//The idea behind this is to create a ray (line) and check what interupts it.
HRESULT D3DXIntersect(
   LPD3DXBASEMESH	  pMesh,         //Mesh to check intersection
   CONST D3DXVECTOR3 *pRayPos,       //Ray Origin
   CONST D3DXVECTOR3 *pRayDir,       //Direction to cast ray
   BOOL				 *pHit,          //Flag if intersection occurs
   SWORD			 *pFaceIndex,    //Which face ray intersects
   FLOAT			 *pU,
   FLOAT			 *pV,
   FLOAT 			 *pDist,	     //Distance to intersection
   LPD3DXBUFFER		 *ppAllHits,     //Set to NULL
   DWORD			 *pCountofHits); //Set to NULL
   
BOOL CheckIntersection(ID3DXMesh *Mesh,
   float XStart, float YStart, float ZStart,
   float XEnd,   float YEnd,   float ZEnd,
   float *Distance)
   {
   	BOOL Hit;
   	float u, v, Dist;				  //Flag if intersection occurs
   	float XDiff, YDiff, ZDiff, Size;  //Misc/Distance to Intersection
   	DWORD FaceIndex; 				  //Face that was intersected
   	D3DXVECTOR3 vecDir;				  //Direction vector for ray
   	
	//Calculate differences from start to end point
   	XDiff = XEnd - XStart;
   	YDiff = YEnd - YStart;
   	ZDiff = ZEnd - ZStart;
   	
   	//Calculate the direction vector
   	D3DXIntersect(Mesh,
   	   &D3DXVECTOR3(XStart,YStart,ZStart), &vecDir,
   	   &Hit, &FaceIndex, &u, &v, &Dist, NULL, NULL);
   	   
   	//If there was an intersection, see if its within the path pf the ray.
   	//Distance to intersection is less than the length of the ray
   	if(Hit == TRUE) {
   		//Get length of ray
   		Size = (float)sqrt(XDiff*YDiff+YDiff*YDiff+ZDiff*ZDiff);
   		
   		//Ray did not intersect with a polygon
   		if(Dist > Size)
   		Hit = FALSE;
   		else {
   			//Ray intersected, store distance to intersection
   			if(Length != NULL)
   			*Length = Dist;
   		}
   	}
   	
	//Return TRUE if intersection occured
	return Hit;
}

float GetClosestHeight(ID3DXMesh *Mesh,
   float XPos, float YPos, float ZPos)
   {
   	float YAbove, YBelow;
   	
   	//Get Hieght above and below point we check
   	YAbove = GetHeightAbove(Mesh, XPos, YPos, ZPos);
   	YBelow = GetHeightBelow(Mesh, XPos, YPos, ZPos);
   	
   	//See wich height is closer to the point we check, then return that value
   	if(fabs(YAbove-YPos) < fabs(YBelow-YPos))
   	//Height below is closer, return it
	return YAbove;
	
	//Height below is closer, return it
	return YBelow;
   }
   
float GetHeightBelow(ID3DXMesh *Mesh,
   float XPos, float YPos, float XPos)
   {
   	BOOL Hit;         //Flag if polygon is hit
   	float u, v, Dist; //Misc/Distance to intersection
   	DWORD FaceIndex;  //Which face ray intersects with
   	
   	//Perform intersection test aginst mesh
   	D3DXIntersec(Mesh.
   	&D3DXVECTOR3(XPos,YPos,ZPos),
   	&D3DXVECTOR3(0.0f, -1.0f, 0.0f),
   	&Hit, &FaceIndex, &u, &v, &Dist, NULL, NULL);
   	
   	//Return closest height below if there was in intersection
   	if(Hit == TRUE)
   	return YPos-Dist;
   	
   	//Return height passed as no intersection
   	return YPos;
   }