PhysX Question about Raycasting - Setting the shape masks

Question

I am trying to make the raycast give me the distance of the terrain and nothing else. But I'm not sure how to use the group Mask Filter.

virtual NxShape* raycastClosestShape (
   const NxRay& worldRay,
   NxShapesType shapeType,
   NxRaycastHit& hit,
   NxU32 groups=0xffffffff,
   NxReal maxDist=NX_MAX_F32,
   NxU32 hintFlags=0xffffffff,
   const NxGroupsMask* groupsMask=NULL,
   NxShape** cache=NULL) const = 0;

\param[in] groups Mask used to filter shape objects.

Let's say my terrain is set in group 1. How would I use the function above?