PA/APV Glidepath Descent and Obstacle Clearance Slope's lowest point determination I am studying the US Terminal Instrument Procedures manual (TERPS) and as I was skimming through the guidelines about Sloping Obstacle Clearance Surface, I came across the image below from section 2-1-4 (the red circle remark is mine):



The problem is, I can not understand how the lowest point of the OCS, which I have marked with the red circle, is determined. It seems to be a point close to Departure End of Runway (DER), but where exactly?

The only things TERPS states is:


  Descending on a PA/APV glidepath. The obstacle evaluation method for descent on a glidepath is the application of a descending OCS below the glidepath. The vertical distance between the glidepath and the OCS is the ROC; thus ROC = (glidepath height) - (OCS height). The ROC decreases with distance from the precise final approach fix (PFAF) as the OCS and glidepath are converging towards the landing surface (see figure 2-1-3). The OCS slope and glidepath angle (GPA) values are interdependent:  OCS Slope = 102/GPA; or GPA = 102/OCS slope.


Could someone point me in the right direction?

EDIT: What I mean is, why (and how do I calculate the extra distance) does the first image apply, rather than the second?
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