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Move svg:transform mapping to its own spec
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| <h1>svg:transform for mapping</h1> | ||
| <pre class='metadata'> | ||
| Level: 1 | ||
| Status: ED | ||
| ED: http://dev.w3.org/fxtf/motion-1/ | ||
| Shortname: transform-mapping | ||
| Group: svg | ||
| Editor: Satoru Takagi, KDDI Corporation, sa-takagi@kddi.com | ||
| Abstract: This specification gives guidance how the SVG transform attribute in the SVG namespace can be used in other context than SVG. | ||
| </pre> | ||
| <pre class=link-defaults> | ||
| spec:svg2; type:element; text:svg | ||
| spec:css-transforms-1; type:property; text:transform | ||
| spec:css-transforms-1; type:type; text:<transform-list> | ||
| </pre> | ||
|
|
||
| <h2 id="GeographicCoordinates">Geographic coordinate systems</h2> | ||
|
|
||
| In order to allow interoperability between SVG content generators | ||
| and user agents dealing with maps encoded in SVG, the use of a common | ||
| metadata definition for describing the coordinate system used to | ||
| generate SVG documents is encouraged. | ||
|
|
||
| Such metadata must be added under the <a element>metadata</a> element of | ||
| the topmost <a element>svg</a> element describing the map, consisting of an | ||
| RDF description of the Coordinate Reference System definition used to | ||
| generate the SVG map [[RDF-PRIMER]]. Note that | ||
| the presence of this metadata does not affect the rendering of the SVG | ||
| in any way; it merely provides added semantic value for applications | ||
| that make use of combined maps. | ||
|
|
||
| The definition must be conformant to the XML grammar described in | ||
| <a href="http://portal.opengeospatial.org/files/?artifact_id=20509"><cite>GML 3.2.1</cite></a>, | ||
| an OpenGIS Standard for encoding common CRS data types in XML | ||
| [<a href='refs.html#ref-GML'>GML</a>]. In order to correctly map | ||
| the 2-dimensional data used by SVG, the CRS must be of subtype | ||
| <strong>ProjectedCRS</strong> or <strong>Geographic2dCRS</strong>. The | ||
| first axis of the described CRS maps the SVG <var>x</var>-axis and the | ||
| second axis maps the SVG <var>y</var>-axis. | ||
|
|
||
| The main purpose of such metadata is to indicate to the user agent | ||
| that two or more SVG documents can be overlayed or merged into a single | ||
| document. Obviously, if two maps reference the same Coordinate Reference | ||
| System definition and have the same SVG 'transform' property | ||
| value then they can be overlayed without reprojecting the data. If | ||
| the maps reference different Coordinate Reference Systems and/or have | ||
| different SVG 'transform' property values, then a specialized | ||
| cartographic user agent may choose to transform the coordinate data to | ||
| overlay the data. However, typical SVG user agents are not required | ||
| to perform these types of transformations, or even recognize the | ||
| metadata. It is described in this specification so that the connection | ||
| between geographic coordinate systems and the SVG coordinate system is | ||
| clear. | ||
|
|
||
| <h2 id="SVGGlobalTransformAttribute">The <span class="attr-name">'svg:transform'</span> attribute</h2> | ||
|
|
||
| <div class="adef-list"> | ||
| <p><em>Attribute definition:</em></p> | ||
| <dl> | ||
| <dt id="SVGGlobalTransformAttributeDefinition"><dfn>svg:transform</dfn> = <<transform-list>> | ''transform/none''</dt> | ||
| <dd> | ||
| <dl> | ||
| <dt><<transform-list>></dt> | ||
| <dd> | ||
| <p>Specifies the affine transformation that has been | ||
| applied to the map data. The syntax is identical to | ||
| that described for the 'transform' property.</p> | ||
| </dd> | ||
|
|
||
| <dt><span class="attr-value">none</span></dt> | ||
| <dd> | ||
| <p>Specifies that no supplemental affine transformation has been | ||
| applied to the map data. Using this value has the same meaning as | ||
| specifying the identity matrix, which in turn is just the same as | ||
| not specifying the <a>svg:transform</a> the attribute at all.</p> | ||
| </dd> | ||
| </dl> | ||
| <p class="anim-target">Animatable: no.</p> | ||
| </dd> | ||
| </dl> | ||
| </div> | ||
|
|
||
| This attribute describes an optional additional affine | ||
| transformation that may have been applied during this | ||
| mapping. This attribute may be added to the OpenGIS | ||
| <em>CoordinateReferenceSystem</em> element. Note | ||
| that, unlike the 'transform' property, it does not indicate that | ||
| a transformation is to <em>be applied</em> to the data within the file. | ||
| Instead, it simply describes the transformation that <em>was already | ||
| applied</em> to the data when being encoded in SVG. | ||
|
|
||
| There are three typical uses for the <a>svg:transform</a> | ||
| global attribute. These are described below and used in the examples. | ||
|
|
||
| <ul> | ||
| <li>Most ProjectedCRS have the north direction represented by | ||
| positive values of the second axis and conversely SVG has a | ||
| <var>y</var>-down coordinate system. That's why, in order to follow the | ||
| usual way to represent a map with the north at its top, it is | ||
| recommended for that kind of ProjectedCRS to use the | ||
| <a>svg:transform</a> | ||
| global attribute with a ''scale(1, -1)'' value as in the | ||
| third example below.</li> | ||
|
|
||
| <li>Most Geographic2dCRS have the latitude as their first | ||
| axis rather than the longitude, which means that the | ||
| south-north axis would be represented by the <var>x</var>-axis in SVG | ||
| instead of the usual <var>y</var>-axis. That's why, in order to follow | ||
| the usual way to represent a map with the north at its top, | ||
| it is recommended for that kind of Geographic2dCRS to use the | ||
| <a>svg:transform'</a> | ||
| global attribute with a ''rotate(-90)'' value as in the | ||
| first example (while also adding the ''scale(1, -1)'' as for | ||
| ProjectedCRS).</li> | ||
|
|
||
| <li>In addition, when converting for profiles which place | ||
| restrictions on precision of real number values, it may be | ||
| useful to add an additional scaling factor to retain good | ||
| precision for a specific area. When generating an SVG | ||
| document from WGS84 geographic coordinates (EPGS 4326), we | ||
| recommend the use of an additional 100 times scaling factor | ||
| corresponding to an <a>svg:transform</a> | ||
| global attribute with a ''rotate(-90) scale(100)'' | ||
| value (shown in the second example). | ||
| Different scaling values may be required depending on the | ||
| particular CRS.</li> | ||
| </ul> | ||
|
|
||
| Below is a simple example of the coordinate metadata, which | ||
| describes the coordinate system used by the document via a | ||
| URI. | ||
|
|
||
| <div class="example"> | ||
| <pre><code class=html> | ||
| <?xml version="1.0"?> | ||
| <svg xmlns="http://www.w3.org/2000/svg" | ||
| width="100" height="100" viewBox="0 0 1000 1000"> | ||
|
|
||
| <desc>An example that references coordinate data.</desc> | ||
|
|
||
| <metadata> | ||
| <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" | ||
| xmlns:crs="http://www.ogc.org/crs" | ||
| xmlns:svg="http://www.w3.org/2000/svg"> | ||
| <rdf:Description rdf:about=""> | ||
| <!-- The Coordinate Reference System is described | ||
| through a URI. --> | ||
| <crs:CoordinateReferenceSystem | ||
| svg:transform="rotate(-90)" | ||
| rdf:resource="http://www.example.org/srs/epsg.xml#4326"/> | ||
| </rdf:Description> | ||
| </rdf:RDF> | ||
| </metadata> | ||
|
|
||
| <!-- The actual map content --> | ||
| </svg> | ||
| </code></pre> | ||
| </div> | ||
|
|
||
| The second example uses a well-known identifier to describe | ||
| the coordinate system. Note that the coordinates used in the | ||
| document have had the supplied transform applied. | ||
|
|
||
| <div class="example"> | ||
| <pre><code class=html> | ||
| <?xml version="1.0"?> | ||
| <svg xmlns="http://www.w3.org/2000/svg" | ||
| width="100" height="100" viewBox="0 0 1000 1000"> | ||
|
|
||
| <desc>Example using a well known coordinate system.</desc> | ||
|
|
||
| <metadata> | ||
| <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" | ||
| xmlns:crs="http://www.ogc.org/crs" | ||
| xmlns:svg="http://www.w3.org/2000/svg"> | ||
| <rdf:Description rdf:about=""> | ||
| <!-- In case of a well-known Coordinate Reference System | ||
| an 'Identifier' is enough to describe the CRS --> | ||
| <crs:CoordinateReferenceSystem svg:transform="rotate(-90) scale(100, 100)"> | ||
| <crs:Identifier> | ||
| <crs:code>4326</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| </crs:CoordinateReferenceSystem> | ||
| </rdf:Description> | ||
| </rdf:RDF> | ||
| </metadata> | ||
|
|
||
| <!-- The actual map content --> | ||
| </svg> | ||
| </code></pre> | ||
| </div> | ||
|
|
||
| The third example defines the coordinate system completely | ||
| within the SVG document. | ||
|
|
||
| <div class="example"> | ||
| <pre><code class=html> | ||
| <?xml version="1.0"?> | ||
| <svg xmlns="http://www.w3.org/2000/svg" | ||
| width="100" height="100" viewBox="0 0 1000 1000"> | ||
|
|
||
| <desc>Coordinate metadata defined within the SVG document</desc> | ||
|
|
||
| <metadata> | ||
| <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" | ||
| xmlns:crs="http://www.ogc.org/crs" | ||
| xmlns:svg="http://www.w3.org/2000/svg"> | ||
| <rdf:Description rdf:about=""> | ||
| <!-- For other CRS it should be entirely defined --> | ||
| <crs:CoordinateReferenceSystem svg:transform="scale(1,-1)"> | ||
| <crs:NameSet> | ||
| <crs:name>Mercator projection of WGS84</crs:name> | ||
| </crs:NameSet> | ||
| <crs:ProjectedCRS> | ||
| <!-- The actual definition of the CRS --> | ||
| <crs:CartesianCoordinateSystem> | ||
| <crs:dimension>2</crs:dimension> | ||
| <crs:CoordinateAxis> | ||
| <crs:axisDirection>north</crs:axisDirection> | ||
| <crs:AngularUnit> | ||
| <crs:Identifier> | ||
| <crs:code>9108</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| </crs:AngularUnit> | ||
| </crs:CoordinateAxis> | ||
| <crs:CoordinateAxis> | ||
| <crs:axisDirection>east</crs:axisDirection> | ||
| <crs:AngularUnit> | ||
| <crs:Identifier> | ||
| <crs:code>9108</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| </crs:AngularUnit> | ||
| </crs:CoordinateAxis> | ||
| </crs:CartesianCoordinateSystem> | ||
| <crs:CoordinateReferenceSystem> | ||
| <!-- the reference system of that projected system is | ||
| WGS84 which is EPSG 4326 in EPSG codeSpace --> | ||
| <crs:NameSet> | ||
| <crs:name>WGS 84</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>4326</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| </crs:CoordinateReferenceSystem> | ||
| <crs:CoordinateTransformationDefinition> | ||
| <crs:sourceDimensions>2</crs:sourceDimensions> | ||
| <crs:targetDimensions>2</crs:targetDimensions> | ||
| <crs:ParameterizedTransformation> | ||
| <crs:TransformationMethod> | ||
| <!-- the projection is a Mercator projection which is | ||
| EPSG 9805 in EPSG codeSpace --> | ||
| <crs:NameSet> | ||
| <crs:name>Mercator</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>9805</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| <crs:description>Mercator (2SP)</crs:description> | ||
| </crs:TransformationMethod> | ||
| <crs:Parameter> | ||
| <crs:NameSet> | ||
| <crs:name>Latitude of 1st standart parallel</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>8823</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| <crs:value>0</crs:value> | ||
| </crs:Parameter> | ||
| <crs:Parameter> | ||
| <crs:NameSet> | ||
| <crs:name>Longitude of natural origin</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>8802</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| <crs:value>0</crs:value> | ||
| </crs:Parameter> | ||
| <crs:Parameter> | ||
| <crs:NameSet> | ||
| <crs:name>False Easting</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>8806</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| <crs:value>0</crs:value> | ||
| </crs:Parameter> | ||
| <crs:Parameter> | ||
| <crs:NameSet> | ||
| <crs:name>False Northing</crs:name> | ||
| </crs:NameSet> | ||
| <crs:Identifier> | ||
| <crs:code>8807</crs:code> | ||
| <crs:codeSpace>EPSG</crs:codeSpace> | ||
| <crs:edition>5.2</crs:edition> | ||
| </crs:Identifier> | ||
| <crs:value>0</crs:value> | ||
| </crs:Parameter> | ||
| </crs:ParameterizedTransformation> | ||
| </crs:CoordinateTransformationDefinition> | ||
| </crs:ProjectedCRS> | ||
| </crs:CoordinateReferenceSystem> | ||
| </rdf:Description> | ||
| </rdf:RDF> | ||
| </metadata> | ||
|
|
||
| <!-- the actual map content --> | ||
| </svg> | ||
| </code></pre> | ||
| </div> |
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