thoughts-on-extending-the-geo-microformat
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A proposal for amendment of the geo microformat
- Editor/Author
- DimitriosZachariadis 17:03, 17 Jan 2007 (PST)
Abstract
A compact microformat for defining locations and events is proposed, as an ammendment to the draft geo microformat. A tolerant five part, semicolon separated, spacetime and reference-system value, is argued to be sufficient to provide accurate information for uniquely identifying a place or an event in a four dimensional universe.
Introduction
Spacetimes are the arenas in which all physical events take place — for example, the motion of planets around the Sun may be described in a particular type of spacetime, or the motion of light around a rotating star may be described in another type of spacetime. The basic elements of spacetime are events. In any given spacetime, an event is a unique position at a unique time. Examples of events include the explosion of a star or the single beat of a drum.Spacetime, Wikipedia (annotation by the author)
LOCATION and geo referencing microformats proposed on this site, namely geo, luna and mars have at present (Jan 2007) a status of draft. With this in mind, a number of issues regarding time and the geo microformat reference system are presented for discussion.
Default values and ambiguities
Disambiguation of the values used for tagging is an issue if the data tagged are to be valid, accurate and and useful outside the assumptions made at the time of their creation.
Omitting dimensions from a 4d coordinate system introduces assumptions about their default values. To demonstrate the problems that need to be dealt with, the following example of a markup, tagged with the geo microformat, might prove useful:
In 1687, a Turkish ammunition dump <a class="geo" href="http://maps.google.com/maps?q=37.971508,23.72658&ie=UTF8&z=18&ll=37.971508,23.72658 &spn=0.002584,0.006083&t=k&om=1&iwloc=addr"> inside the building <abbr class="latitude" title="37.971508"></abbr>, <abbr class="longitude" title="23.72658"></abbr> </a> was ignited by a Venetian cannonball. The resulting explosion severely damaged the Parthenon and its sculptures.
which may be rendered as:
In 1687, a Turkish ammunition dump inside the building was ignited by a Venetian cannonball. The resulting explosion severely damaged the Parthenon and its sculptures. Wikipedia, The Parthenon
In the case of the markup above, a human reader would assume that the coordinates involved in the code have been taken using WGS84, the most known and used datum today, due to its global validity and the proliferation of the GPS receivers. The ambiguity of altitude leads to an assumption of a default value of 0, which is acceptable for 2D cartography. No assumption needs to be made about time, since the human reader will read the year written in the text.
Time
It is noticeable, that most of the location related information, whether about the Earth or another celestial body, have a time related aspect attached; historical places and events, geographical features, but also street names and addresses are tightly related to an act or an observation made at some point in time; addresses used today may have not been in use a century ago, or may well change to something different tomorrow. Landmarks existing for centuries have an age, which means they have a birthday, and most probably a death. A vast amount of location information used by people on a daily basis on the web, is in fact an aggregation of events; stories, news and descriptions are most of the time nothing more than annotations of events. As such, most of this information can be accurately tagged, stored and used for as long as it exists, if its spacetime dimensions are known.
It is evident that the inclusion of a time dimension in the geo markup moves the focus of geo tagging from the realm of two dimensional cartography to the realm of events. This should come as no surprise: events are about stories; cartography is about navigation. People are interested in stories, news and events; few people can successfully navigate using a map and even fewer are interested in navigation and cartography as a science or art. Geographic coordinates shown in texts are quite useless without a map. Humans use names to identify places, not numbers. The reason geographic coordinates exist in text, is merely to help humans manually identify places on maps. If things can be done electronically, then numbers don't seem to matter a lot any more.
A human visiting the page with the markup above (which was chosen for the length of the time dimension and the connotations involved), will have a chance to read the text (and the year of the event, which however is un-tagged), click at the link and get a Google Map centered at the Acropolis and the Parthenon. Semantic connotations for humans enhance their understanding of information presented to them.[1] However, a machine seeking information in the Semantic Web, will have no way of finding out if it is the 5th century BC Athenian state or the contemporary Greek state that this particular piece of information is referring to. Semantic information about that particular markup that could otherwise be cataloged by the machine, will pass unnoticed. Had this geo markup been amended with a time dimension, a robot crawling this and other similar pages could better "understand" the data based on nothing more but the information provided by the geo microformat.
Dealing with time in multiple reference systems is not an easy. It might not be necessary for humans in their daily life, but it is important when when information on a global, or universal basis is involved. Different cultures, use different reference systems to tell the time. Islamic calendar These need to be taken into account in a Semantic Web.
In a similar manner, trying to use a Martian reference system, while living on Earth, involves more than a simple addition/subtraction of a few hours. A Martian day, is not of the same duration as a Terran day, and the same is true for the duration of the seasons; a number of connotations, that would otherwise help in getting a gut feeling, fail helplessly.
The reference system
It should be noted that the assumption about the datum used in expressing the lat-long coordinates mentioned above, is not a trivial one: Only 20 years ago, the same coordinates would possibly point hundreds of meters away from the Parthenon, since the reference system used at that time was different than WGS84. Furthermore, assuming WGS84 was the datum for the markup above, these coordinates may not be accurate 10 years from today, when the WGS84 reference system will have been revised once again.
A markable indication of the importance of the reference system, when expressing geo coordinates, is the fact that the Greenwich Observatory, which was by definition the origin for the longitude coordinate for more than a century, lies now about 102.5m West of the WGS84 0.0 meridian, at N 51° 28' 36.71, W 0° 0' 5.18", (in WGS84 datum) according to Wikipedia, Prime_Meridian. Interestingly, Google maps and Wikipedia do not seem to agree on these coordinates map
For a microformat to be able to convey accurate information, well defined and known reference systems should be identified together with the data. Transformations among the various reference systems can make content referenced in one system understandable and usefull on another.
Other celestial bodies
The amended geo microformat can be easily used on any celestial body for which a reference system, even rudimentary, has been defined. To that extend, Mars related content can be readily microformatted using Google Mars. Obviously, matters regarding reference systems of other celestial bodies require expert knowledge, however, reading the news on NASA's site, e.g. for Titan, Two Sides of Dunes, or geo-extension-strawman reveals that reference systems are in place for all celestial bodies that man made spacecrafts have visited, and probably for a lot more than those.
Proposed specification amendments
Values
The value of an amended geo location/event resides in the title attribute of an (X)HTML tag as a semicolon separated 5-tuple:
v1;v2;z;t;u where: v1 is either latitude or x, depending on u, mandatory v2 is either longitude or y, depending on u, mandatory z is the altitude, optional t is the time, optional u is the reference system code, mandatory if not defined globally
Dimensions z, t, and u can be omitted. If ambiguities arise from such an omition, e.g. omiting the z and t dimensions but defining the u dimension, then the relevant field values must be left empty, e.g.: v1;v2;;;u
Default Reference System
The default reference system is WGS84 and the Gregorian calendar.
The time dimension is expressed in the ISO-8601 format ("YYYY-MM-DDThh:mm:ss.ssZ"), in any of its abbreviations and forms. W3C, Date and Time Formats and Wikipedia, ISO-8601
An interesting provision of the ISO-8601 time format is that it allows time durations to be defined in a consistant manner.
Meta data
The default reference system may be set, using the name and content attributes of the meta (X)HTML element, as follows:
<meta name="geo.reference" content="reference-system"/>
Default coordinate values
When a dimension is omitted, the following default values are implied:
u: reference system, no default v1: latitude, expressed in decimal degrees, no default v2: longitude, expressed in decimal degrees, no default z: altitude, expressed in meters, defaults to 0.0 t: time, no default
The default reference system for an (X)HTML page MUST be set by using meta data properties, if the geo data defines no reference system
It is strongly recommended that web pages include a meta data element that defines the default reference system, in their HTML head section. The inclusion of the meta data element ensures the validity of the 4d data for as long as the page exists, while keeping geo values compact. By adding the geo.reference metadata property, a transition to the amended geo microformat would require no further change to the markup.
Reference systems
Other reference systems include:
- UTM:zone
- v1 is x, expressed in meters
- v2 is y, expressed in meters
- t is expressed in ISO-8601
...more
Examples
Example 1
Hidden 4d coordinates, no content
The Parthenon was ruined by a Venetian cannonball in 1687 <abbr class="geo" title="37.971508;23.72658;;1687"></abbr>
Example 2
Hidden 4d coordinates, free text content
<span class="geo" title="37.971508;23.72658;;1687"> The Parthenon was ruined by a Venetian cannonball in 1687 </span>
may be rendered as:
The Parthenon was ruined by a Venetian cannonball in 1687
Example 3
Visible 4d coordinates
<abbr class="geo" title="37.971508;23.72658;;1687;WGS84">N 37° 58' 17.43, E 23° 43' 35.69</abbr>
may be rendered as:
N 37° 58' 17.43, E 23° 43' 35.69
Example 4
Minimal HTML page:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="geo.reference" content="WGS84"/>
</head>
<body>
<div class="geo" title="51.476864;-0.000518">
The Greenwich Observatory
</div>
</body>
</html>
Example 5
A fictitious Mars reference system, using Mars coordinates from Google Mars:
<span class="geo" title="18.302380;-133.472900;;;MARS:J2000"> Olympus Mons, Mars: The highest mountain in the solar system. </span>
rendered as:
Olympus Mons, Mars: The highest mountain in the solar system.
Real world example
[http:// A crafted page with the examples appearing here and a map demonstration]
Notes
- Umberto Eco, A Theory of Semiotics, 1976
- RFC2426
- W3C, Date and Time Formats
- Wikipedia, ISO-8601
Further discussion
Comments and criticism about this proposal: