*
The graphical presentation of experimentally or theoretically generated data
sets frequently involves the construction of contour plots. A general
computer algorithm has been developed for the construction of contour plots.
The algorithm provides for efficient and accurate contouring with a modular
approach which allows flexibility in modifying the algorithm for special
applications. The algorithm accepts as input data values at a set of points
irregularly distributed over a plane. The algorithm is based on an
interpolation scheme in which the points in the plane are connected by
straight line segments to form a set of triangles. In general, the data is
smoothed using a least-squares-error fit of the data to a bivariate
polynomial. To construct the contours, interpolation along the edges of the
triangles is performed, using the bivariable polynomial if data smoothing was
performed. Once the contour points have been located, the contour may be
drawn.*

This program was released by NASA through COSMIC as ARC-11441. The italicized text above is from the official NASA release.

- There are no references for this program.
- Go to the download page to download the source code and two test cases that will demonstrate the use of the Conplot procedure.

There are a number of other algorithms that compute contour lines. I would suggest a search of netlib. This algorithm permits the input of scattered data; many other algorithms require the data on a rectangular lattice.