Tuesday 10 January 2012

Plaques, blue or not

I came across a reference to a blue plaque recently and found openplaques in the process. Blue plaques get put up to commemorate something or someone. I somehow assumed that they were only in London so I was surprised to find 60 in Hull. When I looked a bit further there seems to be some in East Yorkshire too, including 25 in the small town of Howden alone.  This is just something else to look out for in our travels.

We then discovered that the next village has at least one (pictured) and maybe more judging by the heritage trail inscription. If I want to add these to OSM, and why not, I wondered what tags to use. The plaques are marking something of historic significance; something happened here, or someone special lived or worked here, or a building of significance is or was here. The historic tag seems appropriate, but what else? The Map Features wiki page lists many historic tags, the closest seems to be memorial, but that is not really what the plaques are. It does suggest it can be  a 'plate on a wall', but I wonder if historic=plaque or historic=marker might not be better. I have used historic=memorial with memorial=blue_plaque too.

I have emailed Jez at openplaques.org to discuss this. He suggested connecting the OSM object to the openplaques object, which others have thought of too, especially Mappa-mercia. I like this idea, but as always there is no generic way to do this. I have used openplaques:id=* for now. Jez suggested ref=openplaques:id=xxxx. At first I liked the ref idea, but it is not verifiable on the ground - the id is not written on the plaque because it is made up by openplaques not the people who put them up. I think ref=* should be verifiable. I also don't like the extra "=" sign. I wonder if OSM needs a link_ref tag as a general linkage to outside references. That could be link_ref:openplaques=1234 but also link_ref:ERoY=xyz if the council (East Riding of Yorkshire) have a separate reference too.

It is important that databases outside of OSM do not expect OSM IDs to persist. It would be tempting to reverse the process and have openplaques store the OSM node id of the plaque, but these id are not guaranteed to remain unchanged. In the case of a blue plaque it is unlikely to be redrawn as a building described by a way rather than a node, but other objects certainly can be.

If you have plaques (blue or otherwise) near you to commemorate someone or something why not add them to OSM and openplaques.org. Then we can all argue over agree what tags to use.


gom1 said...

Spot on as usual. I have used slightly different tagging to record these, without being completely happy with it. I think they are memorials. They are not always blue though. Really they are "commemorative plaques". Naming them is a bit of a quandary. I used "blue_plaque:person's name" but it's not very satisfactory. Fwiw I think it's best if osm refers to some external id, rather than vice versa. Much better that Open Plaques takes on the management of unique ids, than osm, and the way Open Plaques uses Flickr machine tags is quite neat.

Anonymous said...

Nice thinking. One of the reasons Open Plaques has grown is that it exposes these issues. Ideally, in a semantic web world, we'd be able to find a way to describe an object without using anyone's id scheme.

Our ids are very slow moving, not 100% as we have occasional dupes to delete, but as there is no issuing body to manage plaque ids it may be the strongest alternative to triples that describe the subject(s) which could get painful.

- Jez, from Open Plaques

Anonymous said...

The OSM tag historic=memorial does already seem to cover blue plaques.

"Much like a monument, but smaller. Might range from a WWII memorial to a simple plate on a wall."

Anonymous said...

There is now an OSM Wiki page to discuss this http://wiki.openstreetmap.org/wiki/Key:openplaques_plaque please add some opinion