Overview of KML – OGC Geographic Visualization Standard
KML is the international standard language (actually an XML schema) for expressing geographic annotation and visualization within Internet based, two-dimensional (2D) maps and three-dimensional (3D) earth browsers (or GeoBrowsers). Objective of KML is to encourage broader implementation and greater interoperability and sharing of earth browser content and context.
KML (earlier called Keyhole Markup Language) was developed by Keyhole, Inc which was acquired by Google. Google uses KML as the file format to display geographic data in Earth browsers like Google Earth, Google Maps and Google Maps for mobile. Google submitted KML to the Open Geospatial Consortium (OGC) and KML ver 2.2 has been adopted as an OGC implementation standard. After KML has been accepted as an OGC standard, it is supported (in one way or another) on most of the major GeoBrowsers.
KML focuses on geographic visualization – namely the presentation of graphical data on the globe, control of the user’s navigation on where to go and where to look, and annotation of maps and images. Using KML files, we can specify the features – images, polygons, 3D models, place marks, photo overlays, paths, network links – for display in any 3D GeoBrowser implementing the KML encoding. KML files are used to pinpoint locations, add image overlays, and expose rich data in new ways. Typically, KML files are distributed in KMZ files, which are zipped files with a .kmz extension.
KML can also be used to represent astronomical data and display objects – stars, constellations, planets, moon and galaxies – in the sky using Google Sky or Microsoft’s WorldWide Telescope. You can define tours (using the set of extensions to the OGC KML standard – supported currently by Google Earth) by placing specific elements, in order, into a KML file. Tours basically use the tour primitives like FlyTos, Waits, TourControls, AnimatedUpdates and SoundCues – that run in series or in parallel.
Any KML feature can have time data associated with it, restricting the visibility of the data set to a given time period (TimeSpan) or point in time (TimeStamp). In Google Earth, the time slider controls which parts of the data is visible.
Usage of KML includes:
· Casual users use KML to Place mark homes, plan and document tours, plan cross-country hikes and cycling ventures.
· Scientists use KML to provide detailed mapping of resources, models and trends such as volcanic eruptions, weather patterns, earthquake activity, and mineral deposits.
· Real estate professionals, architects, city development agencies use KML to propose construction and visualize plans.
· Educational community uses KML to explore historic and current people, places and events.
· National Geographic, UNESCO and such organizations use KML to display their rich sets of global data
· Non-profit organizations use KML to highlight problems and advocate change
KML is complementary to other OGC standards including GML (Geography Markup Language), WFS (Web Feature Service) and WMS (Web Map Service). KML 2.2 utilizes certain geometry elements – point, line string, linear ring, and polygon – derived from GML 2.1.2. In the near future, additional harmonization of KML with GML (e.g., using the same geometry representation) is expected. Together KML and GeoRSS makes information more searchable – with KML focus on visualization of resources and GeoRSS on tagging and linking of resources.