AutoCAD .NET :: Rotating 3DPolyline To Check Z-value Of Vertices In Top-view Mode

I am trying to rotate a 3dPolyline, so that I can check(view) the Z-value of vertices in the top-view mode.

The following is the example which I am trying to achieve.

The white 3dPolyline has vertices at each intersecting point with the contour lines. Each vertax has Z-value.

I want to rotate this white 3dPolyline in 3D, so that I can make the red 2dPolyline. (Z-value becomes Y-value)

And the start point of the white 3dPolyline becomes the left most point of the red line.

The end point of the white 3dPolyline becomes the right most point of the red line.

Currently, I am doing this by making a new 2dPolyline, manually inserting Z-values as Y-values.

But it looks like my code can be neater if I use TrnasformBy(Matrix3d.Projection).

I am not good with Geometry, so how to use this Matrix3d.Projection.

Need a quick code snippet to make this function? (preferably in C#)