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With photos stored in JPEG file format, the geotag information is typically embedded in the metadata (stored in Exchangeable image file format (EXIF) or Extensible Metadata Platform (XMP) format). These data are not visible in the picture itself but are read and written by special programs and most digital cameras and modern scanners. Latitude and longitude are stored in units of degrees with decimals[
GPS Latitude : 57 deg 38' 56.83" N GPS Longitude : 10 deg 24' 26.79" E GPS Position : 57 deg 38' 56.83" N, 10 deg 24' 26.79" E
or the same coordinates could also be presented as decimal degrees:
GPSLatitude : 57.64911 GPSLongitude : 10.40744 GPSPosition : 57.64911 10.40744
When stored in EXIF, the coordinates are represented as a series of rational numbers in the GPS sub-IFD. Here is a hexadecimal dump of the relevant section of the EXIF metadata (with big-endian byte order):
+ [GPS directory with 5 entries] | 0) GPSVersionID = 2 2 0 0 | - Tag 0x0000 (4 bytes, int8u[4]): | dump: 02 02 00 00 | 1) GPSLatitudeRef = N | - Tag 0x0001 (2 bytes, string[2]): | dump: 4e 00 [ASCII "N\0"] | 2) GPSLatitude = 57 38 56.83 (57/1 38/1 5683/100) | - Tag 0x0002 (24 bytes, rational64u[3]): | dump: 00 00 00 39 00 00 00 01 00 00 00 26 00 00 00 01 | dump: 00 00 16 33 00 00 00 64 | 3) GPSLongitudeRef = W | - Tag 0x0003 (2 bytes, string[2]): | dump: 57 00 [ASCII "W\0"] | 4) GPSLongitude = 10 24 26.79 (10/1 24/1 2679/100) | - Tag 0x0004 (24 bytes, rational64u[3]): | dump: 00 00 00 0a 00 00 00 01 00 00 00 18 00 00 00 01 | dump: 00 00 0a 77 00 00 00 64
SMS messages
The GeoSMS standard works by embedding one or more 'geo' URIs in the body of an SMS, for example:
I'm at the pub geo:-37.801631,144.980294;u=16
HTML pages
ICBM method
The GeoURL[4] standard requires the ICBM tag[5] method which is used to geotag standard web pages in HTML format:
<meta name="ICBM" content="50.167958, -97.133185">
The similar Geo Tag format allows the addition of placename and region tags:
<meta name="geo.position" content="50.167958;-97.133185"> <meta name="geo.placename" content="Rockwood Rural Municipality, Manitoba, Canada"> <meta name="geo.region" content="ca-mb">
[edit]RDF feeds
The RDF method is defined by W3 Group and presents the information in RDF tags[6]:
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#"> <geo:Point> <geo:lat>55.701</geo:lat><geo:long>12.552</geo:long> </geo:Point> </rdf:RDF>
Microformat
The Geo microformat allows coordinates within HTML pages to be marked up in such a way that they can be "discovered" by software tools. Example:
<span class="geo"> <span class="latitude">50.167958</span>; <span class="longitude">-97.133185</span> </span>
which might display as:
50.167958; -97.133185
(giving a live Geo microformat on this page).
A proposal has been developed[7] to extend Geo to cover other bodies, such as Mars and the Moon.
An example is the Flickr photo-sharing Web site, which provides geographic data for any geotagged photo in all of the above-mentioned formats.
Wikipedia
On Wikipedia it is possible to include geotagged information in articles (and thus also images), using the template {{coord}}. On Wikimedia Commons it is more common to tag camera locations with template {{location}}. The inserted coordinates will be presented in the top right corner as a link on the Wikimedia Toolserver[8], allowing the user to click further on to different geographic content on the Internet. For the article Råbjerg Mile it looks like this:
Geotagging in tag-based systems
No industry standards exist, however there are a variety of techniques for adding geographical identification metadata to an information resource. One convention, established by the website Geobloggers and used by more and more sites, e.g. photo sharing sites Panoramioand Flickr, and the social bookmarking site del.icio.us, enables content to be found via a location search. All sites allow users to add metadata to an information resource via a set of so-called machine tags (see folksonomy).
geotagged geo:lat=57.64911 geo:lon=10.40744
This describes the geographic coordinates of a particular location in terms of latitude (geo:lat) and longitude (geo:lon). These are expressed in decimal degrees in the WGS84 datum, which has become something of a default geodetic datum with the advent of GPS.[citation needed]
Using three tags works within the constraint of having tags that can only be single 'words'. Identifying geotagged information resources on sites like Flickr and del.icio.us is done by searching for the 'geotagged' tag, since the tags beginning 'geo:lat=' and 'geo:lon=' are necessarily very variable.
Another option is to tag with a Geohash:
geo:hash=u4pruydqqvj
A further convention proposed by FlickrFly adds tags to specify the suggested viewing angle and range when the geotagged location is viewed in Google Earth:
ge:head=225.00 ge:tilt=45.00 ge:range=560.00
These three tags would indicate that the camera is pointed heading 225° (south west), has a 45° tilt and is 560 metres from the subject.
Where the above methods are in use, their coordinates may differ from those specified by the photo's internal EXIF data, for example because of a correction or a difference between the camera's location and the subject's.
Geoblogging
Geoblogging attaches specific geographic location information to blog entries via geotags. Searching a list of blogs and pictures tagged using geotag technology allows users to select areas of specific interest to them on interactive maps. [9]
The progression of GPS technology, along with the development of various online applications, has fueled the popularity of such tagged blogging,[citation needed] and the combination of GPS Phones and GSM localization, has led to the moblogging, where blog posts are tagged with exact position of the user. Real-time geotagging relays automatically geotagged media such as photos or video to be published and shared immediately.
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