I received a private message as follows:
I am ignorant about the use of the following terms and was hoping you might clarify. I found your name when you were discussing Geb's 1/2 marathon in Arizona.
I ran the Kaiser 1/2 Marathon in San Francisco and this has a slope of 2.78 m/km and a separation of 14. What does this mean? I felt that it was a net downhill but the uphill finish mitigated much of the earlier downhill.
I would appreciate a real brief description. Thanks for your expertise!
Drop and separation are terms used to describe the geometry of a road course. Drop is the amount of downhill in the course, expressed in m/km, or parts per thousand. For the above example, the Kaiser Half Marathon has a length of 21.0975 km. The course drops 2.78 meters for each kilometer of course length, for a total drop of 58.6 meters or 192 feet. In other words, the course is downhill, with the finish being at a lower elevation than the start.
Separation is the straight-line distance between the start and the finish, expressed as a percentage of the course length. Again using the Kaiser course, its separation is 14 percent of 21.0975 km, or 2.95 km or 1.84 miles. Start and finish lie separated by 1.84 miles as the crow flies.
In the early 1980’s TAC (now USATF) took over road running records from the National Running Data Center. Rules were needed to establish which courses would be suitable for setting records. Obviously a severely downhill course, or one which was susceptible to wind assistance, would give aid to the runner, thus leading to faster times. After a lengthy period of analysis, a standard was set. To assure fairness of competition, record-quality courses were limited to a maximum downhill (drop) of 1 m/km and a maximum separation of 30 percent.
The limits set include about 90 percent of all certified courses.
It was recognized that the limits of drop and separation are not a perfect standard of fairness, as in the case you cite above, where the uphill finish somewhat offsets the net downhill. Still, it was the best we could come up with. No science of which we are aware has come up with a better standard. It is unambiguous and easy to understand.