Spectra Precision FAST Survey Reference Manual - Manual (Page 127)

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Although standard backsight procedure may be to take one or more readings to a single backsight, the Advanced
Occupation command allows multiple readings to be taken for the backsight, to points identified in advance, leading to a
'best fit" backsight orientation computed by least squares methods. Consider the graphic below, showing a proposed
occupied point (tripod) and three known point IDs as originally surveyed and precisely located based on a due North
backsight.
Although it is sufficient to backsight to point 4, from known occupied point 1, taking additional measurements to point 2
and point 3 can lead to an "averaged" backsight and best fit orientation. Furthermore, if point 1 is not known, the
position can be determined by the least squares calculations within Advanced Occupation, similar to a resection. Even
if point 1 is known, if the calculated position is saved and not overwritten, then a redundant position for point 1 can be
stored to the raw file, leading to more precise calculation of point 1 by Network Least Squares methods using office
software.
In the above example, points 2 and points 3 can be considered to be towers and sighted for angle only measurements,
while the measurement to point 4 would include both angle and distance. Furthermore, the data for these points will be
recalled from a control file, so that they can be re-used on multiple jobs. And in this example, the angles will be
recorded in gons/grads, or the "400 circle", with distances in metric. In the new file (Ville2.crd), a control file which we
will call RadioTower.crd is referenced within Job Settings, Options tab. Note that is it not necessary to extract the
backsight points from a control file. However, in developed areas with known control points, it is a useful technique to
store control points to a distinct file so they can be easily recalled in all new jobs. The elevations on the radio tower
points represent only the vertical positions originally measured, and can be ignored when used in Advanced Occupation