Lesson Note On Methods of Grid Levelling
The art of determining relative altitudes of points on the surface of the earth of beneath the surface of earth is called LEVELLING.
A contour is defined as an imaginary line of constant elevation on the ground surface. It can also be defined as the line of intersection of a level surface with the ground surface. For example, the line of intersection of the water surface of a still lake or pond with the surrounding ground represents a contour line.
To determine the positions of the contours on a plan the leveling methods are used to find the relative positions of points on the surface of the ground with reference to certain benchmarks at regular intervals. This technique of making observations and plotting on a plan is called as grid leveling.
Contour Interval:
The vertical distance between any two consecutive contours is known as a contour interval. For example, if the various consecutive contours are 100m, 98m, 96 m etc., then the contour interval is 2m. This interval depends upon,
· The nature of the ground
· The scale of the map and
· The purpose of survey
Contour intervals for flat country are generally small, eg. 0.25m, 0.5m, 0.75 m etc. For a steep slope in hilly area is greater, eg. 5m, 10m, 15 m etc.
Again ,for a small-scale map, the interval may be of 1m,2m,3m etc. and for large scale map,it may be of 0.25m,0.50m,0.75m etc.
It should be remembered that the contour interval for a particular map is constant.
Contouring:
The process of locating these contour lines on the surface of the earth is known as contouring.
Methods of contouring:
The method of establishing / plotting contours in a plan or map is known as contouring. It requires planimetric position of the points and drawing of contours from elevations of the plotted points. Contouring involves providing of vertical control for location of points on the contours and horizontal control for planimetric plotting of points. Thus, contouring depends upon the instruments used (to determine the horizontal as well as vertical position of points). In general, the field methods of contouring may be divided into two classes:
1. Direct methods
2. Indirect methods
Direct Method:
In the direct method, the contour to be plotted is actually traced on the ground. Points which happen to fall on a desired contour are only surveyed, plotted and finally joined to obtain the particular contour. This method is slow and tedious and thus used for large scale maps, small contour interval and at high degree of precision.
Vertical control :
In this method, a benchmark is required in the project area. The level is set up on any commanding position and back sight is taken on the bench mark. Let the back sight reading on the bench mark be 1.485 m. If the reduced level of the bench mark is 100 m, the height of instrument would be 100 + 1.485 = 101.485 m.
To locate the contour of 100.5 m value, the staff man is directed to occupy the position on the ground where the staff reading is 101.485 -100.500 = 0.985 m. Mark all such positions on the ground where the staff reading would be 0.985 m by inserting pegs. Similarly locate the points where the staff reading would be 101.485 -101 = 0.485 m for 101m contour.
The contour of 101.5 m cannot be set from this setting of the instrument because the height of instrument for this setting of the instrument is only 101.485 m. Therefore, locating contours of higher value, the instrument has to be shifted to some other suitable position.
Establish a forward station on a firm ground and take fore sight on it. This point acts as a point of known elevation, for shifting the position of the instrument to another position, from where the work proceeds in the similar manner till the entire area is contoured.
Horizontal control :
The horizontal control is generally provided by method of plane table surveying or locating the positions of points.
Indirect method of contouring:
In this method, the spot levels of selected guide points are taken with a level and their levels are computed. The horizontal positions of these points are measured or computed and the points are plotted on the plan. The contours are then drawn by a process called interpolation of contours from the levels of the guide points. The following are the indirect methods are commonly used for locating contours.
1. Squares or Grid method
2. Cross section method
Square or grid method:
In this method, the area to be surveyed is divided into a grid or series of squares. The grid size may vary from 5 m x 5 m to 25 m x 25 m depending upon the nature of the terrain, the contour interval required and the scale of the map desired. Also, the grids may not be of the same size throughout but may vary depending upon the requirement and field conditions. The grid corners are marked on the ground and spot levels of these comers are determined by leveling. The grid is plotted to the scale of the map and the spot levels of the grid corners are entered. The contours of desired values are then located by interpolation. Special care should be taken to give the spot levels to the salient features of the ground such as hilltops, deepest points of the depressions, and their measurements from respective corners of the grids, for correct depiction of the features. The method is used for large scale mapping and at average precision.
Cross section method:
In these sections, a base line, centre line or profile line is considered. Cross sections are taken perpendicular to this line at regular intervals. After this points are marked along the cross sections at regular intervals. A temporary bench mark is set up near the site. Staff readings are taken along the base line and the cross sections. The readings are entered in the level book the base line and the cross sections should also be mentioned. The RL of each of the points calculated. Then the base line and cross sections are plotted to a suitable scale. Subsequently the RLs of the respective points are noted on the map, after which the required contour line is drawn by interpolation
This method is suitable for route survey, when cross sections are taken transverse to the longitudinal section.
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