Leica GPS1200 Series Technical Reference Manual page 1090

Reference Plane
Tilted plane Any number of points define the plane. The axis of the tilted reference plane are:
X axis: Horizontal and parallel to the plane
Z axis: Defined by steepest direction of the plane
Y axis: Perpendicular to the plane; increases in the direction as defined
Offsets are applied in the direction of the Y axis.
a
Z
P1
Y
GPS12_147
a Height
b Easting
N Northing
P1 Origin of plane
X X axis of plane
Y Y axis of plane
Z Z axis of plane
With four or more points a least squares adjustment is calculated resulting in a best fit plane.
GPS1200
a
X
X
b
GPS12_148
a
b
N
P1 Origin of plane
X
Y
Z
Z
Y
P1
Height
Easting
Northing
X axis of plane
Y axis of plane
Z axis of plane
1090
b