When you submit a mark recovery to the ["NGS"], a window opens to ask you whether or not the mark is suitable for satellite observation. In order to answer this question, you must have checked the surroundings of the mark. A rule of thumb is that, except for perhaps a pole or other very narrow item, the view of the sky above 15 degrees at eye level must be clear in all directions. To measure approximately 15 degrees above eye level, with your thumb pointing straight up, hold your fist out at arm's length so that the bottom of your fist is level with your eye. The tip of your thumb is then at about 15 degrees above eye level.