 I am Mr. Sheshikant B. Gosavi, Assistant Professor, Department of Civil Engineering, Vultured Institute of Technology, Sulapur. I am here to prepare an online educational resource on preparation of contour map. Dear friends, the learning outcomes of today's OER are that at the end of this session student will be able to compare the direct method and indirect method of preparation of contour maps. The learners will also be able to select the appropriate method of contouring for preparation of the contour maps. To begin with, we need to have a little bit background of contouring. What is contour? Contour is a line joining the points of same elevation or reduced levels. Contour interval is vertical distance between two successive contours. The horizontal distance between any two points between two successive contours is known as horizontal equivalent. All of us are familiar with several characteristics of contours. Like contour should form a close loop or cross the boundaries of the map and the distance between the successive contour gives us idea about stiffness of the ground or flatness of the ground. In addition, contours also help us in identifying specific geographical features such as hill or ditch or a saddle or a ridge or valley and so many things which most of us are already familiar. Here in the sketch you can see there is a number 7700 and another number 775 located between these two lines. The total distance between these two is 25 meters. Remember in between there are 1, 2, 3 and 4 lines. That means this is 7700 next to that is 7705 then 7710, 7715, 7720 and then 7725. Thus between two successive contours the vertical distance is 5 meter and therefore we can say that contour interval of this map is 5 meter. This is a classical sketch of a crater. At the center you can see 7600, 28 is a spot level surrounded by 7600, 750, 7700, 7725 that means this is like a pond or a ditch while from here on up to this this is going to fall down further and towards the corner it goes away. Therefore this is a classical example of a contour map. The scale has been shown over here and remember here they have mentioned contour interval as 5 meter. Let us begin with various methods of contouring. The first one is direct method of contouring. The prerequisites needed for starting the direct method of contouring is we need to decide the scale of the proposed map and we will have to decide what contour interval we will have to choose. Normally we use leveling instruments for the vertical control like dump level, auto level and leveling staff and for horizontal control we use plane table. Combine together the thing can also be done by using a total station. The procedure for the direct method of contouring consists of we need to select an arbitrary benchmark close by to your site on which we will have to hold the staff, keep the instrument at a commanding position from where the staff reading can be observed on to the benchmark. Thus we will find out what is the RL of horizontal plane of collimation. That will be RL of arbitrary benchmark plus backside reading. Once RL of horizontal plane of collimation is known to us then we can proceed further for searching the staff reading which will help us in getting the desired contour RL. Supposing 100 meter is the RL of the arbitrary benchmark, 1 meter is the backside reading RL of the plane of collimation will be 101 meter. Thereafter we will decide what is the desired contour RL. Suppose it is 99 meter that means we will have to search for a staff reading of 2 meter. Thus we can get the required point on the ground. So we will have to keep on taking trial and error till we get 2 meter as the staff reading and thus the respective point can be marked on the ground with the help of arrow. Thereafter that particular arrow can be located on to the drawing sheet with the help of horizontal control which can be established by using plane table survey. The advantage of direct method of controlling is it is very very precise. However the limitations are there like it is more laborious, time consuming and costly. You can see people going for a classical holding of the leveling staff and taking the reading on to that. This is a part of the direct method. There is second method which is called as indirect method of controlling which is further subdivided as block contouring and radial contouring. The whole area can be divided into number of squares or blocks and the data can be collected for the corners of each of the block regarding its reduced level. Then that is called as the block contouring procedure and the whole area can be divided into number of radiators at some angular interval and then staff readings can be taken to find out RL at some regular interval along the radiators that is called as a radial contouring. See block contouring is useful when ground is divided in grid of blocks of uniform size. The reduced levels are recorded at the corners of the blocks and then the contours are interpolated. Remember in indirect method interpolation is mandatory while in case of the direct method interpolation is not at all required. The block contouring is suitable for ground with mild slope or flat ground while on the other hand there is radial contouring where ground is divided by radiators. Along the radiators reduced levels are recorded at known distance and then contours are interpolated. This is suitable for hilly area. Hence the instruments used for the radial contouring are normally tachymetry type of instruments while in case of the block contouring we can use any leveling instruments such as dumpy level, auto level, tilting level etc. The advantages of the indirect method of contouring consists of it is cheaper, less laborious, faster than direct method. The limitations of the indirect method of contouring are it is inferior in accuracy as compared to direct method. This is a small example of indirect method of radial contouring. Of course the whole map is not yet prepared but you can see the radiators at every 30 degree angle radiators are drawn. These are also established on the ground and then at some arbitrary interval the leveling staff can be held, staff readings can be calculated and those can be recorded. These numbers are respective staff readings. Once staff readings are available then interpolation can be done between two successive reduced levels to draw the contours. This is a script of interpolation of contours by arithmetic calculation method. Interpolation is possible by using the graphical method as well as by using the estimation method. However arithmetic calculation method is the most accurate out of them so we have shown it over here. You can see the coordinate over here as 40 comma 0. Similarly another coordinate over here at the other end of the block as 40 comma 5. The reduced level of this coordinate is given to you as 499.315 while this is given as 499.375. The reduced level of the contour that we have decided is 499.32 and the distance between these two points is 5 meter so between a length of 5 meter we have to search for a precise point through which this contour will pass. That can be done by using the formula for the interpolation and thus this x is showing you distance 0.417 from the smaller array. This is a excel worksheet which I have shown. This is a classical drawing of a block contouring which is prepared. You can see the blocks at each corner of the block reduced levels are calculated and then by using the interpolation method we can proceed further for precise location of the contours. You can see over here the formula that distance d from the shorter RL is equal to contour RL minus smaller RL divided by larger RL minus smaller RL multiplied by distance between the two ends end RLs of the respective line. Thus we can easily get the respective distance from the smaller RL and thus several contours can be plotted like this. Learners pause a while the video need to be paused for a while and let us take a feedback about how much we have understood about the whole discussions that we have had in today's online educational resource. The question is contour interval is blank space distance between two successive contours and the options available to you are horizontal, vertical, inclined all of these. Obviously the answer is vertical distance. The other is interpolation of contour is required in direct contouring, indirect contouring both of these and none of these. The answer is interpolation is required in case of the indirect contouring. We can use several references but the one which I like the most is National Program of Technology Enhanced Learning. The various PDFs are available from them. So, thank you very much.