PA/APV Glidepath Descent and Obstacle Clearance Slope's lowest point determination
I can't yet find where this is indicated in TERPS, but it appears that the OCS surface starts at the point before the threshold where the GP is 250 feet above the threshold height.

I'm inferring that from this document about calculating VNAV criteria.



It has the above diagram and says: 


  The distance from the runway threshold where the OCS surface starts
  (Section 1) is defined by the formula D = (250-HATh)/.052407779


where HATh is the same as TCH, which is a standard 50', and the number .05240779 is the tangent of 3°. So far I'm unable to reference that back to any TERPS definition, but maybe you can find it.

There's an interesting article discussing how TCH is defined over at Code7700.com