I am having trouble understanding the concept of Departure End of Runway. Most sources state that DER is at the end of the portion of the runway that can be used for takeoff (which, as far as I can tell, excludes the threshold at the end of the runway). But from the official FAA's Aeronautical Information Manual one can clearly see that DER starts at the "utmost" end of the runway (including threshold). Many other sources seem to support this, like this picture. Could someone point me to the right direction?
The picture you linked is from an FAA publication. As pictured the threshold is the end of the runway pavement, it is a thin line that separates usable runway from either taxiway, blast pad, grass, or an emergency overrun area. It is marked at night with a single row of green or red lights (depending on your direction). It can be displaced or reloacated from the original pavment but in this case it will have noticeable markings that include a well defined line marking the exact end. Some ends are usable for takeoff but not landing or vice versa but that is down to individual runways.
The paint bars near the threshold of an instrument runway are simply for visual reference to help indicate the width and length of landing zone during approach, they have nothing to do with takeoff or departure. VFR runways only have a number and dashed center line, no white bars at all.
To summarize: the threshold is a line not an area, it separates usable runway from non-runway, it may be marked with paint or lights but it can also simply be a change in surface such as tall grass to short grass. Standard departure obstacle clearance climb gradients are calculated with the plane at 35feet above the ground, directly over the threshold at the departure end of the runway.
Just guessing out loud here, but if you go to airnav.com and look up any airfield, two points are given for opposite ends of the runway. For MOR or Morristown, TN,
- Runway 5 = Latitude: 36-10.456562N, Longitude: 083-22.968408W, Elevation: 1312.9 ft. and the other end,
- Runway 23 = Latitude: 36-11.069353N, Longitude: 083-22.085700W, Elevation: 1274.4 ft.
As an Engineering Consultant, everything that is done on this runway is done considering these two points - measurements for placing markings, setting up the RSA and all Part 77 surfaces and safety lines, etc. Approach surfaces, departure surfaces, everything is established off these two points, including length of runway, TODA, TORA ASDA, LDA, etc. are all configured off these FAA established runway endpoints, whether there is any extra pavement beyond or not. In my estimation, the DER is going to be the opposite end runway coordinate of the end you are rolling out on.