The origin, I assume, is the center of the curve (bad choice of words – confusing)
The back tangent has a bearing N70E, which means the azimuth is also 70deg
The (inward) radius from PC to center is 90 degrees clockwise from the back tangent, therefore has an azimuth of 70+90 = 160.
Therefore, the outward radius R1 (from center to PC) has an azimuth of 160+180 = 340
Since the outward radius R2 from center to PI is I/2 clockwise from R1, it has an azimuth of 340+17.5 = 357.5
Therefore, the (inward radius) from PI to center has an azimuth of 357.5-180 = 177.5
This, expressed as a bearing is S2.5E
If you have the All In One Second Edition, this is explained on page 725
