Others exist only as faint signals in the soil, invisible to anyone standing at ground level and legible solely from the air, in the right conditions, at the right time of year. The enclosure at Rogerstown, in north County Dublin, belongs to this second category. What is known of it comes from a single aerial photograph, which captured a cropmark tracing the outline of a curvilinear enclosure, one that appears incomplete, its arc breaking off before it closes.

Cropmarks form when buried features, such as the filled-in ditches of an ancient enclosure, affect the vegetation growing above them. In dry summers, crops over a buried ditch may stay greener longer, drawing on residual moisture, while those over a buried wall or compacted surface may scorch and yellow earlier. From above, these differences in colour and growth resolve into shapes that correspond to long-vanished structures. The Rogerstown site was recorded in aerial photograph GB90. BY.05 and documented by archaeologist Geraldine Stout, with the record uploaded in August 2011. Curvilinear enclosures of this kind are generally associated with early medieval settlement in Ireland, the circular or oval ditched boundaries that once surrounded a farmstead or dwelling, known commonly as a ringfort or rath, though without excavation it is impossible to assign a precise date or function to any individual example.

Rogerstown lies in the coastal lowlands of north County Dublin, close to Rogerstown Estuary, a broad tidal inlet that is itself of considerable ecological interest. There is nothing to see at the enclosure site from the ground; the cropmark that revealed its existence is a feature of dry summers and low-growing arable crops, and even then it is only meaningful from altitude. For those curious enough to investigate, the aerial photograph reference GB90. BY.05 is the primary record, held within the national archaeological survey documentation. The site is a reminder that the landscape of even relatively well-studied counties like Dublin still holds features that have left no surface trace whatsoever, known only because someone happened to fly over at the right moment.