Calculating the contours of a 3D heightmap?

I am looking for a function or example to produce a list of lines representing contours at a specific height within a heightmap.

Eg,

Lines[] = GetContours(Heights[512,512], HeightValue)

Where Heights is a 512x512 array of floating point values, HeightValue is the height at which the contour should be drawn. Heights may contain multiple lines for that specific height (eg a Saddle, or island chain)

Does anyone have or know where to get an algorithm to generate this?

From stackoverflow

• What you're actually trying to do is to draw an iso-line.
  see my question about it a while ago.
  The link given in one of the answers was very helpful.

• The standard algorithm is marching squares.

• Algorithm:

  a) Find which polygons have vertices above&below the height you're interested in.

  b) Intersect the edges of those polygons with the plane to get lines on the plane.

  c) Join the lines together in strips.

• Take a look at Srtm2Osm, a tool I've made to generate contours from NASA's SRTM data. You can find the algorithm in the C# code.