Profile Levelling | Longitudinal and Cross Sectioning in Surveying

In this post, we are going to discuss about profile levelling ( Longitudinal and Cross sectioning).

What is Profile levelling?

Profile leveling is one of the most common applications of running levels and vertical distance measurement for the surveyor. The results are plotted in the form of a profile, which is a drawing that shows a vertical cross section. Profiles are required for the design and construction of roads, curbs, sidewalks, pipelines, etc. In short, profile leveling refers to the process of determining the elevation of points on the ground at mostly uniform intervals
along a continuous line.

Equipment used for profile leveling

  1. Dumpy level
  2. Leveling staff
  3. Tripod
  4. Staff bubble
  5. Chain or Tape

Procedure for profile levelling

1.Longitudinal levelling

Profile leveling is essentially the same as benchmark leveling, with one basic difference. At each instrument position, where an HI is determined by a back sight rod reading on a benchmark or turning point, several additional foresight readings may be taken on as many points as desired. These additional readings are called rod shots, and the elevation of all those points is determined by subtracting the rod shot from the HI at that instrument
location. (See figure 1)

Plotting the Profile

The profile drawing is basically a graph of elevations, plotted on the vertical axis, as a function of stations, plotted on horizontal axis. A gridded sheet called profile paper is used to plot the profile data from the field book. All profile drawings must have a proper title block, and both axes must be fully labeled with stations and elevations.

The elevation or elevation scale is typically exaggerated; that is, it is ‘stretched’ in comparison to the horizontal scale. For example the vertical scale might be 10 times larger. The horizontal line at the bottom of the profile does not necessary have to start at zero elevation

Longitudinal and Cross Sectioning

2.Cross sectioning levelling

Cross sectioning levelling is another method in profile levelling. The term cross-section generally refers to a relatively short profile view of the ground, which is drawn perpendicular to the route centerline of a highway or other types of linear projects.

Cross-sectional drawings are particularly important for estimating the earthwork volumes needed to construct a roadway; they show the existing ground elevations, the proposed cut or fill side slopes, and the grade elevation for the road base.

There is really no difference in procedure between profile and cross-section leveling except for the form of the field notes. Cross-section rod shots are usually taken during the route profile survey from the same instrument positions used to take rod shots along the centerline.

Cross-section data are obtained at the same locations along the route that are used for the profile rod-shot stations. (See figure 2 a and b).

profile levelling

Figure 2:

(a) Top view showing the route center line and the line for cross-section leveling at station 1+ 50.
(b) The cross-section showing ground elevations at points left and right of the centerline.

Hope you can understand this post, profile levelling (longitudinal and cross-section levelling)

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