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Passage dimensions measure the size of the passage by taking four measurements to the walls of the passage. The measurements are the distance between the survey station and the ceiling, floor, left and right walls of the cave. This data is usually described as the Left, Right, Up and Down measurements or LRUDs for short. The information is used by various Compass programs to reconstruct the passage shapes to get a better picture of the true size and shape of the cave.
The LRUD measurements are generally taken at a survey station and at right angles to the shot. They can be entered using any of the standard cave survey units. In instances where there is passage instead of a wall, the letter "P" can be entered to indicate passage. The "*" key can also be used to enter the passage designation. This makes it easy to enter data from the numeric keypad because your hand doesn't have to leave the home position.
Associating Passage Dimensions With From Or To Stations. Passage dimensions (Up, Down, Right, Left) can be associated with either the From or the To station depending on your preferences. The Project Manager gives you the option of processing survey data with either standard.
The Last LRUD. There is always one more station than there are shots in a cave survey. Since cave survey data is entered as shots, there will always be one more LRUD than there are shots. If your LRUDs are associated with the From station, you will have one extra LRUD at the end of each survey. If your LRUDs are associated with the To station, you will have one extra LRUD at the beginning of each survey. To solve the problem, you need an extra shot to hold the extra LRUD. You do this by creating a zero-length-shot at the station where the extra LRUD exists. This gives you an extra station for the extra LRUD.
A zero length shot is a shot where the tape length is zero. Since the length is zero, the compass and inclination are irrelevant and are normally left zero. You then enter the Up, Down, Right, and Left for this station. The compiler ignores zero length shots to the same station except for the Up, Down, Right and Left data. If there is more than one zero length shot, the compiler uses the Up, Down, Right and Left data for the last occurrence. You can also use zero length shots to add extra comments associated with a particular station.