Locating Bad Tie-ins
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Another common type of surveying blunder is a "bad tie-in." Bad tie-ins occur when a shot is tied to the wrong station. This can occur if you write the wrong station name in the survey book, if the station tag in the cave is mislabeled or moved or if the survey is renamed. Connecting a shot to the wrong station can cause very large survey errors.
The blunder detecting routines in COMPASS can help you locate bad tie-ins. The routine works by breaking a single shot in the loop. This allows the station at the break to move to its natural location. If there is bad tie-in, this point will move back near the proper connecting station. COMPASS breaks the loop at each station and tests to see if the point has moved close to another station in the cave. It then retests the loop to see if the loop error has improved. Finally, the program records the best improvement in a list.
COMPASS analyzes Tie-ins whenever you Analyze A Loop for blunders. You can access tie-in information by selecting the "Bad Tie-Ins” page in the Blunder dialog box.
The page shows a list of the best tie-ins for the loop. Here is a description of each column:
1. From. This item shows the original “From” station of the tying shot.
2. Old Tie. This item shows the station that was the original connection.
3. New Tie. This item shows the new connecting station that improved the loop error.
4. New Error. This item shows what the new error would be if the new connection were used.
5. New STD. This column shows the new deviation from the predicted error level for this loop.
6. Improv. The final column shows the ratio of improvement for this connection.
If the new connection is the correct, it should dramatically improve the error in the loop. For this reason, COMPASS lists the 25 best fixes so you can examine the best candidates.
What To Look For. One of the most common causes of mis-ties are typos that occur while the data is being recorded or entered into the computer. For example, here are some real-world examples taken from Lechuguilla Cave data:
OLD TIE NEW TIE OLD ERROR NEW ERROR
K6 K6! 174.64 ft. 6.19 ft.
ECKJ11 ECKJ'11 152.78 ft. 1.75 ft.
BNM4 DNM4 127.52 ft. 3.44 ft.
EY52a EY52A 87.47 ft. 5.39 ft.
As you can see, mis-ties often occur because of small difference between station names.
If you see indications of a bad tie-in, carefully check the survey book to see that the shot has been correctly entered. You also may have to do an in-cave check to make sure that station haven't been moved or mislabeled.