witness-post
n.— «It is extremely important that the points S and L of each spiral should be secured by permanent monuments in the centre of the track, and by witness-posts at the side of the road. The posts should be painted and lettered so that they may serve as guides to the trackmen in their subsequent efforts to grade and “line up” the track.» —by William H. Searles The Railroad Spiral (New York) , 1882. (source: Double-Tongued Dictionary)
Further reading
Astronauts returning from space say they experience what’s called the overview effect, a new understanding of the fragility of our planet and our need to reflect on what humans all share as a species. A book about the end of the universe...