"Paris Meridian Line ... created by François Arago (1786-1853). To commemorate [him], Jan Dibbets, created an 'invisible monument' Consists of 135 bronze medallions, each 12cm in diameter, inlaid into the ground along a 10k stretch of the Paris Meridian

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public art/experimental sound project in which citizens exchange mixtapes, found sounds and original recordings via publicly hidden streetside stations

list of spatial annotation projects

a good idea, a clumsy method.

geotagged flickr photos on a map.

flickr / google

the most commercial of the city tagging enterprises

On dencity.net, you can give virtual extensions to real places

mashing flickr and gmaps

The Chicago Tapes Project is based on the planting and sharing of cassette tapes throughout the streets of Chicago, leaving found audio, mix tapes, loops, sound projects and other recordings in hidden locations throughout the city. Hiding places are marke

pretty snappy