Forests and Their ManagementDr. Ankur AwadhiyaDepartment of BiotechnologyIndian Institute of Technology, KanpurModule - 05Forest SurveyingLecture – 13Classical Tools[FL]. Today we begin a new module and this module is Forest surveying. (Refer Slide Time: 00:18)This module will have three lectures - the first one is Classical tools, the second and third are the modern tools of Photogrammetry and LiDAR.(Refer Slide Time: 00:30)(Refer Slide Time: 00:32)So, let us now begin with the first lecture, and let us look at what a survey is?So, a survey is defined as the act of making measurement of the relative position of natural and manmade features on earth’s surface and the presentation of this information either graphically or numerically.So, what we are doing in the case of a survey is that we are making measurement of the relative positions of different features on the planet earth.(Refer Slide Time: 01:04)So, what we are saying here is that suppose you have these hills, and then, suppose these hills are all full of forests, and we have a river that is flowing from here. Probably, this area has certain habitations; surrounded by certain fields and this area suppose also has say a pasture. Now, when we say that we are doing a survey; what we are doing is we are measuring the locations of each and every of these features.So, we in the case of a survey, what we are doing is we are figuring out the exact location of this house, the exact location of this house, the exact location of this house,the exact boundary of these fields. And, if you have say, a few trees here as well, the exact locations of these trees; the boundary of the river; the boundary of the pasture land,and if necessary, the location of these hills, and the location of the trees. Now, in this case, what we are saying is that we are having certain natural features, such as the river, the hills, the trees. We also have certain man-made features, such as the houses, or say the fields, or the pasture lands. And, what we are doing in the case of survey is that we are making measurements of the relative position of each of these. Now, what do we mean by a relative position? Say, if we take a point as our control point; so, let us say that this is the control point, and we are taking the measurement of this point. So, what we are doing here is we are finding out the distance of this point from the point from our control point and the angle that it is subtending from; say, if this is the north direction either the magnetic north or the geographical north. So, depending on your application you could choose one of these, and what we are seeing is we are measuring this angle theta. And similarly, for each and every of these points; so, for this point what is the distance?what is the angle? For this point - what is the distance? what is the angle? So, once you have a knowledge of the distances and the angles, then you can represent it in on a sheet of paper by plotting these distances at a certain scale and by making use of these angles. So, what we are doing here is that we are measuring the relative positions. Now, by a relative position, because nothing there is no such thing, which is having a fixed position. Because, even if you take a point on the planet earth this point itself ismoving, because the earth is going around the sun. But, in this case, in the case of a survey, we are choosing a point and we say that this is our reference point. And, what is the location of each and every feature with respect to our chosen point. So, we do these measurements and then we present this information either graphically or numerically. So, when we present it say graphically - what we are saying is that will mark this point on a sheet of paper, and with the scale will say that this is one point at a distance of.So, when this distance was ‘d’ you will say. So, you are drawing this as the north direction and you are saying that for this point this is and this is your reference on the sheet of paper. And, you are saying that there is one point at the distance of d, and at the angle of theta, on a sheet of paper. So, you are parking the location of this point, and in this case you will make use of a scale. So, in this case, your d on paper is equal to or you will say that 1 centimeter on paper is say 10 meters on ground. So, this is the first point; 1 for the second point you will again draw it like this so, this is at this angle of theta 1 and at a distance of d 1.So, you have representing this point 1 and the point 2, and similarly your you will be representing each and every feature on the sheet of paper. So, this is a presentation of this information. So, you can present this information either graphically or numerically,and when you do all of these you are taking when you are taking the measurements, and you are presenting this information then this whole process is known as survey.(Refer Slide Time: 06:30) A survey is generally done in three stage. The first one is to have a general view of the whole area; you can also refer to this as ‘a reconnaissance.’ So, in the case of reconnaissance what you did was you look at the this whole location - so, you saw that there are hills; there is a river; you have the houses; you have the fields; you have the pasture lands. so, first of all you had a general view of the location. Once you have a general view then you decide which is going to be my reference point. So, in this case, you choose this point as your reference, probably because this was a point from where it was easy to take measurements of all these different features. Now, in certain conditions, you can even go for two reference points. So, in this case, you will say that this is my one reference point, and I am drawing a straight line and this is my second reference point. So, when I have to take the measurements of the pasture, I am making use of the first reference point. When I have to take the measurements inside the village, when I have to have measurements of the houses or of the fields, then I will make use of the second reference point. But then, both of these reference points are correlated to each other. So, what we are saying in that case is that you first decide on the first reference point, and you mark the second reference point by taking this distance D and also measuring the angle. Let us say that this angle is phi with the north; so, that if you have to represent it on the piece of paper, you can directly represent it as - you have this point at a distance D and making the angle phi. So, you get the second difference on the piece of paper, and now you can start plotting the points that are being measured from the second reference. So, the first stage in serving is to have a general view, so that you can figure out what is going to be your reference point, and if you require multiple reference points, what will they be. The second stage is to take the observations and the measurements. so, in the second stage what you are doing is, you are supposed taking a compass, to take the angular readings; you are also making use of tapes to take the linear measurements, and you make you jot down all of these on a piece of paper. So, this is the second stage that you are doing observations and you are taking measurements.Now, the third stage is the presentation of this data. So, you can either present it on a sheet of paper, or you can present or you can feed this data into a computer to get a digital representation of the whole area that you are surveying. Now, when you are when we are doing the surveying, there is another thing that needs to be kept in mind. The surface of the earth is not flat. The earth is having a shape which we refer to as a geoid. So, it is circular in shape which is a bit more elongated in towards the equator, and the shape is referred to as a geoid. (Refer Slide Time: 09:54)Now, what we are doing here is that, if you represent the earth, you will represent it say by using a sphere. But, if you are taking a very small portion of this sphere, and if you look at it in a magnified view, this will look like this. So, you are getting. So, you are representing it with a section of the sphere; but if you take an even smaller point, then what happens is that you can approximate it as a flat surface. Now, this brings us to different types of surveying. (Refer Slide Time: 10:31) The one is known as a plane surveying. So, if you are doing your survey on a very small area on the surface of earth, then you will make use of plane surveying techniques. Now, the case of plane surveying, you approximate the surface to be a flat plane. So, plane surveying the main surface of the earth is considered as a plane, and it is good for smaller areas that are less than 250 square kilometers. Because if you start taking areas that are larger than these, then your approximation that you are working on a plane surface has ended. Now, you will have to take into account the curvature of the earth at that particular point or in that region. So, that will bring us to the geodetic surveying, which takes into account the true shape of the earth. So, these are two different types of surveying.(Refer Slide Time: 11:25) And depending on the instruments, you can have different classes of surveying. So, the easiest one or the most classical one is ‘the chain and the tape survey,’ in which you take linear measurements only without any angular measurements. Now, what we are doing here is that suppose, you have a field and that has say this shape. (Refer Slide Time: 11:50)Now, in the case of your chain survey or a tape survey, what you will do is you will first of all begin with a reference line. So, in this case, let us say that this is our reference line. Now, with this line, now you need to have the locations of different points. So, now you want to say what is the location of this point, this point, this point, because you know two locations that you have chosen as your reference line. So, let us and suppose this the length of your reference line is say 100 meters, so we will say that this is 0 this is 100. And now, you start moving from 0 towards 100, and you reach a point where the first; so, what you have done is that you have added stakes on all of these points, for which you want to take the measurements. And now, you are moving from 0 to 100, and you reach a point where you find out that the first point is exactly to your left; so, it is making an angle of 90 degrees, and you now have reached this position. So, this is your first position, so let us call it position A. Now, you will stop at this position, and now you will take a measurement of what is the distance of the first point from the position A. So, you will measure out in this distance;let us call it d A. Once you have taken this measurement, now you start moving and you also note down the position of A so, let us call it x A.So, x A is the position is the distance of this point a from the starting point. Now, you start moving even further, and then you reach this point from where, so let us call it. So, you have O P Q R S. So, now, when you have reached this point; let us call it B. In this case, you measure the distance of B from your starting position, using a chain or a tape,and you measure the distance of the point, you measure this distance to the right of B. Now you carry on for further, and you have reached this point C. now, here again you will measure the x C, and you will measure the d C.Now, in this case, you are only taking the linear measurements. You are not taking any angular measurement. You just go to a point where the next point is either completely to your right or completely to your left. And, you are then measuring where you are standing and you are measuring the distance of the your stakes from this point, either to the right or to the left. Now, in this case, you will make a table. And, in this table, you say that you began with this point O which was at 0 meters and then there was a point A which was say at 20 meters. Then, there was a point B which was say at 50 meters. You had a point C which was say at 70 meters, and you had this point Q, which was at 100 meters. Now, at position A, you had the distance of d A. Let us say that this was 30 meters at position B, you had d B, which was say 40 meters. At position C, you had d C which was say - 40 meters, and then you had the final position Q.Now, once you have these measurements now, what you can do is that you can take apiece of paper and you can draw a straight line. And, you can say that say 100 millimeteron paper is equal to 100 meter on ground, or 1 millimeter on paper is equal to 1 meter onground. So, what you will do, in this case, is that you will draw a straight line which is 10 meters which is 10 centimeters on the piece of paper, you will start marking these points O and Q, and then the point A is at 20 meters. So, you measure 20 millimeters,and then you draw a line at 30 degree at 90 degrees, which is 30 millimeters.So, here you have the point S. This is point A. next, you have the point B which is at 50 millimeters, and towards the right you have at 40 millimeters you have the point P. Then,at 70 millimeters you have the point C and towards the left you have 40 millimeters and this is the point R. So, you have represented the whole area that you were surveying on a sheet of paper, and then what remains is just to join these with straight lines. And so, now, you have represented the area on the ground on a sheet of paper. Now what can we, now what can be the use of such a measurement?Now, you can take you can measure the area of the region that you were serving. So, for instance, now you can convert this and you will either have triangles or you will have a trapezium, and we know that the area of a triangle is half of base into height. And, the area of a trapezium is half of a plus b into h, where a and b are the parallel sides and h is the separation between these parallel sides. So, in the case of plane surveys, you are only taking linear measurements, and just by using a chain or a tape to take these linear measurements. You are representing the region on a piece of paper and using it for instance to take different areas.Now, the next survey is known as a compass survey. In the case of a compass survey,you not only take linear measurement, but you also take angular measurements. So, you take angular measurements using compass, and linear measurements using a chain or a tape.(Refer Slide Time: 19:07)So, what you are doing in a compass survey is that - you are say standing at this location,and here you have your field that needs to be measured or that needs to be surveyed. Now, what you are doing is that you are taking just one position and you are measuring the angles with respect to the magnetic north; let us represent it on other color.So, this is your north and this is your field A B C D E, now for every point you are measuring the angle. So, this angle let us say that this is theta C, and you measure a you measure the distance of this point C from your reference point. And so, you do it for C,you do it for B, you do it for D, you do it for E, and you do it for A. So, in this case, you have a table where for every point you have the angle that is being subtended and the distance of the point. So, you have A B C D and E. So, you are measuring theta A, theta B, theta C, theta D, theta E, and you are also measuring the distance is x A, x B, x C, x D, and x E. And, when you have these measurements, now you can do the presentation of this data on a sheet of paper. By just taking a piece of paper, marking out a point called O and marking all of these difference. So, you take O you draw a straight line, and you will say that this is the true north, this is the magnetic north, and then you start drawing these different locations on your sheet of paper; at these angles, and at these distances, and what remains is just to connect these with straight lines, and so, you will get a representation of your region on the sheet of paper.So, in the case of a compass survey, we are not only taking the linear measurements using a tape or using a chain or say using a rangefinder, but you are also taking the angular measurements. Now, the third survey is known as a plane table survey, in which case, you take measurements and which are converted into drawing on a plane table. Now, what we do in the case of a plane table survey is that we take measurements from two positions. (Refer Slide Time: 21:58)So, here you have your region, and so, let us call it as A, B and C. So, this is a triangular field that you are trying to survey, what you will do here is that you will take two positions P and Q, and you are trying to triangulate these different locations. What you do is you measure this angle. So, this is theta C, and you measure this angle phi of C. So, you are taking the measurements; you are taking the angles. You do not need to have any linear measurements. In this case, just one measurement that is the this distance of P Q so, these are two points that you have fixed.Now, you go to the first point, and you take the angle of one of your stakes from the first point that is P. So, you had put a stake here; so, there was a stake you went to this point Pand you took a reading from here to the first point, and you measured it as theta C. Now,you go to the second point Q, and here you take the measurement from the line to the point and which is your phi C. And then, on your plane table, you draw a line at a distance of d, and you draw this line at an angle of theta C, you draw another line at the angle of phi C. And, the point where both of these are meeting you mark it as point C. And, you repeat this for each and every point on your field that needs to be surveyed. So, you do it for A you do it for B. So, in all the cases, you are just measuring two angles. So, in this case, your A is roughly at 90 degrees your from the P, and address add this angle from this. So, now you have this location of A, and similarly you will measured angle her. And,this angle and then you have this location B, and then you draw you join all these three with straight line, and you have a representation of the field on a piece of paper. So, in the case of a plane table survey, you just take two points know that note down their distances, and now, you only need to take angular measurements of each stake that you have put on your field. And, in this way you will be able to represent or present the points on the field on a piece of paper. And, once you have this or drawing at a particular scale, now you can make use of a graph paper to find out the area of your field. So, this is another way of surveying. The fourth one is the ‘theodolite survey’, which measures horizontal and vertical angles. Now till now, we were talking about those regions that were lying on a flat plane, but suppose you are measuring or you are surveying a building; so, in the case of a building you might even want to know the height. So, in the case of the height, you are not only taking the angular measurement of the point, but you are also the angular measurement on the horizontal plane, but you are also taking the vertical measurements.(Refer Slide Time: 25:59)So, if you are; say, this is your reference point. You have a building and this is a wall; so, you take the measurements and you see this is the north. So, you are measuring not only this angle with the north, but you are also measuring this angle. So, in this case, you can take the first measurement of the teak stand; so, the number of trees in the average number of trees in the teak stand. The average number of trees in the solid stand, and the average number of trees in the grassland stand. The area of this is suppose 80; here you have A s and here you have A g. So, in this case you will say that the total number of trees is A t into x t bar plus A g into x g bar plus A s into x s bar. So, in this case, your readings will be much more precise. (Refer Slide Time: 58:48) Next, we have a multistage sampling, which is the procedure of first selecting large sized units, and then choosing a specified number of subunits from the selected large units, and this is known as sub sampling.(Refer Slide Time: 59:07)So, in this case, you are saying that suppose you have to choose between 1 to 10000, and what you are saying is that I will divide it in to 10 different stages. So, we I take 1 to 1000, then I have 1001 to 2000, 2001 to 3000, and so on. And in each of these, I will be taking random samples. Now, this is because when you are just taking random samples it is possible that by chance your random numbers come that all your random numbers come between 9000 and 10000. But, if you take this multistage sample, in this case, you will have a much better representation of the whole population. (Refer Slide Time: 59:52)Then, we also have the probability proportional to size sampling or PPS sampling. “When units vary in their size and the variable under study is directly related with thesize of the unit, the probabilities may be assigned proportional to the size of the unit. And, this type of sampling where the probability of selection is proportional to the size of the unit, is known as PPS sampling.” (Refer Slide Time: 60:20)So, for instance, you want to measure the biomass of your forest; and in your forest, you have these large size trees, and you also have certain small trees. Now, if you want to take a measurement of the biomass, then because the large size trees have a much greater representation in the total biomass of the forest, you can say that, I will choose a sample in which these large sized trees are proportionally represented, based on their sizes and the smaller trees are less represented, so that I can have a much better idea of the total biomass in this forest. So, if you take such a procedure, in which the probability of us of a unit getting into your sample is proportional to the size of that unit, then you refer to it as a probability proportional to size sampling.So, in this lecture, we started with surveys; what is the survey? what are the different kinds of surveys? what are the different ways, in which we do surveys? and then, we moved into how measurements are taken in, or what sorts of errors are there in the measurements. Now, our aim is to reduce this these errors. So, we want to have things which have better precision and better accuracy. Now, if you have large sized samples, then a large sized area then a way to economize on your measurements. So, you want to get good measurements without spending too much amount of money or time or other resources; so, in that case we go into sampling in the whole of the surveyed area, and so, we take small samples and we take samples in such a way that we have a good representation of the total population. So, that is all for today. Thank you for your attention [FL].
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