Updating XNA BoundingSphere - sphere moves in opposite Y direction?

Question

In my players update method I calc the new world matrix then I update a boudingsphere by:

MyBoundingSphere = MyBoundingSphere.Transform(World);

However when the the model's World has moved vertically down MyBoundingSphere moves vertically up?! (i.e. MyBoundingSphere.Center.Y increases instead of decreases);

Answer 1

During a BoundingSphere.Transform(Matrix m) operation, the bounding sphere moves in the same direction as m's Translation vector tells it to. Your problem must be elsewhere.

Have you checked that the sphere starts out in the right place? (before the downward movement of the model).