of all the surveying operations used in construction levelling is the most common practically every aspect of a construction project requires som app;ication of the levelling process the general are as follows.
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 project , the route centre - line is set out using pegs at 10 m , or 30 m intervals . levels are then taken at these peg positions and at critical point such as sudden changes in ground profiles road crossing , ditches , bridges , culverts , etc , A 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 centre - line only , cross - sectional levelling extends that information at 90 ُ to the centre - line for 20-30 m each side . at each centre - line peg the levelsare taken to all point of interest on either side . where the ground is featureless , levels at 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
|longitudinal section of proposed route|
showing the road details and side slope is plotted at formation level , a cross-sectionaal area is produced , which can later be used to compute volume of earthwork . when plotting cross-section the vertical and horizontal scales are the same , to permit easy scaleing of the area and side slopes
from the above it can be seen that sectional levelling also requires the measurement of horizontal distance between the point whose elevations are obtained . As the process involves the observation of many points , it is important to connect to existing BMs at regular intervals . in most cases of routs construction, one of the earliest tasks is to establish BMs at 100 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 levelling .
A contour is a horizontal curve connecting points of equal elevation . contours graphically represent , in a two-dimensional format on a plan or maap , 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 interval chosen depends on :
(1) the type of project involved : for instance , contouring an airstrip requires an extremely small contour interval .
(2) the horizontal separation between contour lines indicates the steepness of the ground . close spacing defines steep slopes , wide spacing gentle slopes .
(3) Highly irregular contours define regged , often mountainous terrain .
(4) concentric closed contours represent hills or hollows , depending on the increase or in elevation .
(5) the slope between contour line is assummed to be regular .
(6) contour lines crossing a stream from V,s pointing upstream .
(7) the edge of a body of water forms a contour line .
contour are used by engineers to :
(1) construct longitudinal sections and cross- sections for initial investigation .
(2) compute volumes .
(3) construct route line of constant gradient .
(4) delineate the limits of constructed dams , road , railways , tunnels ,etc
(5) delineate and measure drainage area ,
if the ground is reasonably flat , the optical level can be used for contouring using either the direct or indirect methods , in steep terrain it is more economical to use heighting ,as outlined later .