@Xibanya
Next one shows that different mountain ranges can have the same level set (marker line at specific altitudes) because the marker line itself does not care about what's on either side of the fence, as long as it never crosses the altitude in any other place than where the marker line is.
And then one gets to the βidealβ mountain range, where the slope is unit, so the amount you walk away from the fence matches how much you go above or below it.
2/3