Height Measurements for coasters

How are height measurement for coasters made, especially to the inch. I'm not talking about how they actually come up with the number, but what the number is supposed to represent. For instance, if I went out and cut the grass under a coaster, would that increase it's height by an inch or two? Or if I went out under the highest point under MF, and dug a 15' hole, would it then be taller than SD2K?

Is the height supposed to represent the highest point on the track, to the ground, or to the top of the footers or...?