Chapter 2 Topo

CHAPTER 1 TOPOGRAPHIC SURVEYING 1.1. Introduction Topographic surveying is the process of determining the positions, on the earth's surface, of the natural and artificial features of a given locality, and of determining the configuration of the terrain. The location of the features is known as planimetry and the configuration of the terrain is known as topography. The purpose of the survey is to gather data necessary for the preparation of the topographic map that displays both the planimetric and topographic features. The topographic map shows by means of suitable symbols The spatial configuration of the earth's surface, which includes such features as hills and valleys; Other natural features such as trees, streams, oceans, seas, etc. and Man-made features such as buildings, roads, canals, cultivation, etc. The distinguishing feature of topographic map is that it represents terrestrial relief. The principal data required for topographic mapping are elevation and distance. Several ground methods that require the use of transit, theodolite, plane table and alidade, level, hand level, tape leveling rod in various combinations are available for furnishing data necessary for topographic mapping total station EDMs and photogrametric methods are also employed where available. Use of Topographic Maps They are necessary aids in the design of any engineering project that requires consideration of landforms, elevations, or gradients. They furnish necessary data for economists, geologists, and others interested in the general development of natural resources. Representation of Topography Topography may be represented on a map by relief models, shading color gradients, hachures, form lines, or contour lines. Of these representation techniques, only contour lines indicate elevations directly and quantitatively. The rest of the chapter is mainly devoted to representation of topography by contour lines and conventional symbols. But first the uses of the plane table instrument will be discussed, as it is the most versatile instrument used in compiling topographic map by any or the field methods. Plane table surveys The plane table and alidade combination can be used for various surveying operations such as traversing leveling and compilation of topographic maps. The combination consists of a drawing board mounted on a tripod in such a manner that the board can be leveled and rotated in azimuth without disturbing the tripod. This condition is realized by the Johnson head tripod which has two nuts that control leveling and rotation of the board independently of one another. The alidade is mounted on top of the board, and is the device used for sighting. Standard board sizes are 18 x 18in, 18 x24 in and 24 x31 in. During sighting the board provides the lower motion for the alidade, and the movement of the alidade over the face of the board can be considered as the upper motion. The rotation of the board is used in back sighting whereas the rotation of the alidade over the face of the board is used in fore sighting. The plane table and alidade is more versatile than the theodolite for map compiling. the drawing sheet fitted on top of the table should be of good quality to withstand adverse weather effects and repeated erasures. It should have a reasonable roughness to take pencils without undue grooving. 1.2.1 Plane-table Traverse Horizontal control for compiling maps can be established by plane table traversing rather than by a more precise theodolite tape traverse. If high quality, durable drawing sheet is used, and if the plane table operator exercises care in taking stadia readings and in plotting the points, a highly satisfactory traversing can be accomplished by using the plane table. The operation can be carried out during map compiling or before the commencement of the work. The latter has the advantage of adjusting the traverse before starting mapping and eliminates much erasing and map revision in the event there is a relatively large closure. The following general techniques are used in orienting the instrument in plane table traverses. 1.Orienting by backsighting This method is applicable to large-scale mapping. Each traverse station is occupied and the board is oriented by backsighting at the previous station. Each line in the traverse is observed from both ends, thus providing checks on observations. Set up and observation procedures for this method are as follows: - Select the traverse stations (A, B and C). Set up the instrument over one of them (A) and level it. Measure the H.I by holding a rod alongside the alidade. Measure the distance from the instrument station to rear station (C) and forward station (B) with the stadia method. Then measure the DE between the instrument station and the rear and forward stations respectively. Using the blade of the alidade, draw a line parallel to the line of sight on the drawing sheet. To the chosen scale mark the respective positions of the rear and forward stations (c and b) on the drawing sheet. Move the instrument to the next station (B) and level it. Orient the board by backsighting to the previous station (A). Then take observations to the rear (A) and forward (C) stations by repeating (2) and (3) above. Note: The position of each station is plotted after taking observations from both ends of a line. a c b Fig.1.1 Orienting by backsighting Any misclosure is adjusted by the graphical method. Let aa’ be the closure at stations b and c, draw a line parallel to aa'. Locate adjusted stations b' and c' on the line bb' and cc' respectively in proportion to their partial distance from a. bb’=(lab/L)*aa’ cc’=((lab+lbc)/L)*aa’ The adjusted traverse is shown as a’b’c’. 2. Orienting by Compass needle: The figure below shows a traverse to be carried out by plane table. The positions of the stations A through F are to be located by using the compass needle of the alidade. Station A is occupied and the board is so oriented that the entire traverse will fall on the sheet The compass needle is unclamped and the alidade is rotated in azimuth until the compass needlepoints to the magnetic north on the compass. A line representing the magnetic meridian is drawn the full length of the blade. The alidade is then pivoted in turn to F and B and the rays af and ab are drawn to scale representing the distances AF and AB. Differences in elevation are computed for determining elevations of F and B. F f e a E d b c f D A a f b a d B C b c Fig . 1.2 Orienting by compass needle The plane table is next set up at station C and leveled. The compass needle is released and the blade is aligned with the line representing the magnetic meridian. The board is rotated in azimuth until the compass needle points to the magnetic north of the compass. The board is now oriented. The alidade is rotated in turn to Band D and the rays obtained are drawn to scale representing CB and CD. The procedure for the set up at E is the same as that of C. 3. Method of Radiation In this method, the plane table is set up at only one station and various points are located by radiating (drawing) a ray from the instrument station to each of the points and plotting a scale along the ray the distance measured from the station to the point sighted. The radiation method is suitable for surveys of small areas, which are likely to be commanded from a single station. It is useful in large-scale works, if used in combination with other methods. The procedure of plane tabling by radiation method is as follows: Fig 1.3 Orienting by method of radiation *Select an instrument station O such that all points to be located re visible from it; *Setup and level the table and clamp the board; *Mark the point o on the sheet exactly over the station O on the ground by means of plumb bob; *Mark out the direction of the magnetic meridian with the help of trough or circular box compass on the top right hand corner of the sheet. *With the alidade touching o, sight the various points A,B,…etc to be located and draw the radial lines towards them along the fiducial edge of the alidade lightly with a sharp pointed pencil. *Measure the distance OA, OB…etc from station to the various points with a chain tape or by stadia hairs; *Plot the distance to scale along the corresponding ray and then join the points a ,b ,…etc to give the outline of the survey. Notes Care should be taken to see that the fiducial edge of the alidade should touch the station point o on the paper while taking the sights on different points. This can be best done by erecting a pin on the point o on the sheet and keeping the alidade’s ruling edge just touching it. To avoid the confusion, the various rays should be referenced. The fieldwork can be satisfactorily checked by measuring the distances AB, BC, etc on paper. Intersection or triangulation method of plane tabling This method is widely employed for plotting the details on the maps. It can also be used for plotting the position of points to be used at subsequent stations. The various points can be located by the intersection of rays drawn from two different stations P and Q forming a base line The only linear measurement required is that of the base line the distant and inaccessible objects, the rivers, in survey of hilly country (where distances cannot be measured easily), and for checking the distant objects. Fig 1.4 Orienting by intersection Procedure The procedure of plane tabling by the method of intersection is as follows: Choose two instrument stations P and Q such that all the points may be commanded from both the stations. Set up and level the plane table at station P and mark the point P on paper so that it is vertically over station P on the ground. Mark the direction of the magnetic meridian by means of the trough compass. With the alidade centred on point P, sight towards Q and then all other objects A, B, C and D (to be located) and draw rays towards them along the fiducial edge of the alidade. Mark all the lines by letters a1,b1,c1,q1 , d1 while sighting towards A,B,C,Q, and D for reference and avoiding any confusion. Measure the distance from P to Q accurately with a steel tape and set it off to scale along the ray drawn to Q, thus fixing the position of point Q on the paper as q (at the first station P only). Shift the table, set it up at another chosen station of the base line Q. Center the table so that point q is directly over the station point Q on the ground, and then level it. Place the alidade along qp and turn the table to sight the ranging rod at P. This is called the orientation of the table by backsighting. Clamp the board. With the alidade centered at q, sight the same points A, B, C, and D to intersect the rays a1, b1, c1 and d1, respectively. The points of intersection will be a, b,c, and d on the paper as shown in fig which will represent the corresponding points A,B,C and D on the ground. Note The intersection method of plane tabling is termed as “ graphic triangulation”. Care should be taken that intersection angles are not very acute or obtuse. The angles of intersection should be possibly within the limits of 30oand 120o. 1.3 Contour and Contour Lines A contour is an imaginary level line that connects points of equal elevation. It may be defined as the line of intersection of a level surface with the surface of the ground. Thus, every point on a contour line has the same elevation as that of the intersecting surface. If the contour lines determined by several equidistant level surfaces are imagined to be traced out on the surface of the ground and surveyed, the resulting plan will indicate the contours in their relative positions and thus give information on the character of the ground. So, the configuration of the ground and the elevations of points are most commonly represented by means of contour lines. The contour interval of a map is the vertical distance between contour lines. The interval is determined by the purpose of the map and by the terrain being mapped (hilly or level). 1.3.1 Characteristics of contour Lines The principal characteristics of contour lines are: Horizontal distance between contours is inversely proportional to the ground slope. Hence, on a steep slope, contour lines run close to each other. On uniform slopes the contour lines are spaced uniformly. Along plane surfaces such as embankments, the contour lines are straight and parallel. As contour line represent level surfaces, they are perpendicular to lines of the steepest slope. Thus, they are perpendicular to ridge and valley lines where they cross such features. Contour lines do not simply end. They must close on themselves. Therefore, a closed contour line on a map always indicates either a summit or a depression. As contour lines represent contours of different elevation on the ground, they cannot merge or cross one another on the map except in the case of vertical surfaces (e.g. retaining wall) or overhanging ground (e.g. a cave). A single control line cannot lie between two contours of higher or lower elevation. When a contour line crosses gully/stream/ ravine on other drainage structures, it forms a modified V pointing upstream. For ridges, the V points downstream. The forms of the V's depend on the type of bed material. For clay bed the V is smooth and rounded; for coarse, granular bed, it is sharp. 1.3.2 Scale and Contour Interval The scale of a map is the ratio of distance on the map to the corresponding distance on the ground. It can be stated as a ratio (e.g. 1cm= 100m) or a representative fraction (e.g. 1:10000). The following factors are to be considered while selecting scale for a map Purpose of the map (the desired accuracy) The cost of the work Clarity/legibility Correlation of map data with related maps Desired size of the map sheet Physical factors to be show on the map Factors to be considered while selecting a contour interval for a map are: Desired accuracy of elevations read from the map Characteristic features of the terrain-coarse or fine textures Legibility/ clarity of the map Cost of the work In general the following scales and contour intervals are recommended. Large scale 1:100 to 1: 2000 Contour interval 0.1 to 2m Intermediate scale 1:2000 to 1:10000 Contour interval 0.2 to 5m Small scale 1:10000 to 1: 1000000 Contour interval 5 to 2000m 1.4 Field Methods for compilation of topographic maps Factors that influence the selection of field methods to be used in the compilation of topographic maps are the scale of the map, the contour interval, the type of terrain, the nature of the project, the equipment available, the required accuracy, the type of existing control and the extent of the area to be mapped. There are four field methods for compiling topographic maps. Examples under which each method is suitable are given below. Direct method of contouring: For study of drainage, irrigation or water impounding structures, each contour has to be carefully located in its correct horizontal position on the map by following it along the ground. This is the Method of radial lines (trace contour method.) Indirect type of contouring: For highway/railway/ canal construction, a strip (30 to 300m wide) needs to be mapped. Control lines are the sides of a traverse which has been established by a previous survey and which has been stationed and profiled. Vertical control is provided by profile leveling along the centerline. The method of locating the topography most commonly used is the cross section method. For an area of limited extent with many constant slopes, the grid method is employed. Points forming grids are located on the ground and their elevations are determined. For an extensive area mapping, the contour lines are located by determining the elevations of well chosen points from which the position of points on the contours are determined by interpolation. This is the controlling point method. Direct method of contouring In this method various points are located on each contour by using a level or hand level and these are surveyed and fixed up on the plan. This method is very slow and tedious as a lot of time is wasted in searching points on the same contour. But this is the most accurate method and is suitable for small areas where great accuracy is required. i. Method of radial lines The method of radial lines can be very conveniently adopted when the area to be surveyed is not very large and when all the points may be commanded by the same position of the instrument. A point is selected some where at the center of the area to be surveyed and various radial lines are laid out from this point. The relative positions of these lines are fixed up by the measurement of horizontal angles or by bearings. Temporary benchmarks are first established near the radial lines and the staff readings to get the various contours are calculated. Contour points are located along these radial lines by pegs and the positions of the various pegs may be determined by measuring their distances from the center. It is most effectively run by the use of plane table and alidade, although the transit stadia can be employed. Control is provided by a suitable traverse, which is computed, adjusted and plotted on the plane table sheet. The elevations of the control points are directly entered on the sheet. The positions of the control points S and R have been plotted on the sheet as s and r, respectively. The ground elevation of S is 995. 2m. The plane table is set up over S and is oriented by backsighting at R. It is desired to plot contour lines on the sheet at whole 1-m interval. The H.I at S is established by holding the rod alongside the alidade, and the alidade is, say 1.4m above the station. The H.I is 995.2+1.4=996.6m. If a 995-m contour is desired the rodman backs off until a reading of 1.6m is obtained through the telescope. At the point the foot of the rod is on the 995-m contour. The topographer reads the interval, draws a ray from s to the direction of the rod using the blade and scales the distance along this ray to plot the point. The rodman walks along this contour, and the topographer reads intervals to successive points along the contour. When the 995-m contour is traced out as far as practicable from this set up, the topographer locates the 994-m contour by guiding the rodman until he obtains rod reading of 2.6m through the telescope. Then he traces out this contour in the same manner as the 995-m contour. All points on the same contour are joined by sketching on the sheet. When the ground is beyond the limit of the rod the topographer runs a short traverse to a new set up, from which additional contours are traced. This method is the most accurate and also the costliest and most time consuming. Application the high water level of an impounding reservoir or a dam may be obtained by a combination of plane table traversing and trace contouring. The desired contour is at the elevation of the top of the spillway of the reservoir dam. It is determined by traversing along the water line. When the theodolite is used for trace-contouring the instrument is set up over the control point and the H.I is determined. The azimuth of the backsight line set on the horizontal circle and backsight is taken to the other control point. This orients the horizontal circle. All subsequent sighting is done by the upper motion. With the telescope leveled, he directs the rodman to a contour by getting the proper reading of the middle cross hair, reads the interval and the azimuth to the point, and roads these data along with the rod reading. The sketching may be done in the field or in the office using a 3600 protractor to plot the azimuths. Indirect type of contouring This metod is known as contouring by spot levels or heights and is less laborious, cheaper and is quicker than the direct method. Spot level or spot height may be defined as the reduced level of a point on the ground. i Cross-section Method In the cross section method cross section lines are established along the center line at intervals of 100-m or ft (full station), 50-m or ft (half station), 30-m or ft, 20-m or 10-m and additionally at all points where prominent features occur (e.g. change in direction of the center line or slope). The cross section method can be carried out by any one of transit and tapes, transit stadia level and tape, hand level and tape, plane table and tape or plane table stadia. Horizontal control is established by a theodolite tape traverse; vertical control by profile leveling. When transit and tape is used, each station or plus station is occupied. The H.I is determined by holding the staff against the instrument. A right angle is turned off the center line, and the rod man, holding one end of the tape, proceeds along this cross line until a break in the slope occurs. If possible the instrument man takes a level, a sight on the rod and the distance is taped. If the rod cannot be sighted with the telescope level, a vertical angle is read and the slope distance is recorded. The rise or fall of the line of sight equals the slope distance times the sine of the vertical angle. The rod man proceeds along the same cross line to the next break in the topography and the process of observation is repeated. The stadia method is used for measuring distances and elevations to cross-sections. When the stadia method is used only to obtain elevations, two persons can measure the distances to the left and to the right of the centerline with a tape. Perpendicularity is estimated by rodman. This avoids setting up the theodolite at every station. When the level and tape or EDM are used for cross section the H.I is determined by backsighting to a point of know elevation. The distances are observed and recorded with station elevations. When the rod is out of sight, a new set up is needed and the H.I is determined by backsighting to another point of known elevation or to a turning point. Be careful of collimation errors. The operation with plane table and tape is similar to that with the theodolite and tape, except that the plotting is carried out in the field directly on the plane table sheet in the former case. Control points can be prepared beforehand or concurrently with the mapping. Once cross section data are produced by field operation, contour lines can be plotted at desired contour intervals and scale by interpolation. Linear interpolation between values is the most commonly used mathematical method to locate a missing contour line on a map. Many mechanical methods are also available. All desired details could also be shown on the map. In compiling topography by the cross section method, the position of all planimetric features, such as buildings, fences, streams and property corners must be located with respect to the control line and plotted on the topographic map. The positions of the features can be located by transit stadia or plane table methods by observing the interval and azimuth to the various points. ii. Grid Method The grid method may be used for mapping of areas of limited extent with a fairly regular topography. A level is usually used for determining the elevations of the grid points. The following steps are used in running the grid method. Establish the boundary of the area to be surveyed by running a traverse around the area. Establish control points at the corners. Divide the area into uniform rectangles or squares. The dimensions of these divisions are governed by the required accuracy and the regularity of the topography. The size of the divisions will be such that, for the most part, the ground slopes can be considered uniform between the grid points at the corners of the divisions. The point of intersection of the grid lines is defined by a letter and a figure of the respective intersecting grid lines as shown is figure 3.3. Data are entered into notebooks against these respective designations of the grid points. It is not usually necessary to mark all grid intersections, but enough points should be marked by stakes to permit the remaining points to be located easily and quickly by ranging them from the points that are marked. The rougher and more irregular the surface, the more stakes must be set. Levels are taken at all points and at all intermediate points where the slopes change abruptly. Such intermediate points are generally located in a direct line between two intersections by its distance from the intersection having the lower letter or figure. The distance may be measured or estimated. After the fieldwork is completed, the control points, the boundary and the grid are plotted to the desired scale. The elevations of the grid points are then written at the corresponding map positions of the intersections. The positions of the desired contour lines are located by interpolation between the grid intersections. Exercise: The elevations of grid points for an area as follows: F E D C B A 690 696 702 707 709 711 709 715 723 726 725 721 726 729 733 732 723 715 732 736 742 732 722 715 735 740 737 729 730 732 740 737 737 740 738 730 743 739 745 740 731 719 747 740 747 730 727 722 1 2 3 4 5 6 7 8 The elevations are in m. The readings are taken at 15-m intervals. Plot a 5-m interval contour lines for the area. Use a scale of 1: 200. On your map locate the intermediate point B+3.5m, 3+2m with a spot elevation of 720m. 3. Controlling- point Method The compilation of a topographic map by determining the positions and elevations of carefully selected controlling points is applicable to nearly every condition economy realized. It can be used for mapping of a strip of land for route location studies. The experience and judgment of the topographer determines the accuracy of the map, the speed of progress and the faithful delineation of the true shapes of the contour lines. The method is the most difficult to master, but it is also the most valuable method due to its universal application. In addition to the accuracy of the technical operations of making observations on the controlling points, the topographer's knowledge of land shapes, slopes and stream gradients, his facility for making maximum use of equipment, and his ability to decide where to select points so that he takes neither too many nor too few observations, all affect the success of the mapping operation. Various combinations, of instruments can be used for this method although the plane table and alidade is most desirable. The horizontal control for an area to be mapped by this method can be established by making a simple traverse which is computed adjusted and plotted on the plane table sheet. Vertical control is established by leveling. The accuracy of control depends on the scale to which the map is to be plotted, the contour interval and the required map accuracy. In general horizontal positions on basic control points should be located to within 1/200 in. (0.10m). The basic vertical control should be established to within one-tenth of the contour interval. Once the horizontal and vertical controls are plotted on the sheet, the topographer sets the plane table over a control point and orients the board. He determines the H.I points are selected along ridges, draws, streams and drainage channels, at tops and bottoms of lines with constant slopes, and at points between which the topographer can estimate the crossing of the contour lines. For a topographer, drainage has more influence on land shape than any other feature. Hence, drainage lines have to be located fairly accurately. The positions of the desired contour lines as they cross the drainage lines are obtained by interpolation and are sketched in. The elevations of the points at the changes in slopes will allow the positions and elevations of the controlling points by the plane table and alidade. 1.5 Map Finishing In addition to contours, the positions of other natural features (e.g. trees, streams, lakes, etc.) and man made features (e.g. buildings, dams, roads, etc.) should appear on topographic maps. For this, their positions are observed in the field. Symbols are used to represent these features on the map. Most of the symbols used for object representation on a map are conventional. Where feasible colored symbols are used. The following list gives various colors and objects they represent. Black- for man made or cultural features: roads, buildings, names and boundaries. Blue-for hydrographic features: lakes, seas, oceans, rivers, canals, glaciers, etc. Brown-for relief configuration of ground in terms of contour lines, hachures, etc. Green-for wooded or vegetative covers, with typical patterns to show such features as scrub, vineyards or orchards. Red- to emphasize important roads and public and division lines, and shows built-up areas. Nature and source of data used in the mapping ought to be known. Other information to be shown on the finished map includes. The direction of the meridian and the basis for directions (grid, magnetic, true meridian). A graphical scale of the map with a corresponding note stating the scale at which the map is drawn. A legend or key to symbols other than the common conventional signs. This should be brief but should not leave doubt to the map users. An appropriate title. On topographic maps, a statement of the contour interval. A statement giving the datum to which horizontal and vertical control are referenced. A statement giving the mapping projection. A statement giving the co-ordinate systems for which grid lines are published on the map. Chapter 1 Topographic Surveying PAGE 13 Lecture note a b c b a c a’ a b’ c’ P Q D C B A