While biking yesterday, along the Rhine frontage of the Urdenbach nature preserve, I came across this small number on the shore of the Rhine:
It was about 100 meters from the eastern shore, about where the gray location pinpoint is:
As I rode the trail north, there was a "2" sign after about 100 meters, then a "3", then a "4", but then I turned off the riverside trail.
These are obviously not the large black-and-white kilometer markers along the Rhine (g). Does anyone know what purpose they serve? Thanks in advance for any help.