This weekend I am in Venice in between an Networked Media and Search Systems Concertation meeting and ICMR 2011. I am in the processing of preparing the slides for a talk  that I will give on Tuesday morning on tagging and geotagging. Once again, I find myself busy with the question of what does it mean for a geotag to be relevant to a photo.
At first blush, the answer to this question is obvious: A geotag is relevant to a photo if it gives the exact longitude and latitude of the position of the camera when the photo was taken. I have this information for the photo above, which was taken earlier today -- but it reflects so little of the larger story. On the one hand our cameras (in this case we're using a new Android phone) pin us down and on the other they tell not much at all.
I originally asked this question about the relevance of geotags after the talk of Pavel Serdyukov at SIGIR 2009, "Placing flickr photos on a map." I garbled the formulation, Pavel didn't understand it, and I have since come to understand the large number of dimensions involved in the answer. "Where does a photo truly belong on a map?" isn't an obvious question and doesn't have an obvious answer, either.
It's generally accepted that knowing the position of the photographer underspecifies the location of the photo since it is not known in which direction the camera was pointing or on what it was focused. It could possibly be the case that the subject of the picture is quite far away from the photographer, making the geo-coordinates of the photographer not the most relevant position to the photo in terms of the depicted content.
The photo from today is a good example. It was taken in Venice, but it is also relevant to Trento, since I am here in Italy on a trip to Trento and only happened to come to Venice for the weekend. I will remember the trip as a "Trento" trip. But the Trento association goes beyond my own personal sense of the proper place of the photo -- anyone who was on a business trip to Trento might find Venice related since they are possibly flying into a Venice airport or like myself also planning a touristic stay in Venice.
The sign in the background says "Per Rialto" and is pointing the way towards the Rialto bridge. We're on our way to the bridge and have taken the photo along the way -- it is relevant to the geocoordinates of the Rialto bridge in the sense that this was the final destination of the walk.
The photo was taken for my family back home. My great uncle is sick and in the hospital at this moment. He is a central node in my extended family and ties us together across space and also across time -- and for years has passed along to us the lore of the generations that preceded us -- a mixture of stories, poetry and family values -- a nourishing flow that sustains us and binds us together. One of the things that he does is write email newsletters to the family members and this newsletter has a section that is called "From the Rialto", which describes the current activities of various family members. Here I am sending my own thoughts out from the Rialto to my family back home and my great uncle in the hospital. The picture has more to do with them and where they are than here.
Further, I don't really remember why the section of the newsletter is called "From the Rialto". It might not be the Rialto in Venice (which is actually an entire district and not just a bridge). It might be an entirely different Rialto. My picture is actually related to the original Rialto used by my uncle, except that I don't know where that is. I will admit that there are probable not a lot of photos out there that enjoy this sort of relationship to place -- but the main point is the existence of possibly relevant places that the person who takes the photo is not necessarily aware.
Actually, I just wanted a picture with a Rialto sign in the background. These signs are numerous in Venice...in fact they sell T-shirts on which they are depicted. In a certain way, the photo is a "Woman in from of Rialto sign" picture and is also relevant to anywhere else a Rialto sign is located. In a way, I am standing at place that has many different geo-coordinates.
The renown of the Rialto bridge is attested by the numerous replicas that exist, for example, in Las Vegas. The original and its imitators are related -- photos of one are of interest to people visiting the location of the other. It would be useful for a picture of the Las Vegas Rialto bridge to bear a tag that relates it to the one here in Venice.
I briefly entertained the idea that it is generally accepted that the relevant geocoordinates for a photo are the ones at which the photographer is standing, since those geocoordinates would be considered relevant to the widest range of people, i.e., be the most objective. Indeed, this is the relevance criteria for geotags that we apply within the MediaEval Placing Task. My association of the photo with the folks back home is relevant to a handful of souls, whereas anyone could recognize the connection with this corner of Venice. However, replicas are part of a common shared culture across the world and the connection between an image of a replica and the geo-coordinates of the original could be considered of possible interest to anyone and not just a personal circle.
Finally, there is the question of the relevant geographical resolution. For me, this is a "Venice" picture. I don't need to know the exact corner again -- nor will I try to find it. We overheard a group of Americans talking about their vacation -- and noticed that they referred to themselves not as being in Venice or in Italy, but in Europe. Europeans hardly ever reflect upon the fact that they are in Europe...they take it for granted.
Wandering through Venice its tempting to just fall into people watching -- making up small stories about couples out enjoying the good weather and the beautiful city: how they met, how they fell in love and what the future has in store for their mutual happiness. The relevance of the geo-coordinates of the photographer is obvious for those who want to take people watching to the next level -- if the couple standing next to us a bridge near San Marco uploads their pictures to Flickr, we will know even then more about them...we could check if their trip to Venice provided the solid basis for a stable relationship, or whether they ended up parting ways. Taking the people watching to the extreme: It would finally be possible to check whether kissing under the Bridge of Sighs does its work in guaranteeing everlasting bliss.
 Larson, M., Soleymani, M., Serdyukov, P., Rudinac, S., Wartena, C., Murdock, V., Friedland, G., Ordelman, R. and Jones, G.J.F. Automatic Tagging and Geotagging in Video Collections and Communities. ACM International Conference on Multimedia Retrieval 2011.