angle from curve to center

[ketanco]

Given: When moving up
on a horizontal curve to
the right (i.e. set up
anywhere on point on
curve POC).

Find: The clockwise angle to turn off to hit the radius point (i think radius is the center of circle) is:

and the answer is

© Deflection angle measured from PC to that point plus 90 degrees

i didnt understand why

please help

 

[ptatohed]

This is hard without drawing it. But, to help make the case, let's assume the instrument is set up all the way on the EC (PT). Now, the deflection angle instead of being alpha / 2, it is now I / 2, right? So, if the instrument is set up on the EC facing to the right, parallel with the Long Chord, with the back of the instrument to the BC (PC), what angle would the instrument need to be turned (CW) to site on the center of curve? 90^ + (1/2) I = 90^ + Deflection Angle from BC

It's hard to put in words, sorry.