Roofs are horizontal or slanted walls. In 1925, it was reported in the Hartford Courant an article that caught my eye,

PAINT SIGNS OF ROOFS TO GUIDE MAIL PLANES.

Washington, Dec. 20.—(Associated Press.}—Appreciation of the successful efforts of the St. Joseph, Mo, chapter of the National Aeronautical Association in getting towns along airways in the central part of the country to paint identifying names on roofs as sign posts for air pilots was expressed in a statement issued tonight by the Army Air Service. (The Hartford Courant Vol LXXXVIII Dec 21, 1925. Hartford Conn.)

There are road signs and air signs and maybe even planet signs as has been suggested by some for the Nazca Lines in Peru.

In 1973, I received permission to  photograph the monuments in the old cemetery at Kerameikos in Athens, Greece, I was particularly interested in capturing the light on the tombs and stelae in the evening light. The Kerameikos is the name of the deme or section of Athens northwest of the Acropolis. The cemetery at Kerameikos is enclosed with a high wall and has a high solid metal gate at the entrance. On attempting to leave the cemetery after photographing, I found the gates of the cemetery  locked. Not wanting to spend the night in the cemetery and after an hour or so of searching, wooden crates were found at the back wall and when piled one on the other, permitted scaling the wall. On descending from the wall I found myself in a church yard and was inspired to enter the church. My intuition said go in. In the church was only a priest, a mother and an infant child. A baptism ceremony appeared to be in progress. I sat in a pew and remained silent. I realized that I had gone from the oldest of Greece to the newest of Greece, simply by scaling a wall. It was an exhilarating experience. Here, time was separated by a wall. I have learned to trust my intuition. The art piece below is based on a response to this event.

   

JD Sage, Personal Journey through an Ancient Burial Ground, #20-90, 1988