Spectra Precision FAST Survey Reference Manual - Manual (Page 184)

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A minimum of 3 angle-only measurements can also lead to an accurate calculation of the resection point, as shown
below:
Select the Angle Only toggle before taking an angle only measurement with no distance. If the angle only measurements
are accurate in the zenith angle to a specific location, an accurate elevation can also be calculated from as few as 3 angle
only measurements with no distances. Within Angle Only mode, you do not need to change to reflectorless mode, since
by definition no distance will be measured. You can stay in prism mode. If measurements are taken to steeples and
radio towers and only x,y position is known or the precise elevation point to sight is not known, then only the x,y
position of the occupied point will be determined accurately, and high Z RMS values will result.
Set Collection
The Set Collection routine allows you to collect and average sets of angles to multiple foresights.
Setup
The opening dialog is the standard setup dialog shown below that allows the user to specify the instrument and backsight
information.