CastRay question

I'm trying to write a simple routine to do a castray directly below a point, and see if there is a collision. I am most likely doing something very silly, and I was hoping someone could look at the code below and point it out.
It may be something as simple as it not being correct to assume that z-.5 is directly below z, if x and y don't change.

Thanks,

//
// JSL - check below the camera to see where the ground is
//
double BumpIntoTheGround(Point3F& start)
{
	Point3F end;
	bool	result;
	RayInfo rinfo;

  	end.x = start.x;
  	end.y = start.y;
  	end.z = start.z - .5;

	result = gServerContainer.castRay(start, end, TerrainObjectType, &rinfo);
	
	if (result)
		return(rinfo.point.z);
	
	return(0.0);
}