below. Whole Circle Bearing (W.C.B) Reduced Bearing (R.B) 135° S45E 37° N37E 65° N65E 125° S55E 215° S35W 300° N60W
  2. Traversing It is the method of establishing horizontal controls. Traverse Traverse is a series of connected lines forming or not forming a loop. In the first case it is called closed traverse (when the loop is formed) and in the second case it called open traverse (when loop is not formed). Vertical Control That is the reference point in vertical plane, it includes series of bench marks and points of known elevations. Horizontal Control It is the series of points in the horizontal plane of known co-ordinates. Types of Traversing Traversing can be further divided into two categories depending upon the type of instrument used, Compass Traversing Theodolite Traversing 1) Compass Traversing When prismatic compass is used for determining the direction of line, the method is called compass compass Traversing. 2) Theodolite Traversing When theodolite is used for measurement of angles or directions, the method is called theodolite traversing. By direction of line we mean the bearing of that line.
  3. Angle measurement For angle measurement with theodolite vertical hair is used. Basically there are two methods horizontal angle measurement, Repetition method (For single angle) Reiteration method (For more than one angle) 1 - By Repetition method Let suppose it is desire to measure the angle A from the following figure. We will use repetition method for this purpose. Procedure Setup the theodolite at station A. Bisect the point B with vertical hair of theodolite and move telescope in clockwise and direction to bisect at point C. Note this circle reading in the book and fix this circle reading, then again bisect the point B by keeping the circle reading fixed. Now, release the circle reading and rotate the telescope again in clockwise direction till it bisect again point C. Similarly get 3rd and 4th repetition and note the circle reading after 4th repetition in the book. Change the face of telescope and repeat the above steps, an example and method of booking observations have given below, Inst. Station Angle Face Repetition Circle Reading (° ′ ″) Angle value(° ′ ″) Mean of faces (° ′ ″) A BAC L 1 25 20 00 25 20 10 25 20 9.5 4 101 20 40 R 1 25 20 03 25 20 09 4 101 20 36 2 - By Reiteration method This method is used if there are more than one angles to be measure from a certain station point. Consider the following fiqure, we will measure angles AOB and BOC using this method. Procedure Setup the theodolite at station O, bisect the point A with a certain circle reading with face left. Rotate the instrument in clockwise direction and bisect B, note the circle reading. Then rotate and the telescope till it bisect the point C, note this circle reading also. All these reading will book into face left position. Transit the telescope and rotate the instrument through 180°, this time bisect the point C firstly and then rotate telescope in anti clockwise direction towards B and then ultimately towards A. Put these readings in face right position. You can do more than one sets of measurements for the accurate results, i have done one set and booking method is as follows, Inst. Station Stn. Sighted Face Circle reading(° ′ ″) Mean of faces(′ ″) Angle value(° ′ ″) O A L 10 20 05 20 06 AOB 37 10 05 R 190 20 07 B L 47 30 10 30 11 R 227 30 12 BOC 41 10 14 C L 88 40 20 40 25 R 268 40 30 One should start observation with some initial circle reading say 25°, if we start our observation with zero circle reading our calculations for computing mean will be little bit difficult.
  4. History of Electronic Distance Measurement In surveying distance measurements were always a challenge for surveyors specially when long distances were to be measured with high accuracy. In 1950 scientist tried to calculate the distance by using light beam to travel over unknown distance with measured time. Ordinary lights travels at a velocity of 186,000 miles per second, therefore the time taken will be very small to cover a short distance . This idea was soon dropped but the scientists succeeded in finding a low velocity light beam in form of Infra Red Rays generated by solid state Gallium Arsenide Diode (GAD). This was put into laboratory experimentation in 1960 and finally instrument called Electronic Distance Measurement came into existence. Initially the instruments were very expensive but as the demand increased the price was within the reach of most professionals. Revolution in Surveying due to EDM Modern EDM equipment contains hard-wired algorithms for reducing the slope distance to its horizontal and vertical equivalent. For most engineering surveys, Total stations combined with electronic data loggers are now virtually standard equipment on site. Basic theodolites can be transformed into total stations by add-on, top-mounted EDM modules. The development of EDM has produced fundamental changes in surveying procedures e.g Traversing on a grandiose scale, with much greater control of swing errors, is now a standard procedure. The inclusion of many more measured distances into triangulation, rendering classical triangulation obsolete. This results in much greater control of scale error. Setting-out and photogrammetric control, over large areas, by polar coordinates from a single base line. Offshore position fixing by such techniques as the . Deformation monitoring to sub-millimeter accuracies using high-precision EDM The latest developments in EDM equipment provide plug-in recording modules, capable of recording many thousand blocks of data for direct transfer to the computer. There is practically no surveying operation which does not utilize the speed, economy, accuracy and reliability of modern EDM equipment. For example the EDM instrument Model # LEICA RM100 BUILDER POWER have the following particulars, Absolute circle reading Laser plummet Endless drives 30x magnification Dual-Axis compensation High resolution LCD display Electronic laser distance measurement Graphic sketches EDM measurement with red laser on target Upload and transfer data Data editing and exchange Connectivity to 3rd party devices Hence, the advent of EDM equipment has completely revolutionized all surveying procedures, resulting in a change of emphasis and techniques. Taping distance, with all its associated problems, has been rendered obsolete for all base-line measurement. Distance can now be measured easily, quickly and with great accuracy, regardless of terrain conditions.
  5. Using Rise / Fall Method In this method booking is done in the following manner, B.S I.S F.S Rise Fall R.L Remarks 2.570 - - - - 100.00 A 3.750 - 1.200 1.370 - 101.37 C.P - - 3.750 - 0.70 100.67 B 5.620 - 4.950 1.370 0.70 - - On any page of book the first reading is always a B.S and last reading is always a F.S. If you are not getting F.S as last reading on each page then it means you have done mistake while booking readings. From the B.S next F.S is subtracted. If the answer is +ve it will be Rise and if the answer is -ve it be Fall and put that reading in respective box. In the above table, I have assumed that Reduce Level (R.L) of point A is 100.0 and you can see R.L of point B is 100.67, which shows that point B is .67 units higher than point A. In case of numerous readings the check should be applied at the end of each page while booking reading, ∑ (B.S) - ∑ (F.S) = ∑ Rise - ∑ Fall = R.L of last point - R.L of first point 5.620 - 4.950 = 1.37 - 0.70 = 100.67 - 100 0.67 = 0.67 = 0.67 Using Height of collimation Method (H.O.C) For this method use the following formulas, R.L + B.S = H.O.C and H.O.C - F.S = R.L B.S I.S F.S H.O.C R.L Remarks 2.50 - - 102.50 100.00 - 1.75 1 1.95 102.30 100.55 - - 2.55 - - 99.75 - - 2.70 - - 99.60 - 2.95 - 3.10 - 99.20 - - - 2.75 - 99.40 - 7.20 - 7.80 - - - All other considerations are same as Rise/ Fall method. Below check have applied, ∑ (B.S) - ∑ (F.S) = R.L of last point - R.L of first point = 7.2 - 7.8 = 99.4 - 100 = -0.6 = -0.6
  6. Latitude and Departure In order to do start with Theodolite Traversing you should familiar with the Latitude and Departure which are discussed briefly below, OA is the line with whole circle bearing equal to θ. OC = Latitude = lCosθ OB = Departure = lSinθ By using the above formulae for Latitude and Departure with whole circle bearing, calculator will be giving aljebraic sign automatically for Latitude and Departure. For a closed Traverse ∑ of all Latitude is equal to zero and ∑ of Departure is also equal to zero. Consecutive co-ordinates When the Latitude and Departure are calculated at second point of a given line taking first point as a origin then it is called consecutive co-ordinates. Independent co-ordinates Independent co-ordinates are the Latitude and Departure of points of a traverse with respect to a common origin, so that all the values are +ve. These are used for plotting purposes. Bowdich Rule This rule is used to apply the correction in Latitude and Departure which states that correction in Latitude/Departure is equal to Length of Line multiply by Total correction in Latitude/Departure and then dividing by the perimeter. Traverse Table For the following traverse ABCD, I have applied the correction in Latitude and Departure using Bowdich rule. Line L(m) Beari.(° ′ ″) Latd. Depr. Corrections applied Consecutive co-ord. Independent co-ord. Latd. Depr. Latd. Depr. Latd. Depd. AB 148 115 30 -63.27 133.58 -0.26 - -63.98 133.58 500 500 BC 172 42 25 126.98 116.02 -0.30 - 126.68 116.02 628.68 616.02 CD 201 205 30 -181.42 -86.53 -0.36 - -181.7 -86.53 444.90 529.49 DA 202 306 15 119.44 -162.9 -0.36 - 119.08 -162.9 563.98 366.59 ∑ 723 +1.28 +0.17 -1.28 0 +0.17 As you can see, correction was not applied in Departure as the error was too small to be neglected. Using Bowdich rule we can apply correction in Latitude and Departure for respective line
  7. Fore Bearing It is the bearing of line when the first letter of line say AB is taken as origin. This is to be written as Fore Bearing (F.B). Back Bearing It is the bearing of line when second letter of line say AB is taken as origin and this is to be written as Back Bearing (B.B). Theoretical difference between Fore Bearing (F.B) and Back Bearing (B.B) should be 180°. Local Attraction If the difference between magnetic Fore Bearing and Back Bearing of a line is not exactly 180°, it may be due to presence of local attraction at one of the both stations. If this difference is exactly 180° then both stations are free from local attraction. Local attraction may be due to following reasons. Overhead electrical wires Magnetic materials in the vicinity Practice Problem In the following table observed Bearings are given, we will compute the corrected bearings and Internal Angles. Line Observed Correction Corrected F.B B.B F.B B.B AB 70° 00′ 251° 00′ A= +30′ , B= -30′ 70° 30′ 250° 30′ BC 328° 00′ 145° 00′ - 327° 30′ 147° 00′ CD 225° 00′ 71° 00′ - 257° 30′ 77° 30′ DA 139° 00′ 316° 00′ - 136° 30′ 316° 30′ By observing the table, it may be noted that no line has a difference of exactly 180° between Fore Bearing and Back Bearing. In such a case, a line where the difference is closest to 180° is selected. Such a line is called line of least disagreement, for this line correction is assign to each of the two stations of that line with opposite sign. In the above table line AB is selected for error distribution. Now, we will compute internal angles from these corrected Bearings. A = (360° - 316° 30′) + 70° 30′ = 114° 00′ B = 327° 30′ - 250° 30′ = 77° 00′ C = 257° 30′ - 147° 30′ = 110° 00′ D = 136° 30′ - 77° 30′ = 59° 00′ Before computation of internal angles you need to draw a rough sketch of scheme based on corrected bearings so that you can judge which angle is lying in which quadrant.
  8. Essential parts of a Theodolite

    1 - Tripod It should be of a rigid type capable of fixing the position of the instrument with a small lateral movement on its top when required. 2 - Foot screws These are provided for leveling the instruments. 3 - Plate level Provided for checking the level of the instrument. 4 - Horizontal clamp Provided to clamp the movement in horizontal plane. 5 - Vertical clamp For clamping movement in vertical plane. 6 - Slow motion screws These screws are used to move Theodolite either vertically or horizontally in small fractions. 7 - Telescope In a telescope vertical hair is used for horizontal angle measurement while horizontal hair is used for vertical angle measurement. Focusing arrangement for the object glass is usually provided in the body of the telescope. Collimeter is provided to bring the object in the field of view. 8 - Vertical axis It is the axis around which the telescope rotates in horizontal plane. 9 - Horizontal axis It is the axis around which telescope rotates in vertical plane. 10 - Optical plummet It is provided for centering the instrument over a ground station. 11 - Angle reading arrangement In screen display you can note angle measurements taken with Theodolite.
  9. Introduction to Leveling

    Leveling It is the branch of Surveying in which relative elevations of points are determined. There are following Types of Leveling 1 - Ordinary Leveling It is general purpose Leveling and unless otherwise stated all types of Leveling will come into this category. 2 - Reciprocal Leveling This is done when a site is unusually long, i.e crossing the river. Sights are taken from the two banks by placing the staff on the opposite bank almost simultaneously and finding the average of appearant difference of level. This method eliminates the error due to curvature and refraction. 3 - Precise Leveling This is a special type of Leveling using very precise level fitted with parallel plate micrometer and using precise staff with invar strip This is used for establishing new bench marks and therefore is undertaken by state agencies. 4 - Barometric Leveling This type of leveling is used in higher surfaces of earth like mountains. Application of Leveling Longitudinal Sections (L-Sections) : It is done to determine the levels at given intervals along the center of level road. Cross Sections (X-Sections): These are the levels at a given cross section of a road or any engineering work Contouring Invert levels for sewers Head rooms from bridges: Staff is used in inverted position from the zero end touching the ceiling of the bridge, the reading is entered as -ve and R.L of that position is calculated in usual manner.
  10. 1 - Pacing Permissible error ≤ 1 feet in 20 feet. 2 - Chain Permissible error ≤ 1 in 1000. 3 - Metallic Tape Permissible error ≤ 1 in 1000. 4 - Steel Tape This tape is made of steel alloy of very small co-efficient of thermal expansion. Permissible error ≤ 1 in 1000. 5 - Invar Tape This tape is made of very expensive steel alloy of almost negligible co-efficient of thermal expansion and is used for very precise linear measurements. Permissible error ≤ 1 in 50,000. 6 - Techometry Permissible error ≤ 1 in 50,000. 7 - Electronic Distance Meter Permissible error ≤ 1 in 100,000.
  11. Measuring Personal Pace Length

    Introduction Sometime it needs to approximate the distance between two points. One can do it without using any distance measuring instrument. But firstly you need to compute your own pace length, then you can use your pace length to approximate the actual distance. However, it is not accurate enough to use into the calculations or computations. Procedure open a chain and let it fly in straight position along the piece of ground. Walk along the chain and count the number of steps. The distance being known personal pace length will be equal to length of the chain divided by number of steps. Repeat the observation for two or three times. Example Length of chain No of Paces Pace Length 30m 44 0.68 30m 43 0.69 Application Now, you just need to multiply the number of steps you walked between two points to your pace Average pace length. Approximate Distance = Pace length × No of steps walked Useful Conversions 1 feet = 12 inch. 1 m = 3.28 feet. 1 inch = 2.5 cm. 3 inch = 0.25 feet.
  12. Vertical angle It is the angle in the vertical plane between horizontal line passing through the intersection of cross hairs and inclined line joining intersection of cross hairs and the point being observed. Circle reading in case of vertical angle During Face left vertical angle will be computed in the following manner, When angle of elevation, vertical angle = 90° - Circle reading. When angle of depression, vertical angle = Circle reading - 90°. Now, during Face right vertical angle will be computed in th following manner, When angle of elevation, vertical angle = Circle reading - 270°. When angle of depression, vertical angle = 270° - Circle reading. Example for method of booking Angle Face Circle reading(° ′ ″) Angle value(° ′ ″) Mean(° ′ ″) Remarks * L 69 58 30 20 01 30 20 01 45(+ve) Elevation R 290 02 00 20 02 00 In case of angle of elevation value of vertical angle will be +ve while in case of depression it will be -ve.
  13. For open Traversing Following procedure is adopted in case of open traversing with the help of prismatic compass, We will setup the compass at point A, B, C and so on and note the Fore Bearing and back Bearing of lines. The length of lines or legs are measured by chain twice and mean lengths are calculated. During taking measurements in the field the method used angular measurement and linear measurement should be of same standard of accuracy, i.e either combination of Prismatic compass and Chain or combination of Theodolite and Metallic tape. For closed Traversing In case of closed Traversing while using Prismatic compass the interior angles can be calculated by comparing the bearings of adjacent lines. The above rule also applied in case of closed Traverse with Theodolite. Check for closed Traverse ∑ Interior angles = (2N - 4) × 90°, where N is no of sides of closed Traverse.
  14. By deflection angle method Bearing of the first line AB is measured with the help of prismatic compass or by any other method. Setup the theodolite and point B and with horizontal circle reading bisect point A. Transit the telescope and rotate it in the direction of next station point C and note the angle, this will be θ1 R and is called deflection angle at B. Repeat this procedure for the remaining points of traverse measuring the deflection angle and writing with them letter "L" or "R". For calculation of bearing we have to simply add the deflection angles right ® to bearing of previous line to find out the bearing of next line and subtract the deflection angle left (L) from the bearing of previous line to find out the bearing of next line. Example let θ1 = 35°, θ2 = 55°, θ3 = 45°, Bearing of AB = 65° 00′ 00″ Add 35° R = 35° 00′ 00″ Bearing of BC = 100° 00′ 00″ Subtract 55° L = 55° 00′ 00″ Bearing of CD = 45° 00′ 00″ By direct Bearing method Bearing of first line AB is determined by any method. Setup the instrument at point B. Set the horizontal circle reading at the Back Bearing of of AB and bisect the back station A. Rotate the instrument in clockwise direction and bisect the next point C. The circle reading will give directly bearing of line BC. Repeat the procedure for remaining lines.
  15. Photogrammetry Photogrammetry is the branch of surveying in which measurements are made from photographs. Merits This is a very quick and accurate method of surveying in which the ground observations are almost totally eliminated. This is very accurate method if true interpretations of photographs are made. It also provides means to develop a Contour map. Demerits This method requires fair weather conditions. The instrument is very expensive and staff should be highly qualified and experienced to make full use of this method. Types of Photogrammetry There are Two main Types of Photogrammetry - Aerial Photogrammetry and Terrestrial Photogtammetry. 1 - Aerial Photogrammetry In this photographs are taken from specially manufactured plane. The characteristics of this procedure are following: The plane is made to fly along the center of longitudinal strips marked with the help of clearly visible ground monuments. The speed of aircraft being known the camera speed is adjusted accordingly to provide the requisite transverse and longitudinal overlap between successive photographs. The speed of the aircraft, its height and specification of the camera are already known. The photographs are then developed in laboratory with each photograph being placed in its proper position and by cutting the overlapped edges. This will provide a base map on the basis of actual photographs which can be processed further for particular requirements. The scale of photographs can be established by distances on the ground between two points and this dimension on the graph. The contours can be drawn by putting the photographs under the stereo plotter. Stereo plotter is an optical device which gives three dimensional view of plane photographs. 2 - Terrestrial Photogrammetry In this type, the photographs are taken from elevated ground stations. Further development of these photographs will take into account the elevations of camera and tilt of the axis of photograph. This method is very similar to previous one except that the camera is in stationary position. The camera used in this method is called photo-theodolite as it will require the same features as theodolite. This type of photogrammetry is much cheaper and can be carried out by individual surveying firms also.
  16. Surveying Types and Maps

    Surveying Surveying is a technique in by which measurements are taken on the surface of the earth and presented on the maps or stored in the digital format and vice versa. There are following types Surveying, 1 - Plane Surveying It is meant for small areas where the surface of the earth is taken to be plane surface, i.e curvature of the earth is ignored. e.g for survey inside a city. 2 - Geodetic Surveying In this curvature of the earth is taken into consideration. e.g National surveys, Basic triangulation network of a country. Geodacy is termed as actual shape of the earth. Surveying Maps There are Following Types of Surveying Maps, 1 - Topographic Maps It shows natural and artificial features on the surface of the earch. Surveying done for this purpose is called Topographic surveying. 2 - Engineering Maps Thes maps shows the detail of engineering projects, e.g roads, bridges, dams. Surveying done for this purpose is called Engineering surveying. 3 - Geographic Maps These are about the political boundaries of the country and used by general public. Surveying done of this purposes are undertaken by the state agency. e.g in Pakistan state agency is "Survey of Pakistan" 4 - Cadastral Maps These shows ownership rights of individual or Communities. Surveying done for this purpose is called Cadastral Surveying
  17. Procedure for Leveling

    Procedure Suppose A and B are two distance points whose difference of level is to be determined as in the following figure, Place the staff over point A and set up the instrument at suitable distance towards B. The suitable distance between instrument and staff can be about 25 meters. Take a sight on the staff, that reading is called Back Sight (B.S). Now, move the staff to a new position towards B and take the reading, this will be a Fore Sight (F.S). Care should be taken to make the F.S equal to preceding B.Sm, by doing this we will eliminate the effect of curvature and refraction. Now, move the instrument to a new position and take the reading on the previous position of the staff. This position of staff is known as Change Point (C.P) and this reading will be Back Sight (B.S). Now, shift the staff to the point B and take reading, this reading will be Fore Sight. Setting up of instrument means careful leveling of instrument so, correct reading can be taken. It is not necessary to move in the straight line from A to B. If due to any reason F.S is not equal B.S then this error due to curvature and refraction can still be eliminated by making, ∑ (Back Sight distances) = ∑ (Fore Sight distances)
  18. Object This System provides us with accurate horizontal and vertical measurements and gives us the position of observer in terms of Latitude and Longitude. Advantages This system is fast replacing with conventional methods of surveying like Triangulation, Traversing etc. It no longer requires the inter-visibility of station points. The conventional techniques are still required for detail surveying. The horizontal and vertical control can easily be established with the help of GPS. Instruments This system basically requires the receiver which is setup at the point of observation. The second part of the equipment is no of satellites which are 18, launched into 6 different orbits.Each orbit have three satellites with 120 degrees interval. The height of satellite is about 20,000 km with orbiting velocity of 11hr 58min. The GPS is controlled from an airforce base in California. Procedure The GPS position is achieved by the precise measurement of the distance between the satellite & the receiver at an instant of time. For a three dimensional measurements three or four satellites will be needed depending upon the quality of receiving equipment. It requires highly accurate clocks both in the transmitter and in the receiver to measure the precise distance between them. Applications This system gives us the accurate geographic position required for land surveying. It is used for navigation purposes in Aircraft,Ships,Submarines etc. It is now exceedingly used to locate the enemy targets and subsequently hitting them by GPS information guided missiles. For public use simpler version are available for locating the vehicles, the individuals and the parties, in hiking and mountaineering expeditions and other number of applications.
  19. What is an Adjustment? Adjustment of a theodolite means the operation of tightening or loosening of moveable parts to prepare the instrument for accurate measurement. It also includes other operations meant for this purpose. There are two types of adjustments for a theodolite - Temporary Adjustment & Permanent Adjustment. 1 - Temporary Adjustments These are required for each setting up of the instrument and includes following, a - Centering This is to center the instrument exactly over the ground station which is indicated by optical plummet. b - Leveling It means to make the horizontal and vertical axes in their true position. It is indicated by the central position of plate level. c - Removal of parallax That is to bring the cross hairs and the object in focus simultaneously 2 - Permanent Adjustments These are to be tested after a long interval or at the beginning of an important project. The field party is only expected to carry out the test and adjustment, if required will be done by the trained for this purpose in a workshop. Permanent Adjustments for a theodolite have discussed below, a - Plate level Adjustment Its purpose is that bubble should remain central in all positions after the adjustment. Its procedure is as follows, After making the circular bubble central bring the plate level parallel to any two foot screws. Move the two foot screws inward or outward till the bubble is centeral. Rotate the instrument through 90° and with the help of third foot screw bring the plate bubble in the central position. Repeat the process at least two times so that bubble is central at the end of each step. Now, rotate the instrument through 180°, if the bubble remains central then Adjustment is correct otherwise it is to be done. b - Horizontal axis Adjustment Its purpose is that the horizontal axis should be remain truly horizontal after the instrument has been carefully leveled. Its procedure is as follows, Setup and level the instrument at a position where highly inclined sight is available. Move the telescope in upward direction to bisect the a well defined elevated point. Now, depress the telescope and take a staff reading on the horizontally placed staff below the elevated point. This observation is taken with face left. Now, change the face and again bisect the same elevated point. Depress the telescope and take the staff reading, if the two reading are same then Adjustment is correct. c - Line of collimation Adjustment The line of collimation must pass through the point of intersection of cross hairs and optical center of the object glass (also the geometric center) and line joining the intersection of cross hairs. Its procedure is as follows, Setup and level the instrument carefully at a position from where about 100 ft long sights are available on the opposite sides. Fix the horizontal movement and fix an arrow at a distance of 100 ft and bisect it by vertical hair with face left. Transit the telescope and take the staff reading on the horizontally lying staff at a distance of about 100 ft. Rotate the instrument though 180° and again bisect the arrow. Now, again transit the telescope and take the staff reading, if the two readings are the same then the Adjustment is correct.
  20. Leveling Equipment a - Level There are different types of Levels as follows, 1 - Dumpy Level It is the type of Level in which whole body of level is cast in one unit. 2 - Tilting Level Still being used, Level can be tilted in vertical plane with the help of tilting drum. 3 - Automatic Level In this type the line of sight become horizontal when the Level is within certain limits. This system provides the works on the principal of gravitation. b - Staff It is the graduated rod of maximum 5m length usually available in telescopic form. The gradations are both in feets and meters. Smallest graduation in feet is 0.01 ft or 1/100 ft and smallest division in meters is .005m. Technical Terms in Leveling 1 - Sights A reading taken from a level on staff is called sight. 2 - Back Sight (B.S) It is the first sight taken after setting of the instrument. 3 - Fore Sight (F.S) It is the last sight taken before shifting the instrument. 4 - Intermediate Sight (I.S) These are sights taken between F.S and B.S. 5 - Line of collimation It is the straight line joining the intersection of cross hairs and optical center of object glass. 6 - Level line It is the curved line equidistant from the center of earth at all points. 7 - Horizontal line It is the straight line tangent to observer position. The of collimation obtained by a carefully leveled instrument is a horizontal line. 8 - Reduced level (R.L) It is the level of a point with respect to a certain datum whose level is taken as zero. 9 - Datum It is a certain reference level to which levels of all other points are referred i.e in Pakistan Datum is mean sea level (MSL) at Karachi. 10 - Change point (C.P) It is the last position of staff after which the instrument was shifted.