Keyhole Markup Language

Keyhole Markup Language (KML) is an XML-based language schema for expressing geographic annotation and visualization on existing or future Internet-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Keyhole Earth Viewer. It was created by Keyhole, Inc, which was acquired by Google in 2004. The name "Keyhole" is an homage to the KH reconnaissance satellites, the original eye-in-the-sky military reconnaissance system first launched in 1976. KML is an international standard of the Open Geospatial Consortium. Google Earth was the first program able to view and graphically edit KML files, and other projects such as Marble have also started to develop KML support.

Structure

The KML file specifies a set of features (placemarks, images, polygons, 3D models, textual descriptions, etc.) for display in Google Earth, Maps and Mobile, or any other 3D earth browser (geobrowser) implementing the KML encoding. Each place always has a longitude and a latitude. Other data can make the view more specific, such as tilt, heading, altitude, which together define a "camera view". KML shares some of the same structural grammar as GML. Some KML information cannot be viewed in Google Maps or Mobile.

KML files are very often distributed in KMZ files, which are zipped files with a .kmz extension. Must be legacy (ZIP 2.0) compatible compression (e.g. deflate method) otherwise might not uncompress in all geobrowsers. The contents of a KMZ file are a single root KML document (notionally "doc.kml") and optionally any overlays, images, icons, and COLLADA 3D models referenced in the KML including network-linked KML files. The root KML document is typically a file named "doc.kml" at the root directory level but the first .kml file entry in the KMZ file is the actual one selected in Google Earth regardless of its name. By convention the root KML document is at root level and referenced files are in subdirectories (e.g. images for overlay images).

Example KML document:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Placemark>
  <name>New York City</name>
  <description>New York City</description>
  <Point>
    <coordinates>-74.006393,40.714172,0</coordinates>
  </Point>
</Placemark>
</kml>

The MIME type associated with KML is application/vnd.google-earth.kml+xml; the MIME type associated with KMZ is application/vnd.google-earth.kmz.

Geodetic reference systems in KML

For its reference system, KML uses 3D geographic coordinates: longitude, latitude and altitude, in that order. The longitude, latitude components are as defined by the World Geodetic System of 1984 (WGS84). The vertical component (altitude) is measured from the WGS84 EGM96 Geoid vertical datum. If altitude is omitted from a coordinate string, e.g. (-122.917, 49.2623) then the default value of 0 (approximately sea level) is assumed for the altitude component, i.e. (-122.917, 49.2623, 0) is assumed. A formal definition of the coordinate reference system (encoded as GML) used by KML is contained in the OGC KML 2.2 Specification. This definition references well-known EPSG CRS components.

OGC standard process

The KML 2.2 specification was submitted to the Open Geospatial Consortium to assure its status as an open standard for all geobrowsers. In November 2007 a new KML 2.2 Standards Working Group was established within OGC to formalize KML 2.2 as an OGC standard. Comments were sought on the proposed standard until January 4, 2008, and it became an official OGC standard on April 14, 2008.

See also

• Geospatial Content Management System
• CityGML
• Geography Markup Language
• GPX (data transfer)
• Point of Interest
• Waypoint
• COLLADA

External links

• OGC KML 2.2 Standard
• OGC Official KML 2.2 Schema
• Google's KML Documentation
• KML Validator
• KMLToolbox
• KML Developer Support group