Hello! This part of geometric leveling is dedicated to control the level. The level control is an important operation which is carried out before a construction or before topographic work to ensure that the level is set correctly. Indeed, there may be parallel defects between the guideline of the spirit level and the line of sight. We can draw the following diagram: with a level here, placed on its tripod, its scope, and I have here my axis which should be horizontal and I draw here my directional line with a small parallel defect that I note as angle <i>i</i> that characterize this defected adjustment. In order to avoid a defect in the level settings, we will use the following principle: we ensure that the spans are equal that means that the distance between the level and the front rod is equal to the distance between the level and the rear rod. If this condition is respected, we have a reading on the rear rod which is equal to the value <i>r</i> plus a small error <i>Δr</i> which is the contribution to the defected adjustment. I made the observation on the front rod: I have <i>V+ΔV</i> with my small adjustment error. If I have spans that are equal, the contribution from my angel <i>i</i> will generate a <i>Δr</i> will be equal to <i>ΔV</i>, so when I calculate the difference <i>Δh</i> as <i>r-v</i>, my errror will be eliminated. The procedure of level control is an operation that must be made before any important topographic work. So it can detect an excessive adjustment defect, because the spans is not always equal according to the topography of the field. If this is the case, we must be able to adjust the level, and it is this procedure that permits us to do this. It is described here in a video that was shot on location, and that shows you in detail how to proceed to do this control and to assure proper adjustment of the instrument. The level control of the construction site occurs between two points. The operator place the level in the middle of the section, the wedge, and then performs the measurements on the leveling rod: first a backsight towards point <i>A</i> rA1=1,126m Then the operator moves over to the 2nd point, point <i>B</i> The operator can then make a front sight: VB1 = 1,578m Then, the rod remains in place, during which the operator approach the level. He fix again the instrument, before continuing with the other measurements. He proceeds with a new foresight towards point <i>B</i> : VB2=1,210m From this station, he also performs a backsight towards point <i>A</i> after the operator has moved the rod rA2=0,759m The procedure of level control permits to highlight a defect horizontally to the line of sight and consist of calculating the closing distance <i>w</i> 1st situation: the ranges are equal. The horizontal defect is identical to the front and rear measurements. As a result, the error can be eliminated by calculating the difference <i>rA1-VB1</i> so we get a height difference without error. 2nd situation: the level is close to one of the rods. For the foresight, we make the assumption that the horizontal defect is negligible because the rod and instrument are close. In contrast, the long distance separating the rod and the instrument at the backsight will affect the measurement error if the level is set incorrectly. However, we can compute a value for <i>rA2</i> as a function of the 3 first measurements where the influence of the horizontal defect is negligible. Finally, the closing distance <i>w</i> is the difference between the calculated and measured value of rA2. For leveling a construction site, we consider the setting is good if <i>w</i> does not exceed the tolerance level of 5 mm.
