When is a treasure hunt not a treasure hunt? When it's also a group art project and health campaign using the latest in mapping technology. Peter Jackson explains.
Eye spy: Objects are mapped via GPS to create a Google image
There's nothing like trying to solve a good old-fashioned puzzle - but the latest poser from Londoner Alfie Dennen is nothing like an old-fashioned puzzle.
As the co-founder of camera phone picture site Moblog, Dennen is no stranger to pulling people together with a common artistic cause.
But it was not until he started creating images on Google maps using the co-ordinates of real-life objects, that a new concept was born.
Dennen's prototype was the picture of an eye created by mapping the grid references of 41 every-day objects from his house using global positioning system (GPS) tracking technology on a handset.
The central marker of the eye's pupil was Dennen himself standing on a football pitch near his house.
"The eye was my proof of concept... but I realised the idea would work better at scale. The bigger the area, the better the image," he said.
Hunters can enjoy the hidden art of noticing things, says Alfie Dennen
"I want to push the idea out nationally and internationally. It's such a fun thing to do."
He then struck upon the idea of turning art into a game by hiding objects around London at set locations for people to find using clues hidden within blogs.
When the 43 objects are found, the treasure hunters photograph the whole thing and post their images on Moblog while plotting the co-ordinates on a map to create a communal artwork.
Each object also contains a letter which when put together spell out a question. The person who answers the question first - a 43-character phrase - wins.
Traditional GPS drawing involves people with hand-held GPS devices tracing out different shapes in straight lines simply by walking around.
Dennen said of his project: "This is a new way of drawing. GPS drawing has always been there but doesn't lend itself to a group dynamism.
"A consumer with no knowledge of the net can go and draw pictures on maps to whatever scale they want, with friends.
"What we're doing is using map markers to draw a macro image on a Google map rather than one single line drawn using a TomTom or other navigator. "
He said he has a romantic idea of someone spelling out "I Love You" across a map, or even proposing marriage.
But there was another, far more serious reason behind the reality game - the disease tuberculosis (TB).
Central to Dennen's original idea is American photojournalist James Nachtwey, whose campaign site XDRTB.org raises awareness of "extremely drug-resistant tuberculosis".
Dennen has supported the campaign by using Nachtwey's photographs as his hidden objects and related the cause to his final image on the map.
"James's position is the power of the image which communicates the message, it can do that by itself almost. This underlies the entire concept," Dennen said.
TB kills around 1.7m people worldwide a year and more HIV sufferers than any other disease.
James Nachtwey's stark images of TB aim to raise awareness
One of the game's bloggers, Bill Thompson, said we are only just starting to realise the importance of the online network on philanthropic and charitable work.
"What Alfie's done is a really interesting way of exploiting lots of small activities on behalf of people to create something that's quite big... I think it has a great deal of potential," he said.
The project may have a higher cause, but Dennen said it is above all fun with a "low barrier" to entry and "big value" return.
"I want people to feel enjoyment and to really feel alive," he said.
"It's something that gets me out of the house, on a road I've never been on taking a picture of something I've never seen.
"People can engage in the hidden art of noticing things."
So far, just eight of the 43 photographs have been found with the puzzle expected to be solved by October 30.
Prizes include a Nokia phone, tickets to James Nachtwey's photographic launch party in London on October 30 and others from Moo.com.
Updates and new clues can be found at twitter.com. The maps were created by HomeMade Digital.