Drilling deeper into KML – OGC Geographic Visualization Standard

KML is an XML grammar used to visualize geographic data in an earth browser such as a 3D virtual globe, and 2D web browser of mobile mapping applications. KML has a tag-based structure with names and attributes used for specific display purposes and a KML instance is processed by web browsers in a way similar to that of HTML or XML.

KML can be used to:

• Annotate the Earth
• Specify icons and labels to identify locations on the surface of the planet
• Create different camera positions to define unique views for KML features
• Define image overlays to attach to the ground or screen
• Define styles to specify KML feature appearance
• Write HTML descriptions of KML features, including hyperlinks and embedded images
• Organize KML features into hierarchies
• Locate and update retrieved KML documents from local or remote network locations
• Define the location and orientation of textured 3D objects

KML uses the W3C XML Schema language to describe the grammar of conformant KML data instances. Conformant KML 2.2 files shall validate against the KML 2.2 schema. As stated in an earlier article, KML encoding uses geodetic longitude, geodetic latitude and altitude as define by the GML Coordinate Reference System (CRS) with identifier LonLat84_5773. The vertical datum is the WGS84 EGM96 Geoid.

A spatial extent of a Placemark is defined by extending the <Geometry> element. The spatial extent can include:

• the location of an anchor point on the earth, that serves as an origin for a 3D object in case of Model element.
• the encoding of explicit coordinate tuples in the coordinates element (as in Point, LineString or LinearRing).

In case of LineString, LinearRing and Polygon elements, subclauses are used to describe the interpolation schemes (used to represent Placemark that are not explicitly encoded).

The interpolation used for LineString and LinearRing depend on the values of the child altitudeMode (relativeToGround, absolute, clampToGround) and tessellate (1-true or 0-false) elements. It is worth noting that the control points of every LinearRing must lie on a common plane. The following KML instance, has the projected LineString that follow the earth’s curvature:

<kml xmlns="http://earth.google.com/kml/2.2">
  <Document>
    <Placemark>
      <LineString>
         <tessellate>true</tessellate>
         <altitudeMode>clampedToGround</altitudeMode>
         <coordinates>-135,30,500000 -80,30,500000</coordinates>
      </LineString>
   </Placemark>
  </Document>
</kml> 

The interpolation used for Polygon also depends on the values of the child altitudeMode (relativeToGround, absolute, clampToGround) and tessellate (1-true or 0-false) elements. In the following Polygon instance with the altitudeMode set to clampToGround, translates to a part in the earth’s terrain surface as shown below:

<kml xmlns="http://earth.google.com/kml/2.2">
  <Document>
    <Placemark>
      <Polygon>
        <altitudeMode>clampToGround</altitudeMode>
        <outerBoundaryIs>
          <LinearRing>
            <coordinates>-135,78.5,300000 -135,12.5,300000 -45,12.5,300000 -45,78.5,300000 -135,78.5,300000</coordinates>
          </LinearRing>
        </outerBoundaryIs>
      </Polygon>
    </Placemark>
  </Document>
</kml>

KML Polygon Interpolation - Example

• The outer boundary points of the polygon that are interpolated between the control points in the LinearRing form a rectangular perimeter in the plate carrée plane.
• Then the interior points of the rectangle are filled in linearly in this plane.
• Finally each (long,lat) point of the rectangle is mapped to a (long,lat,alt) point on the earth terrain surface.

Refer to http://blog.opengeo.org/2010/08/10/shape-of-a-polygon/ for Interesting examples on Polygon and their representation in KML.

Refer to http://code.google.com/apis/kml/documentation/kmlreference.html#polystyle for complete reference – including coloring of Polygons.

For more detailed information on KML models and syntax, the OGC Project document titled “KML 2.2 – An OGC Best Practice” with more details can be downloaded from http://portal.opengeospatial.org/files/?artifact_id=23689.

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.