MS RFC 58: Kml Output

Date:2009/03/01 Authors:Dvaid Kana (david.kana at gmail.com) Authors:Thomas.Bonfort (thomas.bonfort at gmail.com) Authors:Yewondwossen Assefa (yassefa at dmsolutions.ca) Authors:Michael Smith (michael.smith at usace.army.mil) Status:Planning Version:MapServer 6.0 Id:$

Purpose

This purpose of this RFC is to provide a KML output for MapServer 6.0. The initial work was done by David Kana during the 2009 Google Summer of Code.

The main task of the project is the implementation of the KML driver for generating KML output used mainly by Google Earth application. Code for KML rendering is based on new renderer API described in MS RFC 54

First intention was to use original KML library libkml provided by Google but due to its complexity libxml2 already included in MapServer was selected for xml generating.

General Functionnality

The general idea is to provide an output format that can be set at the map level and allows to generate a kml or kmz output from a MapServer map file. Output can be generated using MapServer cgi (example mode=map) or through a WMS request.

Output format

The default name of the output format is kml, and this name can be used to set the imagetype parameter in the map file.

The format can also be defined in the map file:

OUTPUTFORMAT
  NAME kml
  DRIVER "KML"
  MIMETYPE "application/vnd.google-earth.kml+xml"
  IMAGEMODE RGB
  EXTENSION "kml"
  FORMATOPTION 'ATTACHMENT=gmap75.kml'  #name of kml file returned
END

kmz output will also be supported through the minizip library.

Build

• On windows: there is a flag KML in nmake.opt
• On Linux: –with-kml
• cairo and agg2 are for now necessary to build with. When agg2 will be finshed, we will drop the cairo link. Driver configuration options

Map

In terms for Kml object, the MapServer KML output will produce a <Document> element to include all the layers that are part of the MapServer map file. Features supported for the Document are:

Document element	Supported	MapServer equivalence/Notes
name	Yes	Name in the map file
visibility	No	Can be supported if needed. Default is 1
open	No	Can be supported if needed. Default is 0
address	No	Could be supported for example using ows_address if available
AdressDetails	No
phoneNumber	No	Could be supported using ows_contactvoicetelephone is available
Snippet	No
description	No
AbstractView	No
TimePrimitive	No
styleURL	No
StyleSelector	Yes	Style element will be supported. All different styles from the layers will be stored here and referenced from the folders using a styleUrl.In addition to the Styles related to features, there is a ListStyle element added at the document level. This allows to control the way folders are presented.See Layers section (styleUrl) setting for more details.
Region	No
Metadata	No
ExtendedData	No

Layers

Each layer of the MapServer map file will be inside a Kml <Folder> element. Supported Folder elements are:

Folder element	Supported	MapServer equivalence/Notes
name	Yes	Name of the layer. If not available the name will be Layer concatenated with the layer’s index (Layer1)
visibility	Yes	Always set to 1
open	No	Default is 0
atom:authoratom:linkaddressAddressDetailsphoneNumberSnippet	No
description	No	Could be supported using ows_description
AbstarctView	No
TimePrimitive	No
styleUrl	Yes	The user can use the kml_folder_display layer or map level metedata to choose a setting. Possible values are ‘check’ (default), ‘radioFolder’, ‘checkOffOnly’, ‘checkHideChildren’.
RegionMetadataExtendedData	No

Each element in the Layer will be inside a Kml <Placemark> element. As described in the Kml reference : « A Placemark is a Feature with associated Geometry. In Google Earth, a Placemark appears as a list item in the Places panel. A Placemark with a Point has an icon associated with it that marks a point on the Earth in the 3D viewer. (In the Google Earth 3D viewer, a Point Placemark is the only object you can click or roll over. Other Geometry objects do not have an icon in the 3D viewer. To give the user something to click in the 3D viewer, you would need to create a MultiGeometry object that contains both a Point and the other Geometry object.) »

For Polygon and Line layers, when a feature is associated with a label, a MultiGeometry element containing a point geometry and the geometry of the feature is created. The point feature will be located in the middle of the polygon or line

<Folder>
    <name>park</name>
    <visibility>1</visibility>
    <styleUrl>#LayerFolder_check</styleUrl>
    <Placemark>
      <name>Ellesmere Island National Park Reserve</name>
      <styleUrl>#style_line_ff787878_w4.0_polygon_ff00ffc8_label_ff0000ff</styleUrl>
      <MultiGeometry>
        <Polygon>
          <outerBoundaryIs>
            <LinearRing>
              <coordinates>
              …
         <Point>
          <coordinates>
      -70.86810858,82.12291871
      </coordinates>
        </Point>
      </MultiGeometry>
    </Placemark>
</Folder>

Supported Features in the Placemark element are:

Palcemark element	Supported	MapServer equivalence/Notes
name	Yes	Label attached with the feature. If there is no label a default name is assigned using the layer name and the shape id (ex. park.1)
visibility	No	Is is by default set to true
open	No
address	No
AddessDetails	No
phoneNumber	No
Snippet	No	This is a short description the feature. If needed It could be supported.
description	Yes	This information is what appears in the description balloon when the user clicks on the feature. The <description> element supports plain text as well as a subset of HTML formatting elements.Two ways of defining the description:
AbstractView	No
TimePrimitive	No
styleUrl	Yes	Refers to a Style defined in the Document
StyleSelector	No
Region	No
Metadata	No
Geometry	Yes	Depends on the layer type

General notes on layers

• Labelcache is turned off on each layer
• Projection block should be set. If not set It will be assumed that the data is in lat/lon projection (a debug message will be sent to the user: Debug Message 1)
• Possible to output vector layers as raster using metadata: “KML_OUTPUTASRASTER” “true”
• The user can use the KML_FOLDER_DSIPLAY layer or map level metedata to choose a setting. Possible values are ‘check’ (default), ‘radioFolder’, ‘checkOffOnly’, ‘checkHideChildren’.
• The user can use metedata KML_DESCRIPTION or KML_INCLUDE_ITEMS to define the description attached to each feature.
• The user can use metedata KML_ALTITUDEMODE to specify how altitude components in the <coordinates> element are interpreted. Possible values are: absolute, relativeToGround, clampToGround. http://code.google.com/apis/kml/documentation/kmlreference.html#altitudemode
• The user can use metedata KML_EXTRUDE to specify whether to connect the LinearRing to the ground. http://code.google.com/apis/kml/documentation/kmlreference.html#tessellate
• The user can use metedata KML_TESSELLATE to specify whether to allow the LinearRing to follow the terrain. http://code.google.com/apis/kml/documentation/kmlreference.html#extrude

Point Layers

• Each layer will be inside a Folder element.
• Each feature will be represented by a Placemark.
• The Geometry element for a Point layer would be represented as a Point geometry element in Kml. Supported elements are:

Line Layers

• Each layer will be inside a Folder element.
• Each feature in the layer would be represented by a Placemark.
• If a label is attached to the line, the Geometry element would be represented as a MultiGeometry that includes a LineString element and a Point element representing the position of the label. If no label is attached to a feature, the Geometry element will be a LineString.

Polygon Layers

• Each layer will be inside a Folder element.
• Each feature will be represented by a Placemark.
• If a label is attached to the polygon, the Geometry element would be represented as a MultiGeometry that includes a Polygon element and a Point element representing the position of the label.

Annotation Layers

Not supported yet.

Raster Layers

• Each layer will be inside a Folder element.
• A GroundOverlay feature is created for the layer, that includes an href link to the raster image generated and LatLongBox for the extents (map extents).
• The href is generated using the imagepath and imageurl settings in the map file.

WMS Layers

Not supported yet.

Styling

As described in Section 4, all different styles from the layers will be stored at the Document level and referenced from the folders using a styleUrl.

Point Layers

Point layers will be styled using the IconStyle styling element of kml. An image representing the symbol will be created and referenced from the IconStyle object. If a label is attached to the point, a LabelStyle element will also be used. The LabelStyle will have the color parameter set.

<Style id="style_label_ff0000ff_symbol_star_13.0_ff000000">
    <IconStyle>
      <Icon>
        <href>>http://localhost/ms_tmp/4beab862_19bc_0.png</href>
      </Icon>
    </IconStyle>
    <LabelStyle>
      <color>ff0000ff</color>
    </LabelStyle>
</Style>

Line Layers

Line layers will be styled using the LineStyle styling element of kml. Color and width parameters of the LineStyle will be used. If a label is attached to the layer, a LabelStyle element will also be used.

Polygon Layers

Polygon layers will be styled using the PolyStyle styling element of kml. Color parameter of the PolyStyle will be used. If an outline is defined in the map file, an addition LineStyle will be used. If a label is attached to the layer, a LabelStyle element will also be used.

Attributes

As described in section on Layers, two ways of defining the description:

• kml_description
• kml_include_items

Coordinate system

The map level projection should be set to epsg:4326. If not set, the driver will automatically set it. Layers are expected to have projection block if their projection is different from epsg:4326.

Warning and Error Messages

• When the projection of the map file is not set or is different from a a lat/lon projection, the driver automatically sets the projection to espg:4326. If the map is is debug mode, the following message is sent: « “KmlRenderer::checkProjection: Mapfile projection set to epsg:4326 »
• If imagepath and imageurl are not set in the web object, the following message is sent in debug mode: « KmlRenderer::startNewLayer: imagepath and imageurl should be set in the web object »

Testing

Development is done in mapserver svn (http://svn.osgeo.org/mapserver/trunk/mapserver/). It was initially in http://svn.osgeo.org/mapserver/sandbox/davidK/

Documentation

Comments from Review period

• instead of KML_DUMPATTRIBUTES we could possibly use the gml/ows_include_items
• error messages should be clarified for the projection and exact wording should be put in the RFC
• clearly define what control the user will have over KML styling (the description balloon, icon styles, and Google Earth TOC)

Voting History

Adopted 2010/04/08 on the condition that this RFC should be cleaned and upadted with more details. This allows to move the source code from the sand box to trunk

• with +1 from DanielM, SteveL, TamasS, AssefaY, TomK, JeffM
• with +0 from SteveW