Standard tags in
SGML and
XML are required for names and for spacetime coordinates on
Earth - what a
spacetime DTD solves.
Even on Earth within historical time (back to roughly 10,000 BCE = 12,000 years ago, some non-obvious considerations apply to specifications of blocks of spacetime. But certainly a key requirement is that coordinates of a particular place on the Earth ought to be as stable as its rocks and strata, so that a 'location' on the Earth's surface has its usual meaning.
Here are some of the non-obvious considerations of a terrestrial coordinate system going back 12,000 years and useful in geology and geography, paleontology and biology, ecology and climate, archaeology and time zone. In any project that crosses time zone borders, as this one does, you have said NOTHING if you have not implied or added the time zone. A default of UTC (Greenwich Mean Time) is the only sane standard when this is ambiguous, which makes many articles that assume PST or EST simply wrong. This must be part of the standard.
Relative blocks of time, e.g. 'that week', 'a day earlier', etc., are unambiguous and actually better referents, if an absolute frame has been established. But those informal characterizations should be tagged to note that "that week means -7 days from reference", "a day earlier means -1 days from reference". This would be pretty easy to do automatically and sum up things that happened on a given day.
Many calendars (e.g. Chinese calendar, Islamic calendar, Persian calendar) are still in use in the world. Others tend to alter the year but accept the Gregorian calendar for the "date". But in the Islamic calendar, for instance, "13 December 1979" is "24 Muharram 1400 A.H.". Chinese and Islamic calendars use lunar months, which is more reliable than Julian/Gregorian dating, so the phase of the moon is actually a better reference in most cases if poor records exist.
Earth latitude and longitude are relative to the date, due to polar drift[?] and continental drift - so a latitude/longitude pair far in the past is not necessarily the same place without some compensation.
Earth ecoregions are the most basic reference for living things, and a scientific and standard map of them now exists. Their borders determine most of what one would want to say about biology or climate, so it is better to organize this information around them, than around say national borders, which is a poor way to organize data about physical geography or ecology. This should be part of the system as early as possible, so that the ranges of animals can be specified in terms of ecoregions rather than countries or gross physical features.
If you know of an existing DTD that actually does this correctly and solves the problems of history and geography above, by all means dig it up and let's talk about it, and improve it here. But it appears at present that the Wikipedia already has problems that we should solve before we get into the issues of a Wikipediatlas[?].
See also
meta:spacetime DTD -- Wikipedia:WikiProject Ecoregions
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