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?