1.15.6. Types of direct levelling

This method is used for finding out the difference between the levels of two nearby points. Figure 1.29 shows one such case in which level of BM is given as 100.000 m. RL of unknown point is computed as- RL = BM + BS - FS = 100.000 + 0.973 – 4.987 = 95.986 m

Simple levelling helps determine the elevation difference between two close points. To find the reduced level (RL) of an unknown point, you need a known elevation point (called a Benchmark or BM) and make observations using a Level device. The formula used is RL = BM + BS - FS, where BS is the back sight reading and FS is the fore sight reading. By plugging in the values, you can calculate the RL of the unknown point.

Imagine you're measuring the height of two buildings in a city. You know the height of one building (the Benchmark) and want to find the height of a second building nearby. By using a measuring device (like a level) and comparing the readings, you can easily determine how much taller or shorter the second building is.

Differential levelling is used for finding the difference between the levels of two far-off points. If case of a large distance between two points A and D, it may not be possible to take the readings on A and D from a single setting of instrument. In this method, the level instrument is set at more than one stations along the survey line, and at each shifting station both BS and FS readings are taken to determine the elevation difference between A and D. The following example in Figure 1.30 shows how the RLs of various points on the ground are calculated from BS and FS readings. The last point is another BM which could be used to check the levelling work.

Differential levelling is necessary when points are far apart. Instead of trying to measure both points from one location (which may not be possible), the level instrument is moved between points, taking multiple readings at different positions. Each time the instrument is set up, back sight (BS) and fore sight (FS) readings are taken to compute the elevation change from one point to another.

Think of a long stretch of highway where you want to measure the heights of various points along the road. Instead of standing at one spot and trying to see every point far away, you can set up your level instrument at intervals, just like a photographer might take different shots from various locations to capture the entire scene.

If the survey site is away from the bench mark, the level instrument is set up near the BM and a back sight reading is taken on the BM. The survey work proceeds towards the site by taking fore sights and back sights on a number of change points till a temporary BM is established at the survey site. Now the levelling work is carried out at the site as per differential levelling method. At the end of the work, the last reading is taken again on the temporary BM. It will help determine the closing error in levelling work. The purpose of fly levelling is to connect the survey site to a BM away from the site, and then carry out the levelling work using differential levelling approach.

Fly levelling connects a remote survey site back to a known benchmark. Initially, the level instrument is set up at or near the benchmark to take a measurement. From there, readings are taken towards the site, establishing temporary benchmarks as necessary. At the end of the survey, a final reading checks for any discrepancies, ensuring accuracy over greater distances.

Imagine navigating from a well-known city to a new village far away. Just like you would need to reference back to the city (your starting point) while taking notes or measurements along the way to the village, engineers use fly levelling to make sure they maintain accuracy while traveling further from established points.

This type of levelling, also known as longitudinal levelling, is required to be carried out in highway, railway, canal, pipeline, transmission line, or sewage line projects to know the profile of the ground along selected alignments. It is most popular levelling method to estimate the amount of earth work (cutting or filling) required in large number of engineering projects dealing with the land surface. At regular interval, level readings are taken and RLs of various points along and across the alignment are determined. For drawing the profile of the route, distance is plotted against x-axis and RLs are plotted along y-axis. The vertical scale is usually larger as compared to scale for horizontal distances. It gives clear picture of the profile of the route.

Profile levelling provides a detailed longitudinal view of the terrain, which is essential for planning roadways, railways, and pipelines. By taking regular measurements along a chosen path, surveyors can calculate changes in elevation and visualize the landscape through profiles. This helps engineering projects assess how much land needs to be moved, either cut away or filled.

Consider the process of designing a roller coaster. Engineers need to know the landscape's ups and downs to create a thrilling yet safe ride. Profile levelling allows them to see how the ground rises and falls, ensuring the roller coaster follows a proper path and meets safety standards.

In levelling, the distance of back sight and distance of fore sight is kept equal, as far as possible. It is done to eliminate errors due to non-parallelism of line of collimation and axis of bubble tube, and errors due to curvature and refraction. While doing the levelling across features, like river, big ponds and reservoirs, it is not possible to maintain equal distance for fore sight and back sight distances. In such situations, the reciprocal levelling is used and the height difference between two points is precisely determined by two sets of correlation observations.

Reciprocal levelling is used when it’s impractical to maintain equal distances for measurements, such as across wide valleys or rivers. In this method, the same instrument is moved to both sides of the feature being surveyed, and readings are taken at both stations to ensure accuracy. The idea is to capture measurements that balance out potential errors caused by environmental effects on the optical instrument.

Imagine you’re trying to measure the height of two points across a large river. Instead of measuring from just one side (which can introduce errors from the water or air above), you take turns measuring both from side A and then side B. This dual approach gives you a more accurate reading by comparing the two measurements, much like if you were double-checking an important answer with a friend.

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