For evaluating of reductions of horizontal and zenith angles,
gama-local computes a helper point P_1 in the center of
the network. Horizontal and zenith angles observed at point P_2
are transformed to the projection plane perpendicular to the normal
z_1 of the helper point P_1. Coordinates (x_2,
y_2) of point P_2 are conserved, but its normal z_2 is
rotated by the central angle 2\gamma_12 to be parallel with
Formulas for reductions of horizontal and zenith angles are given only in the printed version.