PRELIMINARY DRAFT CIRCULATION NOTICE

Transcription

1 PRELIMINARY DRAFT CIRCULATION NOTICE Our Ref: Date WRD 05/T TECHNICAL COMMITTEE: Geological Investigation And Sub-Surface Exploration Sectional Committee, WRD ADDRESSED TO: All Members of Geological Investigation And Sub-Surface Exploration Sectional Committee, WRD 05 Dear Sir(s), As per the decision taken in the 7 th meeting of the sectional committee, we are posting the draft standard as mentioned below on the BIS website for your ready reference. Doc.WRD 05(449) Guide for Topographical surveys for river Valley Projects (second revision) Kindly examine this draft and forward your views stating any difficulties which you are likely to experience in your business or profession, if this is finally adopted as a national standard and kindly provide your specific suggestions for revising the same in view of latest technology. Last date for comments : 31 th January 2005 Comments, if any, may please be made in the format attached to the draft and mailed to the undersigned at the above address. Comments will be appreciated in electronic form at the address bsharma@bis.org.in. In case you have any difficulty in accessing the document at our website, please write to us for a hard copy. Thanking you, Yours faithfully, [ Bhavana Sharma ] Asstt Director (WRD) Note: Please inform your address for faster communication at the address above

2 For official use only Doc. WRD 05(449) December 2004 BUREAU OF INDIAN STANDARDS PRELIMINARY Indian Standard GUIDE FOR TOPOGRAPHICAL SURVEYS FOR RIVER VALLEY PROJECTS (Second Revision) (Not to be reproduced without the Last date for receipt permission of BIS or used as a of comments is STANDARD) FOREWORD (Formal clauses will be added later) Topographical survey forms a very important and vital operation for river valley projects. Any river valley project to be planned and constructed is to be based on certain basic facts and figures. The process of collection of these facts and figures is termed as investigations in general. The first step of investigations is to carry out topographical survey and preparation of maps which are graphical representation of land. Topographical survey enables presentation of a picture of the terrain in the form of maps which provide important and accurate data about topography and other characteristics of the area. These accurate data assist in formulation of correct plans for development of river valley projects for harnessing and utilizing the country's water resources. Topographical surveys for river valley projects, when required, are carried out by the Survey of India, the only national survey organization in the country. All demands for topographical maps and surveys should be placed on the Survey of India. In this standard it has been sought to lay down only the general principles or guidelines as regards topographical survey for river valley projects. No attempt has, therefore, been made to describe the details of actual processes of surveying as it is felt that such details are readily available in all standard text-books on surveying, Geodesy, Photogrammetry and Cartography or departmental handbooks of the Survey of India. This standard was first issued in 1969 and revised in In the first revision specifications regarding scale of maps for different surveys had been modified to reflect current practice. Provisions regarding river survey, flood survey and command area development survey had been added. Assistance had also been rendered by the valuable technical information supplied by the Survey of India. The present revision is proposed to reflect the experience gained on the subject since then.

3 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis shall be rounded off in accordance with IS: The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. 1. SCOPE 1.1 This standard lays down the general principles and guidelines for topographical surveys connected with a river valley project. 2 REFERENCES The Indian Standards listed below contain provisions which through reference in this text constitute provisions of this standard. At the time of publication, the editions indicated were valid. All standards are subject to revision and parties to agreements based on these standards are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below: IS No. IS 2: 1960 IS 5510:1969 Title Rules for rounding off numerical values (revised). Guide for soil survey for river valley projects. 3. TERMINOLOGY For the purpose of this standard the following definitions shall apply. 3.1 Absolute Orientation - The process by which an adjoining pair of relatively oriented aerial photographs are brought to correct horizontal scale and vertical datum. 3.2 Azimuth - The azimuth of a body is the angle between the observer's meridian and the great circle (vertical circle) through the body and the zenith of the observer. NOTE - The meridian of an observer is the great circle passing through the poles the zenith, and the nadir. 3.3 Diapositive - A positive print of a photograph prepared on film or glass transparency. 3.4 Electromagnetic Distance Measuring (E.D.M.) Instruments - These are the instruments based on electro-magnetic waves for precise measurement of distance. These are further divided into two groups, namely M.D.M. ( Microwave distance measuring instruments) and E.O.D.M. ( Electro-optical distance measuring instruments). M.D.M. instruments use micro waves as carrier while E.O.D.M. instruments use light or infra-red rays. 3.5 Height - The height of a point above mean sea-level is the length of the vertical between the mean sea-level surface and the point. 3.6 Latitude - The latitude of a place is the angle between the direction of a plumb line at the place and the plane of the equator. It is marked north or south according to the place being north or south of the equator.

4 NOTE - If earth is assumed a perfect sphere of homogenous density the plumb line shall pass through the centre of sphere. 3.7 Longitude - The longitude of a place is the angle between a fixed reference meridian called the prime or first meridian and the meridian of the place. Longitude is measured from 0 to 180 eastward or westward and is marked E o or W o. 3.8 Mean Sea-Level - Mean sea-level is the average elevation of the sea surface which is determined by continuous observation of the varying level of the sea for as long a time as possible. 3.9 Photogrammetry -The science and art of obtaining reliable measurements by means of photography Photographic Interpretation - The art of examining photographic images for the purpose of identifying objects and judging their significance Relative Orientation - The process by which two successive aerial photographs having common overlap is brought to mutual geometrical relationship between themselves which existed at the time of their exposure Stereopair - Two photographs with sufficient overlap and consequent duplication of detail to make possible stereoscopic examination of an object or an area common to both. 4. CLASSIFICATION 4.1 Types of Survey Topographical surveys required for river valley projects may be broadly classified under five important types, namely, river survey, reservoir survey, site survey, command area survey and communication survey. There surveys are required in different stages of investigation of a river valley project. Flood Control and Command Area Development Survey may also be considered as a part of the River Valley Projects. Methods of topographical survey vary depending on the degree of accuracy to be attained, extent of the project and topography of the project area River Surveys - River Surveys cover longitudinal Section ( L-Section ) and Cross Section (X-Section) of the river upstream and downstream of the proposed structure The L-Section on the upstream side shall extend from the axis of the structure to the point up to which the back water effect is likely to extend or up to the maximum water level ( MWL ) + 5 m whichever is less. If any headworks is situated within the reach, the L-Section shall be taken up to the headworks. On the downstream side, the L-Section shall extend for 10 km from the axis of the structure or up to the nearest headworks whichever is less. The levelling of the L- Section shall be done at 50 m or less intervals along the fair weather deep channel. The following items shall be indicated in the L-Section: a) Date of survey of the particular reach and water level on that day; b) Deep pools and rapids, rock outcrops, etc; and c) Maximum historical observed highest flood level (H.F.L. )

5 4.1.3 The X-Section on the upstream side shall be taken at 200 m intervals up to MWL + 5 m or 1 km on either side of the firm bank, whichever is less, for a distance of 2 km from the axis of structure and thereafter at 1 km intervals corresponding to the length of the L-Section. On the downstream side, X-Section shall be at 200 m intervals and taken up to historical highest flood level + 1 m on either side of firmbank for a distance of 2km to 5 km from the axis of the structure depending upon the meandering nature of the river. An X-Section shall also be taken along the axis of the structure. The levelling shall be done at 50 m or less intervals. The following items shall be indicated in the X-Section: a) Date of survey and the water level on that day; b) Minimum water level; c) Maximum historical/observed water level; and d) Rapids and Rock Outcrops, etc The longitudinal and Cross Sections shall be plotted to a scale of 1 : horizontal and 1 : 100 vertical. 4.2 Reservoir Survey A reservoir survey is necessary to provide the basic data for calculation of water storage capacity of a reservoir, for indicating the limit of submergence areas and for locating the topographic depressions or saddles in the reservoir rim which may cause spilling or leakage of the reservoir. The scale and contour interval for topographical survey for reservoir survey depend on the extent of the area and the topography of the site. The reservoir survey is made with a view to examining all possible alternatives for its locations and to eliminate such of the proposals which become unsuitable from considerations of technical feasibility, economy and practicability. The reservoir survey enables preparation of a fairly dependable project report supported by cost estimates and economic studies. Reservoir survey maps shall be made on scales ranging from 1 : to 1: Site Survey A site survey is carried out to investigate the suitability of a site for a particular structure, such as dam site, power house or tunnel site. A dam site survey should cover a sufficiently large area to include all possible locations of the main dam, coffer dams, spillways, separate outlet structures, and other appurtenances. The scale of dam site survey maps may range from 1 : I 000 to 1 : Power house site survey should include an area for possible construction of plant sites, stockpile areas, warehouse, switchyard areas and camp sites. The scale of maps may normally be from 1 : 250 to 1 : Tunnel site survey maps should be made along possible tunnel alignments on scale 1 : to 1 : Command Area Survey The command area is the area which is likely to be benefitted from the river valley project. The command area survey is very important in order to know beforehand the capability of the command area to make maximum use of the benefits of the river valley project. Sometimes the command area survey is also necessary to assist in designing the layout of main canals and distributaries for irrigation purposes. The scale of command area survey maps should range from 1 : to 1 :

6 4.5 Communication Survey The movement of construction equipment and material to a project construction site will normally require the construction of access roads, as most project sites are not accessible from existing highways. Sometimes branch lines from the existing railway map be required. Existing railways and highways running through the reservoir area may have to be relocated where these are liable to be inundated or submerged by a reservoir. The surveys are required for this purpose and for deciding the alignment and grades of railways and highways. The surveys for the location or relocation of railways and highways are normally performed by the field survey parties of the railway or highways authorities who should be entrusted with the determination of specification and survey instructions. 4.6 Flood Control Survey These surveys are required in some project areas for flood control work, that is, for the planning and execution of flood protection works. These maps are prepared on 1: or 1: scale. 4.7 Command Area Development Survey These are carried out for shaping topography to conserve water resources and for an integrated development to optimise productivity of the area. Scale for such survey maps varies between 1 : to 1 : PROVISION OF CONTROL 5.1 Control Survey The first essential to produce an accurate map of an area is to cover the whole area with a number of carefully determined ground control points which will form a framework on which to base the ensuring detail survey of the area. The accuracy of a detail survey depends on the accuracy of its framework of ground control points and hence the desired accuracy of control survey has to be decided in advance keeping in view the scale, purpose and topography of area. It is also essential to ensure that the control in adjoining river valley projects is consistent. For this purpose, the control surveys are invariably linked to the National control framework provided by the Survey of India Two types of ground control points are required for starting the framework of a survey. These are horizontal control and vertical control. The horizontal control provides the planimetric position of a control point. The vertical control provides the altitude or height of a control point above the mean sea-level. 5.2 Horizontal Control The co-ordinates of horizontal control points should be fixed either in terms of latitude and longitude expressed in degrees, minutes and seconds or in rectangular terms in grid metres in Northings and Eastings relating to predetermined origin and type of projection.

7 5.2.1 Horizontal Control points may be fixed by one or a combination of the following method: a) Doppler Satellite control, b) Triangulation/E.D.M. trilateration, c) Theodolite/E.D..M. traverse, and d) Resection. Of these, the soundest method for topographical operations is a system of accurate triangulation, whereby undue accumulation of error is precluded in the extension of the framework, and at the same time limits are set to the intrusion of error, in the internal details to be surveyed Dopplar Satellite Control By this method currently in vogue, three dimensional co-ordinates of a satellite receiver position can be obtained by observation to the Navy Navigation Satellite System Satellites. The data collected by the receiver from the satellite transmissions is processed with the help of a digital computer to obtain the co-ordinates of the ground station where the receiver is located. Since three dimensional co-ordinates can be obtained, this method can provide horizontal and vertical control simultaneously Triangulation/ E.D.M. Trilateration or Traverse The electronic distance measuring instruments in control surveys operations are useful. In view of the accuracy and speed of operations of these instruments, which avoid the laborious and time consuming methods of linear measurements by chains, their use is recommended. Since these instruments measure distances, they may be used for measuring three sides of a triangle which is known as trilateration. They may also be used for traversing to measure traverse legs in conjunction with theodolite for measuring angles. E.D.M. trilateration of traverse is carried out by the Survey of India According to the procedure of observations, instruments used and quality of triangulation results, a triangulation is classified as primary, secondary and tertiary. There is a variety of electronic distance measuring instruments available to day and are very useful in control survey operations. This classification may also be called as first-order, second-order and third-order triangulation. The primary and secondary triangulations are termed as geodetic. The tertiary triangulation is called topographic. All geodetic data which form the basis of topographical survey are published in triangulation and levelling pamphlets. These pamphlets may be obtained from the Survey of India Primary triangulation is the highest grade of triangulation and is employed for the determination of the shape and figure of the earth and other geodetic investigations. It also constitutes the basic precise framework for mapping and for power order of triangulation. As it is independent of external checks, all possible precautions and refinements shall be taken in the observations and their reduction. The length of a base-line in primary triangulation may be about 8 km to 12 km on the average and the sides of the triangles may range from 16 km to 50 km. The average triangular error ( or the discrepancy between the sum of the measured angles in a triangle and plus the spherical excess of the triangle) should not be in excess of one second. The probable error of computed distance should lie between 1 in to 1 in

8 Secondary triangulation is designed to connect two primary series and thus furnish points closer together than those of primary triangulation. The average triangular error in secondary triangulation should not exceed 3 seconds. The probable error of computed sides should lie between 1 in to 1 in Tertiary triangulation is run between the stations of the primary and secondary series and forms the immediate control for topographical surveys. The average triangular error may range from 3 seconds to 15 seconds and the probable error of computed sides may lie between 1 in 5000 to 1 in The initial data required for commencing topographical triangulation in an area of which a detail survey is contemplated, is a base of known length, an azimuth, latitude, longitude and height above mean sealevel of one station. The triangulation chain starts from this known base and closes on another known base Theodolite Traverse In some stretches of flat country, primary triangulation series may lie over 150 km apart, and even where a secondary series might have been run to connect them, many of the secondary control points might have been destroyed. Owing to the flat- ness of the country the expense of triangulating the intermediate area may be greater and triangulation may take several seasons to carry out. In such cases theodolite traversing is a more expedient and economical method of control survey. Traversing is a more laborious and at the same time a less accurate method of fixing control points than triangulation and, therefore, may only be resorted to in flat ground where buildings, trees, high grass or haze prevent distant vision The length of traverse legs are measured by chains ( Crinoline chains, see Note I ) or steel bands ( Hunter short. base, see Note 2) or subtense bars or invar wires or E.D.M. instruments according to the accuracy aimed at. NOTE I - Crinoline chain is a riband of steel about 3 to 5 mm wide and about 1/2 mm thick, obtainable in lengths of 100 metres to 110 metres. It is calibrated for its length under standard conditions of temperature and pressure. It is used for direct measurement of distances where the accuracy desired is of higher standard. The tension applied during field measurements shall normally be the same as that applied during calibration. NOTE 2 - Hunter short base is a short measured base, invented by Hunter, consisting of calibrated steel tapes hung in catenary, for commencement or carrying forward of control work by triangulation ort raverse. Normally it consists of four tapes calibrated under standard conditions of temperature and pressure. The tapes together with other accessories are easily portable and form a convenient base to commence and carry forward control operations in any type of terrain, particularly bushy and undulating, with meagre existing control points The angles between consecutive traverse legs should be measured by a theodolite set up at each station in turn. Vertical angles should be observed by theodolite from one station to another when heights are required. If the position of one station and the bearing of a traverse line connected to it are known, then the position of the next and all succeeding stations may be computed The rate of accumulation of errors in traverse affects the accuracy and shall be kept under control by closing traverses on well fixed points at both ends and also by introducing bases and azimuths. Topographical or tertiary traverse may have an accuracy of 1 in provided more precise angular and linear measurement devices are used.

9 The observation of azimuths, either from sun or stars, at frequent intervals, provides a direct check against accumulation of errors of angular measurements. These azimuths, as well as the bearings from one station to another, should be computed in the field. If comparison reveals 1 an error larger than 1 minutes in 12 stations in main traverse lines the line shall be 2 reobserved The most important field check in linear measurements is the established custom of measuring each line twice and comparing the values. Each line should be measured once with a long chain 30 metres in length and a second time with a short chain 20 metres in length. These two measurements should agree within 1 in 500 for chains and 1 in for steel bands. The measurements of the length obtained by the longer chain shall be accepted. In E.D.M. instruments field check is obtained by repeat observations. However, the time taken is much less than surface taping and accuracy achieved is of high order Traverse should be computed in rectangular co-ordinates and referred to an origin Resection This method may be adopted to determine the geographical position of a point on the ground occupied by surveyor, by making observations to other stations or points of known co-ordinates without visiting them. This method obviates the necessity of visiting or making observations from other existing control points and stations. When the observations are made with theodolite, it is termed as 'trigonometrical resection' or 'theodolite resection'. In topographical triangulation, stations may occasionally be fixed by this method when it is not convenient to visit other stations for making observations. A judicious selection of the control points is essential while adopting this method. It also involves complicated computations. However, occasionally, it may be necessary to resort to this method in exploratory or active service work. 5.3 Vertical Control Vertical control points are required to provide a general idea of altitude of the area and [or surveying contours where necessary. The heights of vertical control points above mean sea-level may be determined by Dopplar Satellite control or levelling. Levelling, or the determination of the relative altitudes of points on the earth's surface, is very important and useful in acquiring data for the design of all classes of works and during construction Apart from the method of Dopplar Satellite control described in vertical control may be fixed by one or combination of the following methods of levelling: a) Trigonometric levelling, and b) Levelling ( spirit levelling ) Of these, the sounder method for provision of accurate vertical control is by levelling ( spirit levelling). For topographical surveys, the methods of trigonometric levelling and spirit levelling may be used.

10 5.3.2 Trigonometric Levelling The elevations of triangulation stations may be established by observing the vertical angles between stations and computing the. differences in elevation trigonometrically. The angles are taken at the time the station is occupied for the purpose of measuring the horizontal angles. The trigonometrical levelling should start from and close on certain point of known elevation. From the known elevation of these points the elevations of the triangulation stations and intersected points may be computed by means of the differences in height derived from the vertical angles and the lengths of the triangle sides. In measuring the vertical angles the theodolite should be placed over the centre mark of the station and its height above the station should be measured. The triangulation stations should have definite signals which should be observed for sighting the cross-hair when measuring the vertical angle. From the known height of the distant signal above the ground, the hight of the triangulation station sighted may be computed The main difficulty in obtaining accurate results in this kind of work arises from the uncertainty of the atmospheric refraction. It may be nearly eliminated by taking reciprocal observations between two stations when the refraction is steady. The best time to observe vertical angles is during the middle of the day, from h to h as the refraction is then much less variable than in the morning or late afternoon Trignometric levelling may be used for fixing vertical control points where the terrain is steep and undulating and the contour interval aimed at is 5 metres or more Levelling Levelling or the determination of the relative altitude of points on the earth's surface, is an important operation; both in acquiring data for the design of all classes of works and during construction. Like triangulation, depending on its quality, levelling is classified as high precision, secondary, double tertiary or tertiary High precison levelling is carried out to provide the main vertical framework of the country and for the detemination of the shape and figure of the earth and other geodetic investigations. It also constitutes the basic precise framework for less precise levelling. Secondary and double tertiary levelling may be used for estabilishing the elevations of the controlling bench marks in the project area for all subsequent levelling. Tertiary levelling may be used for providing a dense network of vertical control points in the project area The simplest operation for levelling consists in determination of the difference of level between two points so situated that, from one position of the instrument, readings may be obtained on a staff held successively on the two points. The precise location of the instrument is immaterial, but to eliminate the effects of possible instrumental error, refraction and curvature effects the two sights should be nearly equal in length as may be judged. Precision levelling also requires observations for the values of gravity along the path of levelling, for accurate computations The closing error for the different classes of levelling shall not exceed those given below: Error of Closure in mm a) High precision ± 4 K b) Secondary ± 6 K c) Double tertiary ± 12 K

11 d) Tertiary ± 24 K where K is the distance in kilometres. 5.4 Field Observations For achieving good results the records should be maintained in a systematic way in order to facilitate computations and reduction of results. All field observations should be recorded neatly in ink in the observation forms provided for the purpose. The required number of measures laid down for the order of accuracy to be aimed at should be observed. On completion of observations at a station the mean should be calculated and compared for agreement. On completing this, the observer should scrutinize the results, and when large differences appear, a repeat set of observations of the stations, which gave doubtful results, should be taken before doing other works. If this important precaution is omitted, errors will accumulate subsequently and there will be no means of rectifying them. Measures which do not comply with the prescribed criterion should be repeated before leaving the station of observation For triangulation stations all three angles of each triangle must be observed and the observer should keep up a record of completely observed triangles, to enable him to note his triangular error, that is, the difference of the sum of the three observed angles of a triangle from 180, spherical excess being considered as necessary, and to see that it is within the specified limits. Any repeat observations of triangles, if necessary, should be completed before leaving the area of work. 5.5 Field Control Charts Field Control Charts should be maintained both for horizontal and vertical control points for showing their location and also for assisting in computations. For triangulation, a field triangulation chart should be prepared either on a smaller scale existing map or even on blank paper with the existing triangulation stations shown on it. A scale of 4 1 to 2 1 of that of the detailed survey will generally be found suitable. For levelling, a field levelling chart should be maintained on an existing large scale map of the area All great trignometrical (G. T. ) stations or other stations or well defined intersected points of old topographical triangulation falling in the area of work should be plotted and inked on the field triangulation chart in a distinctive colour. When one series of triangulation is joined to another, two adjacent stations common to both the series should be opserved and clearly indicated in the triangulation chart for facilitating computations All new stations and their connecting rays should be inked clearly on the chart All high precision and secondary level lines and bench-marks should be clearly marked on the field) evelling chart in a distinctive colour. All new level lines and their alignments along with the position of benchmarks should be inked clearly on the chart.

12 5.6 Survey Marks and Stations Whenever possible, stations should be placed on rock in-situ with a mark which should consist of a hole of about 5 mm diameter drilled into the rock about 20 mm deep with a circle engraved around it. For more precise work with E.D.M. instruments the hole drilled in the centre may be of 1mm to 2 mm diameter, 3 cm to 4 cm deep with a copper or brass wire of the same diameter plugged into it and made flush with the top surface When rock is not available, a large stone should' be embedded about one metre underground, with a circle and dot cut on it, with a second mark-stone similarly treated, and having its dot vertically above the dot of the lower mark, placed flush with the top surface of the platform, Or, if the triangulator may obtain some old 2.5 cm piping, about one metre length, it may be driven vertically into the ground, the centre of this being' taken as the actual mark, Surrounding the mark, a platform should be built of earth and stones, at least 0'5 m high and about 3 m square, The sides of the platform should be revetted with big stones or gently sloped, For. traversing wooden pegs 8cm x 8cm square in section and 30 cm long may be used. A circle and dot shall be engraved on top of each peg which are driven vertically into the ground with the top flush with the ground. Traverse stations may also be made by a circle and dot on parapets of culverts, wells or verandahs of public buildings, like inspection bungalows, dak bungalows, etc In levelling, the levelling staves are generally placed on iron crow-foots firmly placed on the ground so that there is no sinking of the levelling staff during observations. In precise leyelling, wooden pegs are driven in ground in place of iron crow-foots for stability of staves during the levelling operation. However, for a bench-mark a circle with B and M written on either side may be used for semi-permanent and permanent bench-marks engraved on parapets or culverts, well or verandahs or steps of public buildings like inspection bungalows, dak bungalows, etc The descriptions of G.T. stations and high precision bench marks are given in the G.T. pamphlets and levelling pamphlets (published by Survey of India). These pamphlets covering the area of project should be obtained from the Survey of India during investigations All the permanent stations and bench-marks established by the Survey of India are required to be protected, maintained and periodically inspected by the local authorities in collaboration with the Survey of India. Any of these marks disturbed, damaged or missing must be reported to the Survey of India immediately For river valley surveys, certain reference control points may be required to be established for controlling subsequent surveys and construction programmes. The actual specifications of these control points will depend on their importance and type of soil on which to be constructed. These should be finalized in consultation with the Survey of India arid constructed at least one season in advance to make allowances for settling down.

13 5.7 Survey Computations Apart from carrying out good field work and computing them correctly, it is also important to maintain the records in the most systematic manner. All computations should be carried out in duplicate and independently. The duplicate computations should be compared to check any gross disagreement. Both sets of computations should be signed and dated by both computers. The completed computations should be scrutinized by a responsible officer. Use of electronic scientific pocket calculator in the field helps to expedite survey computations. Computations can also be carried out with the help of micro and large computers. To guard against computer failure, computer programmes should be used with check data and results On completion of computations the following records of control survey should be prepared: a) History sheet of control surveys, b) Description of stations/bench-marks, c) List of co-ordinates/heights, and d) Traces of triangulation/traverse levelling charts A short narrative account of the control survey, mentioning the object of the control survey, the instruments and signals used and other facts of interest should be included in the history sheet All basic data collected during the survey and ail computations made as a part thereof should be preserved and filled in such a manner as to be readily available and easily understood. The records should be classified job-wise and kept safe. All original records and computations should be preserved carefully. 6. TOPOGRAPHICAL SURVEYS 6.1 Importance of Topographical Survey Surveying and mapping are the first pre-requisites for starting any programme of development. Maps are basic to all planning and should be prepared before any planning or design may be carried out. Information regarding existing departmental mapping cover on different scales may be had from the map catalogue published by the Survey of India. Information about existing large scale survey and photographic coverage of the project area may be obtained from the Survey of India. The standard topographical series of maps on 1 : scale though helpful at preliminary stage, may not be adequate for detailed planning of river valley projects. Maps on larger scale are necessary for the planning and design of different types of engineering structures. Topographical maps of the project area on 1: or 1 : scale should be obtained from Survey of India at the preliminary investigation stage. The cost of surveying and mapping is only a minor fraction of the total cost of the project and when undertaken at initial stage will save a lot of infructuous expenditure arising out of incomplete or faulty data. 6.2 Scale According to the principles of map making a map should normally be prepared at the smallest scale with the largest contour interval that will serve the purpose of the project. The scale of a map is ordinarily governed by accuracy, purpose and terrain.

14 6.2.1 A map needs to be sufficiently accurate so that the measurements made from it are accurate for the purpose. A map required for determining the capacity of a proposed reservoir does not normally. require high accuracy specially in steep areas. But the highest contour near the top height of the dam has to be surveyed very accurately because it defines the limit of the submerged area which in turn indicates the limit of land acquisition and is a fair index of the amount of compensation and rehabilitation involved. The top contour should be surveyed by ground spirit levelling, especially if the terrain is fairly flat Another significant factor for determining the scale of the map is the requirement of space. A map prepared on a large scale requires higher accuracy in surveying which costs more. Therefore, in such cases the scale of survey should be kept smaller than the scale of the final map. 6.3 Contour Interval While deciding the contour interval it may be noted that the survey of small contour intervals takes more time and costs more. The contours are mostly required for the representation of the configuration of the ground. When the contours are closer, the drawing of the contours becomes difficult and the cost of fair mapping increases. A judicious selection of contour interval for river valley projects is very important The accuracy of contours is also a very important factor in map specifications. Normally, an accuracy of half the contour interval is required in all topographical maps. In flat areas, the requirements of positional as well as height accuracy of contours would be of the same degree and, therefore, a smaller contour interval becomes necessary. In steep countries larger contour intervals may be adequate. Contours closer than 2 mm are difficult to draw The contour interval is largely influenced by the steepness of the country and a visit to the ground is, therefore, essential before drawing up the survey specifications. For plain areas, contour intervals of 0.5 m to 1 m are required but in undulating or hilly areas contour intervals of 2 m to 5 m are suitable. 6.4 Methods of Survey The topographical maps required for river valley projects may be prepared either by ground survey method or by photogrammetric method. The ground survey method is usually resorted to when the extent of area is small and there is difficulty in timely procurement of aerial photographs. The latest trend is to adopt photogram metric method for topographical surveying. The aerial photographs contain large amount of details all of which cannot be picked up by ground survey. Photogrammetric survey is generally quicker and more accurate. 6.5 Control Requirements for Ground Survey If the project area undertaken for survey is hilly, the horizontal control should be provided by theodolite triangulation. If the project area is flat, the horizontal control should be provided by theodolite traverse. The density of control points should be adequate for subsequent detail survey by plane-tabling If the project area is undulating or hilly, and the contour interval is 5 m or larger, the vertical control may be provided by trigonometrical levelling connected to spirit levelling

15 wherever possible. If the project area is flat and the contour interval is one metre or smaller, vertical control points have to be provided by spirit levelling. A regular net work of levelled spot heights at intervals of about 200m should be provided as vertical control Plane-tabling may be resorted to for ground survey. The project area should be divided into several parts so that each part may be conveniently accommodated and surveyed on a planetable. The sheet limits are indicated by rectangular projection lines and the horizontal and vertical control points are plotted on the plane-table section. There should be sufficient horizontal and vertical control points all along the borders of the plane-table section in order to ensure the desired accuracy, of survey at the edges. I f the contour interval is large, both the details and contours may be surveyed by plane-tabling. If the contour interval is small, the details may be surveyed by plane-tabling and the contours by interpolation based on the network of spirit levelled spot heights. Contouring can also be done with the help of clinopole (see Note under 6.6) for which the levelled spot heights net work may be 500 metres to 750 metres apart, depending upon the terrain The surveyor should first carry out reconnaissance of the area of his plane-table and fix sufficient auxiliary points well distributed in the area, in addition to those already fixed by triangulation, in order to enable him to resect his position, almost anywhere. After this, the surveyor is in a position to commence sketching details by intersection and interpolation from his resected positions or fixings. In hilly area, the survey of details and contours should be carried out together. In flat or undulating country representation of contours may be treated separately from detail The survey may be carried out in pencil in approved symbols and then inked in proper colours. The plane-table sections should be kept free from colour washes which tend to confuse the details. Consequently separate colour trace and height trace should be prepared on tracing cloth, covering the area of each plane-table section to facilitate fair mapping later on. The survey should be tested on the ground by field inspection before withdrawing from the area. 6.6 Control Requirelnent for Photogralnlnetric Survey Aerial photographs are used in stereo-pairs with a forward overlap of about 60 percent and lateral overlap of about 20 percent. The survey should be carried out from the overlapping portions only. The overlapping portion of two adjacent aerial photographs is called the stereomodel. The requirements of minimum ground control for aerial photogrammetric suvery should be four plannimetric control points on each stereomodel. If the contour interval is 5 m or larger, the minimum ground control required for aerial triangulation should be provided in the field and control on aerial photographs should be extended by aerial triangulation. For smaller contour intervals, each stereomodel should be provided with the required four control points on the ground. The planimetric ground control points should be sharp and definite, for example, track junctions and cultivation corners. The height control points should be selected as far as possible on level ground. The accuracy of the ground control must be commensurate with the scale of final map and its contour interval Aerial Photography The Surveyor General is the coordinating authority for the execution of all aerial photography in India. Before ordering fresh photography it is advisable that information about existing photography should be obtained from the Surveyor General of India. Fresh aerial photography

16 should, be asked for only when the scale and date of the existing photography do not cater to the requirements of the indentor. Aerial photography in scales 1: to 1 : is normally required for river valley projects Identification of Control Points In aerial triangulation, the requirements of ground control horizontal and vertical, depends on the final accuracy required and the method of aerial triangulation employed. Where large computers are available analytical adjustment covering the entire block of photographs is carried out. If aerial triangulation is not contemplated, because of higher accuracy requirements or any other reason, each stereoscopic pair of photographs should be controlled The horizontal and vertical control points should be carefully identified on aerial photographs by a visit to the ground. The accuracy of photogrammetric survey is dependent on accurate identification of ground control points and should, therefore, be done very carefully and augmented with sketches made on the spot Topographical Survey by Photogrammetric Method The photogrammetric survey should be carried out on precision photogrammetric restitution instruments using aerial photographic points on film or glass. The ground control points should be clearly marked on these diapositive prints. The stereoscopic pair of photographs or diapositives are first inserted on holders of photogrammetric instruments and then relative orientation is carried out to bring the two consecutive photographs in their correct relative positions. The absolute oreintation should be then carried out which ties down the aerial photographs to the correct geographical coordinates and correct height datum with the help of the ground control or aerial triangulation control points appearing on the stereoscopic pair. On completion of absolute orientation, the detailed plotting should be completed first and then the contours. If the contour interval is small, the contours should be interpolated separately based on spirit-levelled spot heights. Normally, the river valley project area contours may be surveyed on a separate transparent astrafoil base so that the registration with the details may be easily checked.. The plotting of details should be carried out on dimensionally stable material Field Completion Survey The names of places and features, pereniality of rivers, classification of roads, etc, are not clear on aerial photographs and should be collected on the ground. For this purpose, field completion surveys should be carried out either by ground verification of aerial photographs before the photogrammetric surveyor by ground verification of the blue prints obtained from photogrammetric survey. This step is important and should always be done. 6.7 Fair Mapping and Printing After the photogrammetric plotting and field verification are completed, the fair mapping of fair drawn originals or scribed negatives should be taken up. For river valley projects, the maps are normally printed in two colours, namely, black for all details, names and heights; brown for contours. Accordingly, two separate originals are fair mapped or scribed for printing in black, and brown. During fair mapping or scribing of the outline original, the roads, canals etc, should be drawn or scribed according to the laid down widths and line thicknesses. The names, milestones, heights, etc, should be nearly type written or printed stick ups used. In the contour

17 original, the contours should be drawn or scribed both thick and thin, every fourth or fifth contour being thick. The contour values are inserted at regular intervals After the fair mapping of the two originals is completed, the registration of the two originals for correct colour fitting is carried out. The shapes of contours should conform to the configuration of the ground and fit well with the streams and rivers. The contour values should not foul and obscure other important details, names or heights. These should be checked and corrected. The fair drawn originals are then ready for printing. 7. RESERVOIR SURVEY 7.1. Importance of Reservoir Survey Reservoir survey may be carried out to find the capacity of the proposed reservoir along different contour levels and to determine the extent of area that will be submerged consequently. The scale and contour interval of the survey depends upon the topography of the reservoir area. 7.2 Purpose of Survey The survey serves two main purposes. The capacity of the reservoir may be calculated on the basis of data collected by the survey. It also helps to decide whether the capacity at various levels of the reservoir will enable to provide the desired yield in terms of hydroelectricity or supply of water for irrigation, navigation, to towns or for industry, etc. From the limit of submergence area at different levels, the amount of compensation to be paid and the cost necessary for rehabilitation of the affected population may be calculated. The technical data and economic aspect of reservoir construction are, therefore, obtained from the survey. The reservoir survey map remains a record of the topography of the submerged area after the lake is formed and is of value in assessing the feasibility of the project as well as the subsequent sedimentation. 7.3 Selection of Reservoir Site A reservoir site shall be so selected that it serves the desired purpose of the project becoming economically feasible. Topographical, geological, hydrological, meteorological, soil, forest cover, possible sedimentation, etc, shall be carefully considered for assessing the economic feasibility of the dam. The area that will be submerged, compensation to the local people, any further expenditure for their re-habilitation, any loss due to submergence to roads, railway lines, mines and quarries and other such expensive installations and construction are economic factors which shall be weighed carefully against the overall benefit, economic or otherwise, which will result from the construction of the dam and reservoir. 7.4 Provision of Control The horizontal control shall have an accuracy of 1 : or higher and may be done by triangulation or traversing depending upon the type of terrain. In narrow river valleys where triangulation sides are required to be kept shall due to topography, triangulation stations with larger sides should be established in the area to which the narrow triangulation series should be connected and adjusted for better accuracy Vertical control should be provided by spirit levelling in flat areas. The highest level of the submergence area should be accurately fixed, since the payment for compensation depends on