Plaques on the ground in Budapest and marking the line of fortresses, camps and stations of the Roman army along the Danube (Pannonian Limes)
The current plaque shows Carnuntum, the Roman name of the multi-period legion camp in Bad Deutsch-Altenburg, Austria
Notice the two lines representing the Danube River in the top-left corner of the plate.
In the next photo taken on the same day, part of the line of fortresses was seen, with the route of the Danube River between the various plates Click for a larger image
More details can be found here Click for the item in The ultimate street signs site