In ka-bāqī al-washm, ka- introduces a comparison. Washm is a tattoo, while bāqī points to what remains of it. The thing compared is the abandoned campsite; the comparison image is a faded tattoo; the shared quality is an enduring but indistinct mark.
The poet does not merely call the ruins lines in the soil. By linking them to the familiar back of a hand, he makes a remote place feel intimate and turns the survival of a physical mark into an image of enduring memory.