Performing a fully automated calculation may not always produce correct results. SPAS 2009 supports the execution of a hand calculation for the determination of Vs based on two selected points. In the following figure there is a case where the software was unable to correctly calculate a Vs value.
Bad signal recording
Signal recordings at the depth of 25 meters for some reason was broken. Reviewing the value calculated by the software is obvious that this value is too low to be correct. In order to change this value we need to perform a hand calculation for signals at depth 25.00 and 26.00 meters. To begin the procedure first enable the helper tool by clicking on the Signal value button 
in the toolbar located above the plot. Now, moving the mouse inside the plot area will display a green circle that snaps on signal values. Move the cursor at the cross over time of the signals at 25.00 meters depth and left click the mouse button holding down the SHIFT key on the keyboard.
Locating first calculation point
The Information panel below the plot will display the properties of the 1st calculation point as shown in the following figure. This way you may review and alter the calculation point as needed.
Figure 32. Properties of the first calculation point
After the definition of the first point, using the mouse we locate the cross over point of the signals at depth 26.00 meters. Then holding down the SHIFT key of the keyboard, right click to define the second calculation point. The properties of that point will appear on the Information panel.
Locating second calculation point
Properties of the second calculation point
Click on the Calculate button and if the result is acceptable click on the Save value... button to save the result in the current calculation. Saving the hand calculation value will result to an automated update of all plots.
Saved hand calculation result