Definition of Contour 2. Contour Interval and Horizontal Equivalent 3. Characteristics 4. Methods 5.
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Of all the surveying operations used in construction, levelling is the most common. Practically every aspect of a construction project requires some application of the leveling process. The more general are as follows. Sectional leveling This type of levelling is used to produce ground profiles for use in the design of roads, railways and pipelines.
In the case of such projects, the route centre-line is set out using pegs at m, m or m intervals. Levels are then taken at these peg positions and at critical points such as sudden changes in the ground profiles, road crossings, ditches, bridges, culverts, etc. The resultant plot of these elevations is called a longitudinal section. When plotting, the vertical scale is exaggerated compared with the horizontal, usually in the ratio of 10 : 1. The longitudinal section is then used in the vertical design process to produce formation levels for the proposed route design Whilst the above process produces information along a centre-line only, cross-sectional leveling extends that information at 90 to the centre-line for m each side.
At each centre-line peg the levels are taken to all points of interest on either side. Where the ground is featureless, levels 5-m intervals or less are taken.
In this way a ground profile at right angles to the centre-line is obtained. When the design template showing the road details and side slopes is plotted at formation level, a cross-sectional area is produced, which can later be used to compute volumes of earthwork.
When plotting cross-sections the vertical and horizontal scales are the same, to permit easy scaling of the area and side slopes. From the sketch above it can be seen that sectional leveling also requires the measurement of horizontal distance between the points whose elevations are obtained.
As the process involves the observation of many points, it is imperative to connect into existing BMs at regular intervals. In most cases of route construction, one of the earliest tasks is to establish BMs at m intervals throughout the area of interest.
Levelling which does not require the measurement of distance, such as establishing BMs at known positions, is sometimes called fly leveling. A contour line is a horizontal curve connecting points of equal elevation.
They graphically represent, in a two-dimensional format on a plan or map, the shape or morphology of the terrain. The vertical distance between contour lines is called the contour interval. Depending on the accuracy required, they may be plotted at 0. The contour vertical interval is chosen based on: 1 The type of project involved; for instance, contouring an airstrip requires an extremely small contour interval.
Characteristics Properties of the contour lines: 1 Contours are perpendicular to the direction of maximum slope. Close spacing defines steep slopes, wide spacing gentle slopes. Contours are used by engineers to: 1 Construct longitudinal sections and cross-sections for initial investigation. If the ground is reasonably flat, the optical level can be used for contouring using either the direct or indirect methods. In undulating areas it is more economical to use optical or electronic methods, as outlined later 5.
Methods of contouring: In this method the actual contour is pegged out on the ground and its planimetric position located. A staff reading of 0. The staff is then moved throughout the terrain area, with its position pegged at every 0. In this way the m contour is located. Similarly a staff reading of 1. The planimetric position of the contour needs to be located using an appropriate survey technique.
This method, although quite accurate, is tedious and uneconomical and could never be used over a large area. It is ideal, however, in certain construction projects which require excavation to a specific single contour line. This technique requires the establishment, over the site, of a grid of intersecting evenly spaced lines. The boundary of the grid is set out by theodolite and steel tape. The grid spacing will depend upon the ru gosity of the ground and the purpose for which the data are required.
All the points of Intersection throughout the grid may be pegged or shown by means of paint from a spray canister. Alternatively ranging rods at the grid intervals around the periphery would permit the staff holder to align himself with appropriate pairs and thus fix the grid intersection point, for example, alignment with rods B-B and fixes point B2 When the RLs of all the intersection points are obtained, the contours are located by linear interpolation between the levels, on the assumption of a uniform ground slope between each value.
The interpolation may be done arithmetically, using a pocket calculator, or graphically. Consider grid points B2 and B3 with reduced levels of
LEVELLING AND CONTOURING PDF
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Lesson 5 - Levelling and Contouring
Contour: Definition and Methods | Contour |Surveying