Loop Closing Myths
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Many people have misconceptions about what closing a loop does. They often assume that when the computer "closes" a cave all errors will be fixed. This is not at all true. The best the computer can do is to distribute the errors around the cave in a way that minimizes the impact of the error. So, instead of the errors piling up on one shot, it is distributed among many other shots. In other words, after closing a loop, most of the station locations are slightly less accurate and one station is much more accurate.
If you have a large error and a short loop, more errors will be distributed to each station and the accuracy of the station locations will be even worse. The best you can do in these situations is try to isolate or unweight the worst loops so the effects of the errors don’t spread to the rest of the cave. This is what the COMPASS loop closer does.
Since closing loops doesn’t improve the accuracy of the survey, the main reason for closing loops is cosmetic. If loops are left unclosed, all the errors in the loop pile up at the closing station. This creates plots with large offsets in the middle of passages or junctions where the angles are all wrong. This makes it very difficult use the computer plot as the basis for a finished map. Closing the cave allows you to create plots that don’t have major distortions so they can be used for finished maps.
When you have a large error, you should understand that the computer cannot fix the error. The only correct thing to do is resurvey parts of the cave and eliminate the error. Unfortunately, the reality is that caves are difficult environments and it can take years to get a resurvey done. In the meantime, people want maps; so closing loops is a compromise that makes the best out of a bad situation.