What is contouring in Surveying?
What is Contouring in Surveying?
Contouring in surveying is the process of determining the elevation of different points on the ground and plotting them in a contour map. This is done by using vertical control leveling and horizontal control such as chain survey, compass survey or plane table survey. If a theodolite is used, both horizontal and vertical controls can be achieved from the same instrument.
Types of Contouring in Surveying
Contouring in surveying can be classified into different groups depending on the instruments used. These instruments include vertical control leveling, chain survey, compass survey, plane table survey and theodolite. Each of these instruments are used for different purposes in the contouring process.
Benefits of Contouring in Surveying
Contouring in surveying provides many benefits. It allows for accurate elevation measurements, as well as providing a visual representation of the terrain. It also helps to identify potential hazards, such as steep slopes or bodies of water. Additionally, contouring allows surveyors to identify potential resources such as minerals or groundwater.
Methods of Contour Surveying
There are two methods of contour surveying:
- Direct method
- Indirect method
Direct Method of Contouring
Contour Line Control Using Levelling Instrument
Levelling instrument is the commonly used tool to find the vertical control of the points which lie on the selected contour line. The instrument is set on a commanding position in the area after taking out fly levels from the nearby bench mark. The plane of collimation/height of instrument is determined and the required staff reading for a contour line is calculated. To get the required staff reading, the staff man is asked to move up and down in the area.
Establishing Horizontal Control
Once the vertical control is established, the surveyor establishes the horizontal control of that point using the survey instruments. The instrument man then directs the staff man to another point to find the same staff reading and horizontal control is established there. This process is repeated for several points on a contour line on one or two contour lines and suitably noted down. Plane table survey is the ideal method for this work.
Speed Up The Levelling Process
To speed up the levelling process, hand level and Abney levels are also used. This method is slow, tedious but accurate and suitable for small areas. After the required points are established from the instrument setting, the instrument is shifted to another point to cover more area. The level and survey instrument do not have to be shifted at the same time, as it is better if they are nearby to communicate easily.
Indirect Method of Contouring
Method of Contour Interpolation
Contour interpolation is a method used for establishing horizontal and vertical control in a survey. It involves locating points at selected points and then reducing their levels to create a map. This method is commonly used in topographical surveys to create an accurate representation of the terrain.
Method of Squares
The method of squares is one of the most commonly used techniques for contour interpolation. It involves choosing square grid points across the survey area and reducing the levels of these points. Once the levels of the points have been established, contour lines are then interpolated between the points. This method allows for a more accurate representation of the terrain.
Method of Cross-section
The method of cross-section is another method of contour interpolation. This method involves selecting points along a cross-section line and reducing their levels. Contour lines are then interpolated between the points. This method is particularly useful when surveying a large area as it allows for the accurate representation of the terrain across a wide area.
Radial Line Method
The radial line method of contour interpolation involves selecting points along a radial line and reducing their levels. Contour lines are then interpolated between the points. This method is useful when surveying a smaller area as it allows for a detailed representation of the terrain across a smaller area.
