19.1.5 Definition of Axes

For clarity the following axes sets are used in the error model:

Body Reference Frame (tool axes);
The z-axis is coincident with the along hole axis of the survey tool and the x and y-axes are perpendicular to z and to each other. This is axes set used to describe orientations of the various sensors.

Earth Centred Reference Frame (nev);
The x-axis is in the horizontal plane and points toward true north, the y-axis is also in the horizontal plane and points towards true east. The z-axis points downwards.

Borehole Reference Frame (hla);
The z-axis is aligned along the borehole axis. The x-axis is perpendicular to z and points toward the high side. The y-axis perpendicular to both of these and hence is laterally aligned across the borehole.