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Proposal status: Abandoned (inactive)
Proposed by: myfanwy
Tagging: date=*

Draft started: 2007-11-28
RFC start: 2007-11-28

A value tag to define the age of an item. This would allow historic maps to be generated, e.g. the position of a road may change over time. Using this tag, the two routes of the road could be drawn, one tagged '1830' and one tagged '2007'. If a viewer looked at a 2007 map (selected via some menu, similar to the 'Mapnik'/Osmarender' dropdown on the openstreetmap homepage), then the 2007 location of the road would show. If the user were then to look at an 1830 map (again, selected via some menu), the 1830 location of the map would show.

For a building, this tag would consist of it's build date - a map representing a time after this date would show the building, a map representing a time before this date would not.


Applies to all entities: nodes, segments, areas, relations.

Tag Description
date= date that applies to this specific location of the object


No icon would be necessary, either the item should display on a given map, or it shouldn't.


Items are assumed to be at a location from the date specified until the next date for that item

e.g. the A6 (a road in England, present since Roman times) would have one route from 50CE (or whenever it was built), another from 1955 (when the council routed part of it round a village) and so on. Anyone choosing to view a map from (say) 1730 would see the road in it's 50CE location, as that is the last listed date for this item. Anyone viewing a map from (say) 1960 would see it in it's 1955 location.

  • There's already a 'date' construct that, imho, would work better anyway - date_on=* and date_off=* (feature 'start date' and 'end date' repectively) --Cohort
    • That's the wrong construct. Use start_date and end_date. See Map Features.
    • End date for things no longer existing is essential, as it's really not possible in the general case to work it out from the presence of other things with a later start date. Also, for anything like this to be safe to use, we need to have renderers ignore ways with these tags. Also we could do with a way of specifying particular tag values with a time-limited range - e.g. i want to say that this road was called "Waterloo Way" before 1999 but "Tigers Way" afterward, without having two ways sharing the same nodes... Morwen 01:09, 28 November 2007 (UTC)
    • I think we are thinking too much along the lines of OS MasterMap – although it would in fact be very good to have comments here from people who have used the historical features of MasterMap. In many, possibly most, cases (depending on the kind of item that people choose to tag up) the start date and end date are simply not known. What can be stated with some confidence are dates where the feature was known to exist, in effect "samples" from history based on old texts, old maps, paintings, etc. For example, it might be nice to add records for Old London Bridge, that stood from around 1209 to around 1831. We'd also like to record dates for notable structures on the bridge, Nonesuch House, St Thomas' Chapel and so on. Even for such a well-documented structure as London Bridge we can only hope to do this on a "sampling" basis. Note that with multiple samples, you have some kind of "lifespan" data by using min and max. Lorp 01:47, 28 November 2007 (UTC)
    • Since historical data is not verifiable like current data, I'd also like to see us give references for any historical OSM metadata: "date=1209{source:Britannica 1911}", but that brings be onto other ideas for complex tagging... Lorp 01:47, 28 November 2007 (UTC)
    • When using historical sources, always beware of the dating system in use. There's the well-known Gregorian/Julian problem (William of Orange left Holland on November 11, 1688, and arrived in England on November 5), but there's also the practice of the year starting around March 25 rather than January 1. Here's R.L. Poole on the varieties of in use in the Middle Ages: "If we suppose a traveller to set out from Venice on March 1, 1245, the first day of the Venetian year, he would find himself in 1244 when he reached Florence; and if after a short stay he went on to Pisa, the year 1246 would have already begun there. Continuing his journey westward, he would find himself again in 1245 when he entered Provence, and in arriving in France before Easter (April 16) he would be once more in 1244." Lorp 02:03, 28 November 2007 (UTC)
  • I think using the existing start_date and end_date tags as user Cohort points out, with a CE ISO 8601 date value , is a good start for features over the last 100 to 200 years or so ... as long as our current era renderers know about the end_date tag! But I think if we seriously get into historical mapping, we'll need some expressions or enhanced syntax to cover more fuzzy constructs like "appeared sometime around the 12th century" start_date=1100s ? and "certainly existed in 1307 but was built before then" start_date=1307- ? MikeCollinson 12:26, 28 November 2007 (UTC)
  • I'm also very much looking forward to OSM historical mapping as way of showing the evolution of my home town, but I'd urge caution and patience. It will be very confusing to have, say, multiple road layouts for a give area - very hard to edit and may be rendered incorrectly. I personally think we should wait for the basic system to become more mature and then have a separate parallel database for historical features. MikeCollinson 12:26, 28 November 2007 (UTC)