Vertical cross-section of the Sierra Madres, near Oaxaca City.
You can think about the landscape as a scalar field in 2-D (puncture the plane if you need to do this to the whole Earth) with the height being a numerical quantity assigned to any point. In that case the image above records the values the scalar field takes along the 1-dimensional γ. (If the scalar field were discontinuous, that would mean you either just biked off a cliff, or biked into a wall.) Or you can think about it in 3-D — looking around from your bicycle as you ascend the mountain ridge.
- statistical cross-sections (cohorts, longitudinal data, versus “panel” measurements now)