// first vector, the surface vector of the terrain

_terrainNormal = _terrain.GetTerrainNormal( _x, _y );
	
// second vector

Vector3 tankNormal = new Vector3( 0, 0, -1 );

// calculate angles


_angleX = (float) Math.Asin( ( _terrainNormal.X - tankNormal.X ) / ( Math.Sqrt( _terrainNormal.X * _terrainNormal.X + _terrainNormal.Z * _terrainNormal.Z )) );

_angleY = (float) Math.Asin( ( _terrainNormal.Y - tankNormal.Y ) / ( Math.Sqrt( _terrainNormal.Y * _terrainNormal.Y + _terrainNormal.Z * _terrainNormal.Z )) );



that is, im doing the regular sin(angle) = a/b thingy here, and sort of "projecting" the vectors on a 2d picture. like, when calculating angleX i totally forgett about the difference between the vectors in Y. can you do it like this?

if you can do like that, maybe there is something wrong with the code that get the terrain normal?  this code takes three points and makes it a normal:

   

		public Vector3 GetTerrainNormal( float x, float y )
		{
			Vector3 p1 = Vector3.Empty;
			Vector3 p2 = Vector3.Empty;
			Vector3 p3 = Vector3.Empty;

			float fPosX = (x / _spacing);
			float fPosY = (_terrainHeight-y)/_spacing;

			int posX = (int) fPosX;
			int posY = (int) fPosY;

			float percentX = fPosX - (int) posX;
			float percentY = fPosY - (int) posY;

			float balance = percentX + percentY;

			int triangle;

			if( balance < 1 ) triangle = 0;
			else triangle = 1;

			if( triangle == 0 ) // top left triangle

			{
				p1.X = GridXToTerrainX( posX );
				p1.Y = GridYToTerrainY( posY );
				p1.Z = GridZToTerrainZ( GetGridHeight( posX, posY ) );

				p2.X = GridXToTerrainX( posX + 1 );
				p2.Y = GridYToTerrainY( posY );
				p2.Z = GridZToTerrainZ( GetGridHeight( posX + 1, posY ) );

				p3.X = GridXToTerrainX( posX );
				p3.Y = GridYToTerrainY( posY + 1 );
				p3.Z = GridZToTerrainZ( GetGridHeight( posX, posY + 1 ) );
			}
			else // bottom right triangle

			{
				p1.X = GridXToTerrainX( posX );
				p1.Y = GridYToTerrainY( posY + 1 );
				p1.Z = GridZToTerrainZ( GetGridHeight( posX, posY + 1 ) );

				p2.X = GridXToTerrainX( posX + 1 );
				p2.Y = GridYToTerrainY( posY );
				p2.Z = GridZToTerrainZ( GetGridHeight( posX + 1, posY ) );

				p3.X = GridXToTerrainX( posX + 1 );
				p3.Y = GridYToTerrainY( posY + 1 );
				p3.Z = GridZToTerrainZ( GetGridHeight( posX + 1, posY + 1 ) );
			}

			// TextWriter.AddText( "   triangle: " + triangle );


			// calculate the x and y angles to the terrain plane normal from the tank normal


			Vector3 v12 = p1 - p2;
			Vector3 v13 = p1 - p3;

			Vector3 normalTerrain = Vector3.Cross( v12, v13 );

			// TextWriter.AddText( "   normal: (" + normalTerrain.X + ", " + normalTerrain.Y + ", " + normalTerrain.Z + ")" );


			normalTerrain.Normalize();

			return normalTerrain;
		}