Spatial FOG Reference Manual
Page 17 of 144
Version 2.3
03/05/2018
visually in Illustration 1. The arrow indicates the positive rotation direction.
Roll is the angle around the X axis and is zero when the unit is level. Pitch is the angle
around the Y axis and is zero when the unit is level. Heading is the angle around the Z
axis and is zero when the positive X axis is pointing to true north.
5.7.1
Second Right Hand Rule
The two right hand rules are often the best way to memorise the sensor axes and
directions of positive rotation. The first right hand rule gives the positive axis
directions and is described in section 5.6. The second right hand rule shown in
Illustration 3 provides the direction of positive rotation. To use it, point your thumb in
the positive direction of that axis, then the direction that your fingers curl over is the
positive rotation on that axis.
5.7.2
Rotation Order
When multiple axes are rotated, to imagine the final orientation the three rotations
must be performed in the order heading first, then pitch and then roll. To deduce the
final orientation the unit should first be considered level with the X axis pointing north
and the Z axis pointing down. Heading is applied first, then pitch is applied and finally
roll is applied to give the final orientation. This can be hard for some people to grasp
at first and is often best learned experimentally by rotating Spatial FOG with your hand
whilst watching the orientation plot in real time on the computer.
Illustration 3: Second right hand rule
