In order to check we are getting the correct result this page starts with a euler angle, converts it to a quaternion, then converts it back to to euler angle. When we have done the complete rond trip we end up with the same angles which gives confidence that the method is correct.
So lets start with the following euler angle:
heading = 10 degrees
attitude = 20 degrees
bank = 30 degrees
We then convert to quaternion using the first method on this page. Note that most maths libraries use radians so you will probably need to convert degrees to radians by multiplying by PI/180 = 0.01745.
c1
= cos(5 deg) = 0.9962
c2 = cos(10 deg) = 0.9848
c3
= cos(15 deg) = 0.9659
s1 = sin(5 deg) = 0.0872
s2
= sin(10 deg) = 0.1736
s3 = sin(15 deg) = 0.2588
So using the formular gives the following result:
qw
= 0.9437
qx = 0.2685
qy = 0.1277
qz
= 0.1448
We now convert back to euler angles using the method on this page.
First we work out the heading part: this involves using the atan2 function. Implemetations of this function may vary, so be careful about the order of the operands, this page describes the issues.
tan(heading y part): usually the first operand= 0.1633
tan(heading x part): usually the second operand= 0.9254
I
used atan(y/x) instead of atan2(y,x) as I was looking up the
values in 4 figure tables instead of using a computer program.
However, because the angles are all in the first quadrant, I
believe these are equivalent since in fist quadrant:
tan(a)
= sin(a)/cos(a) = opposite/adjacent = y/x
and
atan2(y,x)
= atan2(opposite,adjacent)
tan(heading) = 0.1764
heading = 10 degrees
Calculating the attitude:
sin(attitude) = 0.3419
attitude = 20 degrees
Calculating the bank:
tan(bank y part) = 0.4698
tan(bank x part) = 0.8138
tan(bank) = 0.5773
bank = 30 degrees
An OpenOffice.org with these calulations is here
