To Update the A Priori Standard Deviation

After the adjustment calculation the Chi-square test indicates a possible mismatch between the a priori standard deviation, and the a posteriori mean error. To get a suitable calculation model, you can adjust the standard deviation for the current calculation project.

The Update A Priori Standard Deviation dialog box displays normalized values for the a priori standard deviation, and compares the a priori with the a posteriori values.

To adjust the a priori standard deviation

1. In the Calculation Wizard, step 7, Calculate Adjustment, under Chi Square Test, click the Update A Priori Standard Deviation link.
2. In the Update A Priori Standard Deviation dialog box, specify new a priori values for each measurement group, as shown in the following table.

   Note that the dialog box displays all measurement groups of the current calculation project. There can be multiple measurement groups of the same type.

3. Click Update. Then, the Calculation Wizard steps back to the beginning of step 7, Calculate Adjustment.
   Note:

   This update only modifies the values that are stored in the calculation project, under Measurement. It does not affect the instrument settings.

4. Click Calculate to restart the process.

Update A Priori Standard Deviation	Description
Measurement group	A measurement group summarizes observations that have been observed under the same external conditions, such as instrument. For each measurement group, you can adjust the values.
New Value	Specifies the new value. For example, enter the A-Posteriori value as new value, or enter a value that is close to the A-Posteriori value.
Tachymeter <instrument name>	Compares the values for Tachymeter measurement groups.
Distance	Values for A-Priori and A-Posteriori contain the impact of the centring error, and are based on a distance of 1000 meter (1km).
Distance Param. A Distance Param. B	The distance parameters do not contain the impact of the centring error, however you can enter a new value for the centring error separately.
Horizontal Angle	Values for A-Priori and A-Posteriori contain the impact of the centring error, and are based on a distance of 1000 meter (1km). The new value does not contain the impact of the centring error, however you can enter a new value for the centring error separately.
Vertical Angle Height Difference	A-Priori, A-Posteriori, and New Value do not contain the centring error.
Centring Error	The Centring Error impacts distance measurements, and horizontal angles.
Transformation/GPS <instrument name>	Compares the values for Transformation / GPS measurement groups.
Coordinate	Compares the values for moveable points.

BestPractice:

Use different instruments, if you have measurements with different levels of accuracy.