Contouring Surfaces

I’ve been playing with some ideas, code and Carlson Civil Suite today and I’ve stumbled across an interesting issue with Carlson’s contour routine.

Given this equation for a hyperbolic paraboloid, For more information on this surface

And porting some code to VBA so everyone can follow along

Option Explicit
Public Sub Parabolid()
Dim x As Double
Dim y As Double
Dim z As Double
Dim a As Double
Dim b As Double
Dim pt As AcadPoint
Dim coords(2) As Double
Dim result As Double
a = 1
b = 1

For x = -10 To 10 Step 0.1
    For y = -10 To 10 Step 0.1

        z = x ^ 2 / a ^ 2 - y ^ 2 / b ^ 2

        coords(0) = x: coords(1) = y: coords(2) = z
        ThisDrawing.ModelSpace.AddPoint (coords)

    Next y
Next x

End Sub

And then triangulating and contouring with these options

Produces a couple of visible errors

After a trial run with the “Reduce Vertices” option turned off the results appear correct, or at least visually correct, I’ve not verified them.

So I’m left wondering why the reduce vertices option produced asymmetrical results.