# Trouble regarding 'Angle of Rotation', Tutorial 2.4

New to GPlates and working my way through tutorial 2.4
Exercise 1 - Creating the rotation file
Although the initial steps are self-explanatory, post step 8 there is a note regarding the addition of 360 degrees to some of the rotation angles to eliminate the problem for interpolation of poles between the prescribed time in the rotation file. (Page 11, Bottom para)

Attached here is a screenshot taken from the tutorial module itself. To rightly point out, I have just highlighted the rows of interest in yellow. As per the tutorial, the addition of the 360 degrees is to be only subjected when the finite rotation poles are greater than 180 degrees (Further explanation of this statement will be appreciated).
None of the highlighted rows has a difference of greater than 180 degrees compared to the previous one, however, we find that the rotation angles are changed in the final Excel sheet.
Why or when such changes are required to be done is unclear.

Please let me know if any further explanations are required from my end.
Thanking all for their time and consideration.

Hi Sammy,

First of all, I don’t think this needs to be done anymore since GPlates will make this adjustment automatically when loading a rotation (`.rot` ) file. You can tell if the adjustment was made because the rotation file will then have unsaved changes (and you will be asked to save those changes when closing GPlates).

What GPlates does is make sure the stage rotation of the current total rotation (a line in the rotation file) relative to the previous total rotation (previous line in rotation file) takes the short path. It will adjust the current total rotation if necessary to ensure this. It then proceeds to the next line in the rotation file (next total rotation) and repeats this process, etc.

So that’s when GPlates reads a rotation file. Additionally, when GPlates reconstructs back in time it interpolates to a time that is between two total rotations (adjacent lines in rotation file). And it ensures that the interpolation takes the short path between them. There’s mention of the short vs long path in the wikipedia on quaternion slerp. There’s also mention of this in equivalent rotations in the pyGPlates docs. It can get a little confusing to say the least.

Another thing, regarding the mention of 180 degrees in the tutorial, that’s referencing the total rotation (ie, line in rotation file) whereas the now-automatic correction happens in the stage rotation (which you can’t see just by looking at the rotation file, ie, it needs to be calculated first). Although the same short vs long path concept also applies to total rotations (not just stage rotations), it’s just that the total rotation is from present day (0Ma) to the total rotation’s reconstruction time. That total rotation can still take a short or long path, but we don’t normally care because a rotated geometry ends up in the same position in either case. But the interpolated rotation is another matter because it could be interpolating between one total rotation taking a short path and another total rotation taking a long path (which is why the stage rotation needs to take the short path). So when the tutorial mentions keeping the total rotations on short paths, doing that is likely to ensure the stage rotations between them also take the short path. In any case, GPlates now takes care of that automatically, so it’s not an issue. The tutorial was likely written before that functionality was added to GPlates.