Aquaculture Engineering

Contour surveying - Interpolation of contour

Unit 1- Surveying

Contour surveying

Interpolation of contour

Interpolation of the contours is the process of spacing the contours proportionately between the plotted ground points established by indirect methods. The methods of interpolation are based on the assumption that the slope of ground between the two points is uniform.

Following are the three methods of interpolation.

• By estimation
• By arithmetic calculation
• By graphical method

By estimation
This method is extremely rough and is used for small scale work only. The position of contour points between the guide points are located by estimation.

By arithmetic calculation
This method so accurate and is time consuming. The positions of contour points between the guide points are located by arithmetic calculation e.g. A, B, C and D be the guide points plotted on the map. Elevations at each point are 607.4, 617.3, 612.5 and 604.3 respectively. Let AB=BD, CD=CA= one inch on plan. The vertical difference in elevation between A and B is (617.3-607.4) = 9.9 feet. Hence the distance of the contour points from A will be calculated as follows

i.e., 1/x * y*z
where,
x= Difference in contour elevation between two points
y= The distance between two points
z= The distance between the starting point to contour line

Distance of 610 feet contour point says A1 is calculated by interpolation using the formula,
The difference in contour elevation between two points is (617.3-607.4) = 9.9 feet.
The distance between the two points = 2.0m
The distance between the starting point to contour line is 610- 607.4 = 2.6 feet
Distance from point ‘A’ is = (1/9.9) x 2.6 x 2 = 0.52 m

These contour points may be located on AB the contour points for any lines can be calculated.