 Hello, learners. Best greetings of the time. I am Mr. Sheshikanth Gosavi, assistant professor of department of civil engineering, Valchand Institute of Technology, Sulakpur. I am here to present the online educational resource on Gail Straver's table. At the outset, let us know about the learning outcomes of this online educational resource. At the end of this session, students and the learners will be able to judge the acceptance of data collected during theodolite traverse against permissible limits. In addition to that, the student will be able to classify the order of accuracy of the closed theodolite traverse. At the outset, let us start and understand what is meant by this. Traverse is a skeleton or a network of surveying lines which are connected with each other. If this skeleton or network begins from one station and ends at some other station, the traverse is known as open traverse. However, if it begins at one station and tries to end up at the same station, we call it as closed traverse. Conventionally, in a Gail Straver's table, the traverse is supposed to be run by using theodolite. That means during the fieldwork, the data to be collected will consist of measurement of included angles at each of these stations with utmost accuracy. That means it is done by using the repetition method of measurement of horizontal angle. Besides, the data of magnetic bearing of each of these sides has to be also observed. Thirdly, we will also be measuring the length of sides of the traverse precisely, as has been shown in the table over here. We are supposed to start up with a serial number. It is followed by a station. Name of the station is supposed to be written. Name of the lines are supposed to be written. Then length of each side of the traverse has to be written. For example, I have taken it as small l over here. Observed included angles are supposed to be recorded at that particular station. For example, the station P is shown with an included angle 123 degree 45 minute 20 second. It is followed with corrected included angle. We will try and learn how this particular value are arrived at. In closed traverse with n number of sides, the theoretical sum of included angles has to be equal to 2n minus 4 into 90 degree. Where n is number of the traverse or n is number of stations of the traverse. If this observed sum of the included angle is equal to the theoretical sum we can say that the data is free from instrumental errors and observational errors. However, if there is an error that means if there is a difference between theoretical sum and observed sum of the included angle we call that particular difference as error in included angle. The permissible error in case of the any angular instrument is equal to least count square root of n where least count is the least count of the instrument. For example, for a vernier threshold light it is normally 20 second and square root of n is having n number of sides of the traverse. Whatever is this summation t minus summation o or summation o minus summation t that is error in included angle we can distribute this error equally at all the included angles to find the corrected included angles. That corrected included angle we are recording over here and finally, summation of corrected included angle will always be equal to theoretical sum. Here you can see there is additional data of magnetic bearings. There can be several observed magnetic bearings from each station two bearings are observed one in the forward direction of the traverse one in the reverse direction of the traverse and therefore we may have four bearing and back bearing. Right in the beginning I have shown a traverse with the arrows in anticlockwise direction. Try and remember very well that theodolite traverse is always run in anticlockwise direction. Here we have gone for the calculation of the magnetic bearings from the observed bearings we can find the difference between the forebearing and back bearing and if these two values are differing exactly by 180 degree there is a particular line free from local attraction. We should search for that type of data if unfortunately we do not get that type of data what we can do is we can manipulate with the forebearing and back bearing such that each of them will be adjusted by equal amount and finally the difference between the two must be exactly 180 degree. Thus we will calculate the corrected bearings for remaining sites by using the corrected included angle and the data of one line whose difference is exactly 180 degree. Further we will go ahead with the conversion of corrected forebearing into reduced bearings. All of us are aware that theodolite is capable of measuring the bearing in a whole circle bearing system and finally when the calculation of consecutive coordinates is to be done the value of the angle that we need is required to be in reduced bearing system and therefore whole circle bearing forebearing should be converted to reduced bearing system. Once this theta value is available with us we can proceed further for calculation of consecutive coordinates. See here the figure is indicating line AB corresponding to A and B the distance between those two station is written over here that is the length AB so when we are calculating the consecutive coordinate of B it is nothing but it is the coordinate of B with reference to its preceding station A assuming A to be origin in independent coordinate system the method is different here preceding station is to be assumed as origin please remember this very well the projection of this particular length on meridian is called as latitude of the respective station similarly projection of this particular side onto the line perpendicular to meridian is called as departure of that station so this particular length into cosine of this angle gives us this projection on north south direction or meridian direction which is called as the latitude of line AB similarly length of this side into sine of this angle gives us this projection which is departure of the station B with reference to A so in this way we can calculate latitudes and departures the latitudes can be either northing or southing and departures can be either easting or westing we know each of the station can either be in the northeast quadrant or the respective station may be lying in the southeast quadrant the station may lie in the southwest quadrant or station may lie in the northwest quadrant and therefore either north and east north and west or south and east or south and west can be the combination available for each of the side more important is the calculation of closing error in theodolet traverse for each of the side its latitudes and departures consecutive coordinates can be calculated and summation of the latitude has to be done which ultimately can be shown as the error DL as has been indicated in the figure see this particular summation has to be algebraic summation in a similar way algebraic summation of departure has to be calculated and then we will calculate error of closure by using this formula the degree of accuracy can be calculated as error of closure divided by total perimeter of the traverse pause the video and answer the following multiple choice question these are the answers which you can compare with what you have even here the answer of the first is anticlockwise and answer of the second question is that of on meridian I have referred this particular module of national program of technology and learning as a reference material thank you very much one and all