public boolean blocked(UnitMover mover, int x, int y) {
		// if theres a unit at the location, then it's blocked

		if (getUnit(x,y) != 0) {
			return true;
		}
		
		int unit = ((UnitMover) mover).getType();
		
		// planes can move anywhere

		if (unit == PLANE) {
			return false;
		}
		// tanks can only move across grass

		if (unit == TANK) {
			return terrain[x][y] != GRASS;
		}
		// boats can only move across water

		if (unit == BOAT) {
			return terrain[x][y] != WATER;
		}
		
		// unknown unit so everything blocks

		return true;
	}