In an unrelated answer rbp linked to this Atlanta airport info page, and runway 8R/26L caught my eye. If I understand correctly, these two runways are the same piece of pavement, but refer to the use of the runway in opposite directions. According to that page, 8R has a gradient of "1.0% DOWN," but 26L has a gradient of "0.4% DOWN." It seems like a physical impossibility - if one is sloped down, the other must sloped up, right? So what am I missing?
The slope of a runway isn't always constant. In some cases, for example, if you stand at one end of the runway, you can't see the opposite end, because the runway goes up then down.
In this case, the ICAO (in the airport design manual) computes the runway slope as follows: the runway is divided into four equal segments. The average slope of each segment is computed and the total slope follows this formula:
[(slope of 1st quarter) + 1.33*(slope of 2nd quarter) + 2.33*(slope of 3rd quarter) + 3.33*(slope of 4th quarter)] / 8
The runway slope has a greater influence when the aircraft has a greater speed. Therefore, the slope at the end of the runway is given a greater weight.
In the case that you mention, if you walk from the west side of the runway to the east side of the runway, you will go up a little, then down, and end up at a lower altitude than when you started.