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Tuesday, September 05, 2006

Geotag -Meta Tags

What is a geotag?

A geotag is simply a properly-formed XML tag giving the geographic coordinates of a place. The coordinates can be specified in latitude and longitude or in UTM (Universal Transverse Mercator) coordinates.
Why should I geotag?

We believe that there are many benefits accrued from locating data and news on maps. Geo-located information can answer lots of questions: Where did that happen? Was it near me/my colleagues/my family/my friends? How many are near me and how many are over there? Is that a good place to be or a place to stay away from at all costs? The events of interest can be anything from rock concerts to earthquakes to poltical gatherings to superfund sites to terrorist activities. We can see patterns emerge and watch trails being made.
What can I geotag?

Weblog posts, web pages, RDF resources, even arbitrary HTML and XML elements.
The details

The RDFIG Geo vocabulary from the W3C is the common basis for the recommendations below. It supplies official global names for the latitude, longitude, and altitude properties. These are given in a system of coordinates known as "the WGS84 datum". (A geographic datum specifies an ellispoidal approximation to the earth's surface; WGS84 is the most commonly used such datum; it is utilized, eg for GPS).

To specify that the longitude of something is X, that its latitude is Y, and, optionally, that its altitude is Z, tags of the form


are used. Altitude is specified in meters. The prefix "geo:" represents the RDFIG Geo namespace, whose URI is: If you don't know about namespaces, and are concerned with geotagging web pages or ordinary HTML, then just follow the examples below.

Geotagging an HTML element

The following will pass muster as correct XML in the context of XHTML (the newer dialect of HTML that adheres to the XML standard), but will also work in earlier HTML dialects, in the sense of being tolerated by all modern browsers. To geotag an HTML element, include a span of the following form:

<span style="display:none" xmlns:geo="">

If the geo namespace is defined at an outer level of the document, the namespace definition in the span tag can be omitted, leaving

<span style="display:none"> <geo:lat>46.1</geo:lat><geo:long>124</geo:long></span>

In earlier HTML dialects, omitting the namespace definition is also appropriate, since the objective of adhering to the XML standard is irrelevant.

This technique can be used to geotag a post in a weblog, or elements within any HTML document.
Geotagging XML (including RSS and RDF ).

Simply include elements of the form


as children of the element you wish to tag, and place the definition of the geo namespace at the outermost level of the document (or at the level you prefer).
Geotagging a web page

Use this method to assign a location to a web page as a whole, rather than to its parts. In the element, include meta tags of the form:

<meta property="geo:lat">46.1</meta>
<meta property="geo:long">124</meta>

In XHTML, the document namespace definition should include the geo tag.

This form of meta tag follows the recommendations contained in


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