Locating Individual Blunders

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After you have located loops that have large deviations, the next step is to analyze them looking for the specific measurement that has caused the blunder. The program does this by examining each and every measurement, looking to see if it matches the error signature. The process involves making a test adjustment of each measurement, trying to fix the error. It uses some complicated geometry to find the best fix for each azimuth, length and inclination. Since the program only adjusts one measurement at a time, there is a limit to how much improvement each adjustment can make. Some adjustments will have very little effect on the error. However, adjustments on shots that closely match the error signature will dramatically improve the loop error. The program saves the most successful adjustments. Since these are the adjustments that do the best job of fixing the error, they are most likely candidates for blunders.


You can also make some inferences about the kind of blunder that you have by looking at the size of the error:


Small Errors: 2 - 5 Standard Deviations. Errors between two to five standard deviations generally are seen when there is a misreading of the instruments. For example, you could get this type of reading if you misinterpreted an azimuth of 320 as 330.


Medium Errors: 5 - 15 Standard Deviations. Errors between five to twenty are seen with shot reversal. For example, if you read the wrong end of the compass needle or do a backsight without reversing the stations. This would give a reading that is incorrect by 180 degrees. The length of the shot governs the size of the errors. Long shots produce bigger errors.


Large Errors: 15 - 100 Standard Deviations. Errors greater than fifteen standard deviations are seen in situations where a survey has been tied into the incorrect station. For example, if a survey is supposed to connect to station B12 and, instead, is connected to B13, a large error will probably result. The size of the error will depend on how far the erroneous station is from the correct station.


These numbers are very general rules of thumb. Actual errors can vary depending on the exact circumstances. For example, if you have a shot reversal on a very short shot, the error could easily be less than one standard deviation.