1. Lagrangian Interpolation
Civil Engineering Majors
Authors: Autar Kaw, Jai Paul
Transforming Numerical Methods Education for STEM Undergraduates

2. Lagrange Method of Interpolation http://numericalmethods.eng.usf.edu

3. What is Interpolation ?
Given (x0,y0), (x1,y1), …… (xn,yn), find the value of ‘y’ at a value of ‘x’ that is not given.

4. Interpolants Evaluate Differentiate, and Integrate.
Polynomials are the most common choice of interpolants because they are easy to:
Evaluate
Differentiate, and
Integrate.

5. Lagrangian Interpolation

6. Example
To maximize a catch of bass in a lake, it is suggested to throw the line to the depth of the thermocline. The characteristic feature of this area is the sudden change in temperature. We are given the temperature vs. depth plot for a lake. Determine the value of the temperature at z = −7.5 using the Lagragian method for linear interpolation.
Temperature vs. depth of a lake

7. Linear Interpolation

8. Linear Interpolation (contd)

9. Quadratic Interpolation

10. Example
To maximize a catch of bass in a lake, it is suggested to throw the line to the depth of the thermocline. The characteristic feature of this area is the sudden change in temperature. We are given the temperature vs. depth plot for a lake. Determine the value of the temperature at z = −7.5 using the Lagragian method for quadratic interpolation.
Temperature vs. depth of a lake

11. Quadratic Interpolation (contd)

12. Quadratic Interpolation (contd)

13. Cubic Interpolation

14. Example
To maximize a catch of bass in a lake, it is suggested to throw the line to the depth of the thermocline. The characteristic feature of this area is the sudden change in temperature. We are given the temperature vs. depth plot for a lake. Determine the value of the temperature at z = −7.5 using the Lagragian method for cubic interpolation.
Temperature vs. depth of a lake

15. Cubic Interpolation (contd)

16. Cubic Interpolation (contd)

17. Comparison Table

18. Thermocline
What is the value of depth at which the thermocline exists?
The position where the thermocline exists is given where
Simply setting this expression equal to zero, we get

19. Additional Resources
For all resources on this topic such as digital audiovisual lectures, primers, textbook chapters, multiple-choice tests, worksheets in MATLAB, MATHEMATICA, MathCad and MAPLE, blogs, related physical problems, please visit

20. THE END