Using the three sigma threshold during a PAAT session

The three sigma threshold is a value that represents the mean plus three times the standard deviation. Any residual value that is greater than the three sigma threshold is very likely to be an outlier. The point should be revisited to determine whether it needs to be reread or made inactive.

Any points with total residual values more than three sigma above the mean are flagged in the PAAT Results dialog box.

步骤：

1. Start ArcMap.
2. If necessary, load data in the map.
3. On the main menu, click Customize > Toolbars > Data Reviewer.
4. On the Data Reviewer toolbar, click Data Reviewer > Positional Accuracy Assessment.

   The Positional Accuracy Assessment toolbar appears.

5. Start a PAAT session.
6. Click the Show Results button on the Positional Accuracy Assessment toolbar.

   The Positional Accuracy Session Results dialog box appears.
7. Check and uncheck the Use check box to see how the results are affected when points with large residuals are withheld from the calculation.