Marching Cubes Terrassing/Ridge Effect

I am implementing a marching cubes algorithm generally based on the implementation of Paul Bourke with some major adjustments:

• precalculation of the scalarfield (floating point values)
• avoiding duplicated vertices in the final list using a std::map
• vertex storing to visualize the final mesh in Ogre3D

Basically I changed nearly 80% of his code. My resulting mesh has some ugly terrasses and I am not sure how to avoid them. I assumed that using floating points for the scalar field would do the job. Is this a common effect? How can you avoid it?

calculating the vertex positions on the edges. (cell.val[p1] contains the scalar value for the given vertex):

//if there is an intersection on this edge
        if (cell.iEdgeFlags & (1 << iEdge))
        {
            const int* edge = a2iEdgeConnection[iEdge];

            int p1 = edge[0];
            int p2 = edge[1];

            //find the approx intersection point by linear interpolation between the two edges and the density value
            float length = cell.val[p1] / (cell.val[p2] + cell.val[p1]);
            asEdgeVertex[iEdge] = cell.p[p1] + length  * (cell.p[p2] - cell.p[p1]);
        }

You can find the complete sourcecode here: https://github.com/DieOptimistin/MarchingCubes I use Ogre3D as library for this example.

As Andy Newmann said, the devil was in the linear interpolation. Correct is:

float offset;
float delta = cell.val[p2] - cell.val[p1];

if (delta == 0) offset = 0.5;
else offset = (mTargetValue - cell.val[p1]) / delta;

asEdgeVertex[iEdge] = cell.p[p1] + offset* (cell.p[p2] - cell.p[p1]);