How the Loop Closer Works
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The Loop Closer takes an ordinary survey data file and processes it so that all the loops are closed. The closed data is then written to a file on disk. These files have the extension ".CLP" and are identical to the ".DAT" file except the shots in the loops have been adjusted to close the loops. In fact, you can examine CLP file with the Editor and see how the Loop Closer has adjusted various shots to close a loop. This can give you clues as to where the errors might be.
The COMPASS loop closer uses special loop closing techniques. Click here for detailed explanation of the philosophical and theoretical aspects of loop closing.
The Loop Closing Process. As the Loop Closer processes a cave file, it performs a number of operations on the data. Understanding these steps can help you solve certain problems with the cave data. Here is a detailed explanation of the steps:
1. Locating Loops. Before you can close a loop, you must find the loops in the data. COMPASS locates loops by looking for closing shots. Normally, when you take a survey shot, one of the stations is new and one of the stations has already been defined by the previous shot. However, when you survey the last shot in a loop both stations are defined. This is the closing shot. By walking backward from the closing shot to the first shot in the loop, the program locates all the shots in the loop.
2. Calculating Errors. The COMPASS loop closure technique closes the best loops first. This way, errors from relatively bad loops are not distributed to the good loops, spoiling their accuracy. In order to decide which loops to close first, the errors for each loop must be calculated.
3. Adjusting Shots. The errors in each loop are removed by adjusting each shot so that it compensates for part of the error. Since long shots cause larger errors than short shots, long shots are adjusted proportionately more than short shots. After each shot is adjusted, it is marked so it cannot be adjusted again. This way, shots that are part of a previously closed loop are not adjusted.
4. Merging Data. Once the all the loops have been closed, the newly closed shots are merged with original data to produce a new data file.