Geo Tag Elements

Note! - the ordering of (latitude,longitude) has changed since the first version of this document.

<META NAME="geo.position" CONTENT="latitude; longitude">
<META NAME="geo.placename" CONTENT="Place Name">
<META NAME="geo.region" CONTENT="Country Subdivision Code">

e.g.
<META NAME="geo.position" CONTENT="49.2;-123.4">
or
<META NAME="geo.position" CONTENT="49.2;-123.4">
<META NAME="geo.placename" CONTENT="London, Ontario">
<META NAME="geo.region" CONTENT="CA-ON">

Element Description

Longitude:

Longitude is conventionally given in degrees of arc relative to the Greenwich Meridian, a great circle passing through the poles and Greenwich in London, England. Longitude is either qualified as East or West, or given as a signed numeric value of degrees East. The geo.position tag uses a signed numeric value.

On a Mercator map, Longitude is plotted left to right as the X coordinate, while Latitude is plotted bottom to top as the Y coordinate, with (0,0) in the middle. Thus, positive values for Longitude correspond to locations East of Greenwich, e.g. in Asia. Locations West of Greenwich, e.g. in the United States, correspond to negative values of Longitude.

(Latitude;Longitude) in quadrants

Latitude:

Latitude is conventionally given in degrees of arc relative to the Equator. Latitude is either qualified as North or South, or given as a signed numeric value of degrees North. The geo.position tag uses a signed numeric value.

Region:

The geo.region tag is taken from a controlled list, and may be used for resource discovery. It may also be used as a bounding check to validate the geo.position tag.
For the geo.region tag, the ISO 3166-2 country subdivision codes are used. These codes are formed from the "domain-name" 2-character codes, for example iso3166-countrycodes.txt, together with a regional code. The codes may be found here, while the GeoTag Generator includes a 2-stage script to pick appropriate codes.

For the USA and Canada, the subdivision code is the familiar 2-character state/province abbreviation. See for instance United States Postal Service, Official Abbreviations - States and Possessions and Canada Postal Guide (Province and Territory Symbols)

Country:
If the regional code is not known, the 2-character country code may be used instead.

ISO 3166 codes are reproduced with permission from ISO

Place Name:

The geo.placename tag is provided primarily for resource recognition; it is anticipated that this field be harvested by automated agents and presented to the user in search engine results in a similar manner to the description META tag. This field is free-text, and typically would be used for city, county and state names. It could, however, be used for resource discovery, particularly if names from some controlled vocabulary such as the Getty Thesaurus of Geographic Names is used.

Required Accuracy of Position

The accuracy with which positions need to be determined is largely determined by the character of the resource being described.

While a position given in a gazetteer or thesarus such as the Getty Thesaurus of Geographic Names is convenient, and in many cases adequate, in other cases it is clearly not. For instance, it is probably sufficient to place a branch office of a multinational corporation on a map of the world, or a map of a country. It is, however, clearly insufficient to place a public telephone, filling station or fast-food restaurant. In these cases, position must be measured or researched to greater accuracy.

Accuracy of Elements

Properly expressing the accuracy of geographic positions is too complex for the simple geo.position META tag, yet some clue to the accuracy may be given by the number of digits in each element. One Minute of Arc of Latitude corresponds to one Nautical Mile (1852 metres, or 1.8km).

Latitude	Accuracy	Mile	Km
49	1°	60	111
49.1	0° 6'	6	11
49.30	36"	0.6	1
49.320	3"	0.06	0.1

In a commercial GPS set, the standard accuracy is less than 15 metres, now that SA has been turned off. (May 2000). DGPS sets may allow accuracy better than this

Extent

As currently defined, the geo.position tag describes a point, not a region. It is thus unsuitable for describing an extended area. What constitutes a region may depend on the scale of map or geographic search used; for instance, the country of Andorra (geo.position 42.5;1.5) may reasonably be represented by a point on a map of the world but not on a map of Andorra. It is the responsibility of the user to determine if a point representation is meaningful given the intended audience of the tagged document.

Future versions of geotags may incorporate a region element, as do other metadata standards.

Position Datum

In cases where the resource position is less accurate than a few kilometres, datum issues may be ignored. For accurate positions, however, the WGS-84 datum should be used. In North America, this corresponds to the NAD-83 datum. Many maps and charts still use the older NAD-27 datum, and there is a difference in coordinates of as much as a couple of hundred metres, depending on the exact position. Conversion software is available, and most recently issued topographic maps and charts will give the offsets.

Tag Placement

In accordance with the HTML 4.0 specification, META tags (including geo tags) should be placed in the HTML document header, between the <HEAD> and </HEAD> elements, for instance:

<html><head><title>My Document</title>
<meta name="geo.region" content="US-WA">
</head>

TGN etc. Cut/Paste

The Geo Tag Generator allows geographic positions from certain popular sites to be pasted in using the browser clipsheet. Bold text is required. Examples:

TGN: Lat: 49 21 N Long: 123 05 W
Tiger: Scale: 1:218074 (Centered at Lat: 38.89000 Lon: -77.02000)
CPCGN:  Latitude - Longitude : 49° 16' 00" N - 122° 57' 00" W

Check Map

The Geo Tag Generator incorporates an optional check map which may be used for a quick sanity check on geographic co-ordinates. Co-ordinates may also be checked against country and region codes, if given.

Icon

Pages which have included geotags may include a geotag icon

to indicate that geographic search capability may be available. To use the icon, copy the image to your system (e.g. right-click "Save Image As") and include the following HTML code:

<a href="http://geotags.com/geo/" target="_top">
<img src="geo2t.png" alt="Geo Tagged for Geographic Discovery"></a>

(If someone would like to create a better one, that would be cool..)

Notes

The tags describe the position of the resource described on the page, for instance a beach or restaurant, not the company hosting the page, the company managing the resource, or the server hosting the page (cf. RFC 1876)
The tags are described in terms of current HTML practice, which does not preclude them being represented in another manner such as RDF or XML.
The tags are intended for use by a wide base of authors who are probably not well versed in GIS or formal metadata techniques. They are not intended to supplant, and do not address as many issues, as such formal standards as FGDC, GILS, TC211, Dublin Core etc which should be used where applicable.

Maintained by Andrew Daviel