The borderline is the white stone circular line where the granite pillars and chains are located.  The wood barricades where I am standing on this photo (wearing a cap, standing towards the camera) are for traffic purposes and to funnel people to entry gates, and is situated some ten metres from the border, inside the Vatican state.
The actual border follows the outside of the columns and in a more circular fashion (see below).

The border in the zone of Piazza San Pietro is a white stone strip - not the one we see on the photo above, because we(the blue point on the map).