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Vehicle following terrain

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I''ve implemented one of the techniques discuted in previous threads. But I dont know what is my problem right now, the vehicle do not follow the ground correctly. this is my Tank update method.
D3DXMATRIX CTank::Update(float fDeltaTime)
	//adjusting height of the tank with the heightmap

	m_vctPosition.y = m_pContourTerrain->GetHeight(m_vctPosition.x, m_vctPosition.z) + 40;
	fMinX += m_vctPosition.x;
	fMinY += m_vctPosition.y;
	fMinZ += m_vctPosition.z;

	fMaxX += m_vctPosition.x;
	fMaxY += m_vctPosition.y;
	fMaxZ += m_vctPosition.z;

	D3DXVECTOR3 roue1 = D3DXVECTOR3(fMinX, 0, fMaxZ);
	D3DXVECTOR3 roue2 = D3DXVECTOR3(fMaxX, 0, fMaxZ);
	D3DXVECTOR3 roue3 = D3DXVECTOR3(fMinX, 0, fMinZ);
	D3DXVECTOR3 roue4 = D3DXVECTOR3(fMaxX, 0, fMinZ);

	//Rotating the 4 points according to orientation of the tank

	if ( m_fYaw != 0.0f)


		D3DXVec3TransformCoord(&roue1, &roue1, &matY);
		D3DXVec3TransformCoord(&roue2, &roue2, &matY);
		D3DXVec3TransformCoord(&roue3, &roue3, &matY);
		D3DXVec3TransformCoord(&roue4, &roue4, &matY);		

	//Adjusting height of each of the wheeks

	roue1.y = m_pContourTerrain->GetHeight(roue1.x, roue1.z);
	roue2.y = m_pContourTerrain->GetHeight(roue2.x, roue2.z);
	roue3.y = m_pContourTerrain->GetHeight(roue3.x, roue3.z);
	roue4.y = m_pContourTerrain->GetHeight(roue4.x, roue4.z);
	fMinX -= m_vctPosition.x;
	fMinY -= m_vctPosition.y;
	fMinZ -= m_vctPosition.z;

	fMaxX -= m_vctPosition.x;
	fMaxY -= m_vctPosition.y;
	fMaxZ -= m_vctPosition.z;

	//Extracting Dir and Right vectors

	D3DXVECTOR3 vDir = roue1 - roue3;
	D3DXVECTOR3 vRight = roue4 - roue3;

	D3DXVec3Normalize(&vDir, &vDir);
	D3DXVec3Normalize(&vRight, &vRight);

	D3DXVec3Cross(&vUp, &vDir, &vRight);
	D3DXVec3Normalize(&vUp, &vUp);

	m_matTransformations._11 = vRight.x;
	m_matTransformations._12 = vRight.y;
	m_matTransformations._13 = vRight.z;
	m_matTransformations._14 = 0;

	m_matTransformations._21 = vUp.x;
	m_matTransformations._22 = vUp.y;
	m_matTransformations._23 = vUp.z;
	m_matTransformations._24 = 0;

	m_matTransformations._31 = vDir.x;
	m_matTransformations._32 = vDir.y;
	m_matTransformations._33 = vDir.z;
	m_matTransformations._34 = 0;
	m_matTransformations._41 = m_vctPosition.x;
	m_matTransformations._42 = m_vctPosition.y;
	m_matTransformations._43 = m_vctPosition.z;
	m_matTransformations._44 = 1.0f;

	return m_matTransformations;
The car do follow the ground correctly if i don''t modify my m_fYaw value. but I need to modify it, since I want the tank to freely move around, human controlled...

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