When turn-by-turn directions specify traveling 1 mile down a very steep grade, do they mean 1 mile along the slope, or 1 mile along a flattened map? Only the f

When turn-by-turn directions specify traveling 1 mile down a very steep grade, do they mean 1 mile along the slope, or 1 mile along a flattened map? Only the former would match the car's odometer, of course.