A blunder is an observation that is in error by more than the typical systematic or random error normally encountered. It’s a BIG error that happened because two numbers were transposed or the wrong back sight point was sighted or there was a glitch in the data transfer or something. Blunders are generally ‘one time’ errors that don’t appear elsewhere in the survey.
TPC can report observation blunders it identifies during a Least Squares solution. Once identified, you can exclude the offending observations or coordinates and compute a new solution or you can take whatever steps are necessary to correct the observation or coordinate then compute a new solution.
A 99.7% probably error is often used for detecting blunders. This corresponds to a residual that is 2.965 * standard deviation. You can modify this factor in the Miscellaneous dialog.
The Least Squares solution computes a residual for each observation. The residual is the difference between the observation and the equivalent inverse between the final adjusted coordinates. If an observed horizontal distance of 100 ft had a residual or 0.03 feet, we would say that the solution found 0.03 feet of error in that observation. So the residual value really just reports the probable error for the observation. An observation with a large residual indicates that the observation has a large probable error. If the probable error is large enough, the observation might be a blunder.
Robustness refers to a technique used to filter bad observations. It involves solving once with the estimated standard error values for each observation then averaging the estimated standard error of each observation with its computed residuals and solving again. The result of this technique is that smaller initial errors become smaller and larger initial errors become larger. The errors are thereby magnified, making them more evident.
In the example shown here from the Least Squares report, any residual that is 5X the standard error is displayed as a potential blunder.
[ Horiz Angle Blunders ]
At From To Residual Correct
CP215 CP214 CP107 0°00'24.89" 76°58'54.89"
CP216 CP215 CP107 -0°00'21.70" 29°58'38.30"
2 Horiz Angle Blunders Detected (blunder > 5 X StdErrorYou can turn on robustness in the Solve dialog page. When you choose Solve, TPC will run the solution twice to magnify the errors. You can specify a blunder factor in the Miscellaneous dialog page.
Least Squares
Least Squares Overview
Least Squares Positional Tolerance
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