Shabbat 7 - Height Limits, Public and Private

In a private space, for an object thrown from a public space on Shabbat - landing has no height limitation:

"R. Hisda said: If one fixes a rod in private ground and throws [an article from the street] and it alights on the top, even if it is a hundred cubits high, he is liable, because private ground extends up to heaven’."

But in public space there is:

"Rabbah b. Shila said in R. Hisda's name: If a brick is standing upright in the street, and one throws [an article]16 and it adheres to its side, he is liable; on top, he is not liable. Abaye and Raba both state: Providing that it is three handbreadths high, so that the public do not step on it"

This is because 'public space' is defined as a place where many people go. A brick 3 hands tall will not be stepped on, people will step around it. It's sides, however, serve only as a temporary resting place - eventually, something landing there will fall to the street and therefore is considered a public place.