Spectra Precision Survey Pro v5.4 - Manual (Page 69)

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Solving Calibration Introduction to Calibration
A GNSS calibration is a 2D similarity transformation. The GNSS LLH
coordinates are transformed using a map projection into XY mapping plane
coordinates. The XY mapping plane coordinates are then translated, scaled
and rotated into your local grid using the calibration.
Your choice of projection mode will determine the mapping plane used for this
procedure:
1. If you started your job with no coordinate system, then there is no
projection and no datum available to relate the LLH coordinates to the local
grid coordinates. In this case, Survey Pro will initialize a default map
projection when you collect your first GNSS control point. This default
projection will be created to give ground distances at the height of the first
control point.
Because the orientation of your local grid relative to geodetic North is
unknown, you must collect at least two GNSS control points to solve the
scale, rotation, and translation between the default map projection and
your local grid coordinates.
2. If you started your job with a map projection and datum, then the measured
LLH coordinates can be transformed into local coordinates using this
selected map projection. In this case, because the scale and orientation is
defined by the map projection, you need only to solve for the translation
parameter.
If you set your base on a known point, then no calibration will be required.
If you set your base on a new autonomous point, then you must collect at
least one GNSS control point to solve the translation parameter.
A calibration with multiple points can still be performed to give extra
redundancy to the calibration solution.
A calibration is solved by collecting GNSS control points. A GNSS control point
is a point with an accurate local grid coordinate that you occupy with the GNSS
rover. The LLH from the GNSS measurement is combined with the accurate
grid point to form a calibration point pair. The calibration is then solved from
these control points to give you the best fit between your GNSS measurements
and the local ENE grid coordinate system.