Radius measurements

[jonclox]

Todays silly question........
We know that manufactures supply curved track in set radiuses (radii?) but here is the actual measurement taken from?
Is it the inside curve?
the centre line of the curve?
or
the outer edge?  

Now if I had enough track sections of any radius laying around I could obviously make up a complete circle and measure the results but I havn`t.
Reason............. to make up a ply trackbed 1/2" wider (both sides) to incorperate in a section of a proposed layout  

[MJKERR]

9" is the radius to the centre line of the track

[jonclox]

9" is the radius to the centre line of the track

Many thanks. That gives me a chance to build myself a sorta data base to work from

[MJKERR]

As several modern image items from Graham Farish (BachFar) and Dapol are not recommended for 9" radius I measure to the inside edge of the sleeper
This results in a slightly larger radius (but not much of a difference)

I also have measurements for the clearances on parallel tracks on straights and curves
Reminder to self : add this to website!