Hello! This part of the lesson about geometric leveling is dedicated to traverse. What is traverse? Traverse consist of measuring height differences between benchmarks. For example, we will pick here a mark A, with known altitude, and we have here, at the center, a construction site which is not equipped with elevation points and we are therefore interested in getting the elevations for this region. So we will traverse, and we will measure the height differences, through the area of interest here and thus densify the altimetric points in the region of interest. It can also be used to monitor an unstable zone in which we will position the benchmarks for control. The types of traverses. We have many types of traverses. The first type consist of making back-and-forth measurements. We start here at a point A and we will go to a point B. So these are the benchmarks we have on our path here between A and B. We have first a direction of the traverse that permits us to determine a height difference <i>delta H</i> from A to B, and when returning, we have a <i>delta H</i> from B to A. Taking into account the direction of these height differences, if I take the sum of <i>delta HAB</i> plus <i>delta HBA</i>, I theoretically obtain zero, here I will obtain a deviation and this deviation must be relatively small, in this case a few millimeters for a leveling of the construction site. Another way of traversing consist of relating two known points in altimetry. I have here a point A, of which I know the elevation HA, and I have here a point B, of which I know the elevation HB. So I have here a difference in elevation, a <i>delta HAB</i> that is known through documentation of the leveling benchmarks. So I will walk between these points and I have a traverse that will give me a <i>delta HAB</i> that is measured, that I will compare to my delta HAB which is known, and again, if the difference between the two height differences is consistent, at this moment I can validate the elevation of my intermediate points. The last example is the closed traverse. In this case, we are in a zone where we don't have access to the benchmarks. It may be at the bottom of a pit for example, where we have here a benchmark, that is well defined, and we will do a loop here, that means to densify the altimetric points in the area of interest, and return here to this base benchmark. So here, the <i>delta H</i> of my loop, is theoretically zero. So I can also control my height difference measurements with this principle. This last table summarizes the three principles we have seen, namely the traverse back-and-forth, the connected traverse, so at the two known points, and finally, the loop that close at the starting point. You will now be able to see a video that illustrate this principle of traverse outside with an apparatus and the measuring device. Leveling. Traverse, measuring and calculating the height differences. Located between the first two points of the path, the operator adjust the level then aim the leveling staff. The operator place the staff at the starting point of the traverse materialized by a bolt that is a fixed point of known elevation. The operator make a backsight to point A. The measurement is 1,126 meters. Then, the operator moves to the first intermediate point. The operator simply turns the scope of the instrument a half turn. This point is not materialized, so one resorts to the use of a change-plate giving us a temporary benchmark. The operator place the rod at the head of the change-plate and keep it upright. The operator proceed with a foresight to this point. The measurement is 1,578 meters. This time, the rod is stay in place and it is the operator that install the level in the middle of the range between the first two intermediate points. The operator turns the rod to face the instrument. The operator must re-calibrate the level before the backward sight to this point is made. It is again the operator that move to the next point with the rod and the shoe. The operator do a forward sight to the second intermediate point. The traverse continues then with the principle of the back and front reading. The measurements taken permits to calculate the difference in elevation between two points. For example, between the starting point and the first intermediate point, this difference is expressed in the back reading to point A, 1,126 meters, minus the front reading to point A, 1,578 meters. This gives a height difference of a little under 0.452 meters.
