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SLD

Author:Jeff McKenna
Contact:jmckenna at gatewaygeomatics.com
Author:Yewondwossen Assefa
Contact:assefa at dmsolutions.ca
Last Updated:2011-01-14

Introduction

This document describes the procedures for taking advantage of the Styled Layer Descriptor (SLD) support in WMS GetMap requests with MapServer. SLD support exists for the server side (ability to read an SLD and apply it with a GetMap request) and for the client side (includes sending SLD requests to server and generate SLD files on the fly from MapServer map file). SLD support was added to MapServer in version 4.2.

This document assumes that you are already familiar with the following aspects of MapServer:

  • MapServer application development and setting up .map files.
  • Familiarity with the WMS specification would be an asset. Links to the MapServer WMS documents are included in the next section.

Server Side Support

General Information

There are two ways a WMS request can pass an SLD document with a GetMap request to MapServer:

  • SLD parameter pointing to remote SLD (SLD=http://URL_TO_SLD).
  • SLD_BODY parameter to send the SLD definition in the URL.

These two methods are both available through MapServer. An example of a request would be:

http://demo.mapserver.org/cgi-bin/wms?SERVICE=wms&VERSION=1.1.1&REQUEST=GetMap
&LAYERS=country_bound
s&SLD=http://demo.mapserver.org/ogc-demos/map/sld/sld_line_simple.xml

Test the remote SLD request.

The SLD in the above request follows:

<?xml version="1.0" encoding="UTF-8"?>
<StyledLayerDescriptor version="1.0.0"
  xmlns="http://www.opengis.net/sld"
  xmlns:ogc="http://www.opengis.net/ogc"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.opengis.net/sld
  http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
  <NamedLayer>
    <Name>country_bounds</Name>
    <UserStyle>
      <Title>xxx</Title>
      <FeatureTypeStyle>
        <Rule>
          <LineSymbolizer>
            <Geometry>
              <ogc:PropertyName>center-line</ogc:PropertyName>
            </Geometry>
            <Stroke>
              <CssParameter name="stroke">#0000ff</CssParameter>
            </Stroke>
          </LineSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

Version 1.1.0 of the same SLD

<?xml version="1.0" encoding="UTF-8"?>
<StyledLayerDescriptor version="1.1.0"
  xmlns="http://www.opengis.net/sld"
  xmlns:se="http://www.opengis.net/se"
  xmlns:ogc="http://www.opengis.net/ogc"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.opengis.net/sld
  http://schemas.opengis.net/sld/1.1.0/StyledLayerDescriptor.xsd">
<NamedLayer>
  <se:Name>country_bounds</se:Name>
    <UserStyle>
      <se:Name>xxx</se:Name>
      <se:FeatureTypeStyle>
        <se:Rule>
          <se:LineSymbolizer>
            <se:Geometry>
              <ogc:PropertyName>center-line</ogc:PropertyName>
            </se:Geometry>
            <se:Stroke>
              <se:SvgParameter name="stroke">#0000ff</se:SvgParameter>
            </se:Stroke>
          </se:LineSymbolizer>
        </se:Rule>
      </se:FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

When MapServer gets a valid SLD through a request, it parses this SLD to extract all the styles attached to the NamedLayers, and then it applies these styles to the map before it is returned to the client. When applying the SLD, MapServer compares the <Name> parameter of the NamedLayers in the SLD document to the WMS layer names (WMS layer names are available in a GetCapabilities request).

Note

All the examples given in this document are live uses of valid SLDs and a MapServer installation with SLD support.

Additional WMS features related to SLDs have also been developed:

Table1. Additional WMS Features

Features Supported Notes
Method GET : SLD URL Yes  
Method GET : SLD_BODY Yes Additional item
Describer Layer Yes  
GetLegendGraphic Yes  
GetStyles Yes Uses MapScript to get the SLD

Note

As of MapServer version 4.2.3, the GetLegendGraphic request (see section 12 of the Styled Layer Descriptor Implementation Specification) works as follows: if the RULE keyword is absent from the request, an image containing the entire legend for the specified layer will be returned. This image consists of the layer name and a symbolization graphic and label for each class.

Specific SLD Elements Supported

The following tables give a lot of additional details about SLD support in MapServer.

Table2. Named Layers and User Layers

Features Supported Notes
Named Layers Yes  
User Layers No  

Table3. Named Styles and User Styles

Features Supported Notes
Named Styles Yes  
User Styles Yes  

Table 4. User Styles

Features Supported Notes
Name No This was removed at implementation time, since it does not fit with MapServer
Title No No use in the MapServer environment
Abstract No No use in the MapServer environment
IsDefault No Only one style is available per layer
FeatureTypeStyle Yes MapServer has a concept of one feature type style per layer (either point, line, polygon, or raster)

Table 5. FeatureTypeStyle

Features Supported Notes
Name No No use in the MapServer environment
Title No No use in the MapServer environment
Abstract No No use in the MapServer environment
FeatureTypeName No No use in the MapServer environment
SemanticTypeIdentifier No Still an experimental element in the SLD specifications
Rule Yes  

Table 6. Rule

Features Supported Notes
Name Yes  
Title Yes  
Abstract No No use in the MapServer environment
LegendGraphic Yes  
Filter Yes  
ElseFilter Yes  
MinScaleDenominator Yes  
MaxScaleDenominator Yes  
LineSymbolizer Yes  
PolygonSymbolizer Yes  
PointSymbolizer Yes  
TextSymbolizer Yes  
RasterSymbolizer Yes Applies for 8-bit rasters
  • Filter and ElseFilter

    For each rule containing a filter, there is a class created with the class expression set to reflect that filter. Available filters that can be used are Comparison Filters and Logical Filters (see the Filter Encoding HowTo). The ElseFilter parameters are converted into a class in MapServer and placed at the end of the class list with no expression set. They are used to render elements that did not fit into any other classes.

  • MinScaleDenomibator and MaxScaleDenominator are translated in minscale and maxscale in MapServer.

The following are examples of valid requests using the Filters:

This example enables spatial filtering using the BBOX parameter as a Filter for a selected area (Africa). Note that an ElseFilter will not work with a spatial filter.

Table 7. LineSymbolizer

Features Supported Notes
Geometry No MapServer uses the data geometry for the rendering
Stroke: GraphicFill No Solid color is used
Stroke: GraphicStroke Yes Draws the symbol along the line
Stroke (CssParameter): stroke Yes RGB colors are supported
Stroke (CssParameter): width Yes  
Stroke (CssParameter): opacity Yes Only for AGG driver and mapserver version >=5.2
Stroke (CssParameter): linejoin and linecap No Not supported in MapServer
Stroke (CssParameter): dasharray Yes  
Stroke (CssParameter): dashoffset No  
PerperdicularOffset (only in SLD 1.1.0) Yes Offset values of the style object will be set
InitialGap(GraphicStroke SLD 1.1.0) No  
Gap (GraphicStroke parameter SLD 1.1.0) No  

Note

SvgParameter instead of CssParameter are required for SLD 1.1.0.

The following are examples of valid requests using the LineSymbolizer:

Table 8. PolygonSymbolizer

Features Supported Notes
Geometry No  
Stroke Yes Strokes are the same as for the LineSymbolizer
Fill Yes Was developed to support symbol fill polygons in addition to solid fill
Fill-opacity Yes Only available for AGG driver and mapserver version >=5.2
PerperdicularOffset No SLD 1.1.0 parameter
Displacement Yes SLD 1.1.0 parameter. Sets offsetx/y in MapServer

A Fill can be a solid fill or be a Graphic Fill, which is either a well-known Mark symbol (e.g., square, circle, triangle, star, cross, x) or an ExternalGraphic element (e.g., gif, png) available through a URL. When a Mark symbol is used in an SLD, MapServer creates a corresponding symbol in the map file and uses it to render the symbols. When a ExternalGraphic is used, the file is saved locally and a pixmap symbol is created in the mapfile referring to the this file.

Note

The Web object IMAGEPATH is used to save the file.

The following are examples of valid requests using the PolygonSymbolizer:

Table 9. PointSymbolizer

Features Supported Notes
Geometry No  
Graphic: Mark symbol Yes Well-known names (square, circle, triangle, star, cross, X) are supported
Graphic: ExternalGraphic Yes Was developed to support symbol fill polygons in addition to solid fill
Opacity Yes Support added in MapServer 5.4
Size Yes  
Rotation Yes Support added in MapServer 5.4
Displacement Yes SLD 1.1.0 Paramater. Support added in MapServer 5.4
AnchorPoint No  

Note

Refer to the PolygonSymbolizer notes for how the Mark and ExternalGraphic symbols are applied in MapServer.

The following are examples of valid requests using the PointSymbolizer:

Table 10. TextSymbolizer

Features Supported Notes
Geometry No  
Label Yes  
Font(font-family) Yes Font names used are those available in MapServer font file. If no fonts are available there, default bitmap fonts are used
Font-style (Italic, ...) Yes  
Font-weight Yes  
Font-size Yes If true-type fonts are not used, default bitmap sizes are given
LabelPlacement Yes PointPlacement is supported. LinePlacement is supported for versions >=5.2.1. Only PerperdicularOffset and IsAligned are supported for LinePlacement.
Halo Yes Supported (fill converted to outlinecolor, and radius is converted to outlinewidth. Note that outlinewidth is only available for AGG in >=5.2)
Fill Yes Only solid color is available

Notes on the TextSymbolizer:

  • Font names: when converting Font parameters to MapServer, the following rule is applied to get the font name: FontFamily-FontStyle-FontWeight. For example, if there is an SLD with a Font Family of arial, a Font Style of italic, and a Font weight equal to bold, the resulting MapServer font name is arial-bold-italic. Font Style and Weight are not mandatory and, if not available, they are not used in building the font name. When a Font Style or a Font Weight is set to normal in an SLD, it is also ignored in building the name. For example, if there is an SLD with a Font Family of arial, a Font Style of normal and a Font weight equals to bold, the resulting MapServer font name is arial-bold.
  • A TextSymbolizer can be used in MapServer on a Point, Line, or Polygon layer - in addition to other symbolizers used for these layers.
  • PointPacement: a point placement includes AnchorPoint (which is translated to Position in MapServer) Displacement (which is translated to Offset) and Angle (which is translated to Angle).
  • Angle setting (MapServer version >=5.4): by default the angle parameter is set to AUTO. For point features, users can use the PointPlacement to alter the value. For line features, the user can add a LinePlacement: If an ‘empty’ LinePlacement is part of the SLD, the angle will be set to FOLLOW, If a LinePlacement contains the PerpendicualarOffset parameter, the angle will be set to 0 and the PerpendicualarOffset will be used to set the offset values in the label object. SLD 1.1.0 introduces the IsAligned parameter for LinePlacement: if this parameter is set to false, the angle will be set to 0.

The following are examples of valid requests using the TextSymbolizer:

Table 11. RasterSymbolizer

Features Supported Notes
Geometry No  
Opacity Yes  
ChannelSelection No  
OverlapBehaviour No  
ColorMap Yes  
ContrastEnhancement No  
ShadedRelief No  
ImageOutline No  

The current support in MapServer includes only ColorMap parameter support. It can be used to classify 8-bit rasters. Inside the ColorMap parameters, the color and quantity parameters are extracted and used to do the classification.

Table 12. ColorMap

The following Features are available in SLD 1.0

Features Supported Notes
Color Yes  
Opacity No  
Quantity Yes  
Label No  

The following is an example of ColorMap usage for SLD 1.0.

If we have following ColorMap in an SLD:

<ColorMap>
  <ColorMapEntry color="#00ff00" quantity="22"/>
  <ColorMapEntry color="#00bf3f" quantity="30"/>
  <ColorMapEntry color="#007f7f" quantity="37"/>
  <ColorMapEntry color="#003fbf" quantity="45"/>
  <ColorMapEntry color="#0000ff" quantity="52"/>
  <ColorMapEntry color="#000000" quantity="60"/>
</ColorMap>

The six classes that are created are:

class 1: [pixel] >= 22 AND [pixel] < 30 with color 00ff00
class 2: [pixel] >= 30 AND [pixel] < 37 with color 00bf3f
class 3: [pixel] >= 37 AND [pixel] < 45 with color 007f7f
class 4: [pixel] >= 45 AND [pixel] < 52 with color 003fbf
class 5: [pixel] >= 52 AND [pixel] < 60 with color 0000ff
class 6: [pixel] = 60 with color 000000

Note

The ColorMapEntry quantity parameters should be in increasing order.

The following Features are available in SLD 1.1

Features Supported Notes
Categorize Yes  

The following is an example of and SLD 1.1.0 with a raster symbolizer

<StyledLayerDescriptor version="1.1.0" xsi:schemaLocation="http://www.opengis.net/sld
 http://schemas.opengis.net/sld/1.1.0/StyledLayerDescriptor.xsd"
 xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc"
 xmlns:se="http://www.opengis.net/se" xmlns:xlink="http://www.w3.org/1999/xlink"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<NamedLayer>
<se:Name>landsat</se:Name>
<UserStyle>
<se:Name>xxx</se:Name>
<se:FeatureTypeStyle>
<se:Rule>
<se:RasterSymbolizer>
<se:Opacity>0.7</se:Opacity>
<se:ColorMap>
<se:Categorize fallbackValue="#78c818">
<se:LookupValue>Rasterdata</se:LookupValue>
<se:Value>#ffffff</se:Value>
<se:Threshold>22</se:Threshold>
<se:Value>#00ff00</se:Value>
<se:Threshold>30</se:Threshold>
<se:Value>#00bf3f</se:Value>
<se:Threshold>37</se:Threshold>
<se:Value>#007f7f</se:Value>
<se:Threshold>45</se:Threshold>
<se:Value>#003fbf</se:Value>
<se:Threshold>52</se:Threshold>
<se:Value>#0000ff</se:Value>
<se:Threshold>60</se:Threshold>
<se:Value>#000000</se:Value>
</se:Categorize>
</se:ColorMap>
</se:RasterSymbolizer>
</se:Rule>
</se:FeatureTypeStyle>
</UserStyle>
</NamedLayer>
</StyledLayerDescriptor>

The classes that are created are:

class 1: [pixel] < 22 with color ffffff
class 2: [pixel] >= 22 AND [pixel] < 30 with color 00ff00
class 3: [pixel] >= 30 AND [pixel] < 37 with color 00bf3f
class 4: [pixel] >= 37 AND [pixel] < 45 with color 007f7f
class 5: [pixel] >= 45 AND [pixel] < 52 with color 003fbf
class 6: [pixel] >= 52 AND [pixel] < 60 with color 0000ff
class 7: [pixel] >= 60 with color 000000

Examples using 8 bits and 16 bits rasters can be seen at:

Client Side Support

Client side support of the SLD consists of two parts:

  • The first part is using MapServer as a WMS client to send a GetMap request with an SLD. This is done using two metadata that can be placed at a layer level in a MapServer mapfile. These two metadata are:

    • wms_sld_url, which takes a valid URL as a value and appends SLD=xxx to the GetMap request.
    • wms_sld_body, which takes a valid SLD string and appends SLD_BODY=xxx to the GetMap request. If the value of wms_sld_body is set to AUTO, MapServer generates an SLD based on the classes found in the layer and send this SLD as the value of the SLD_BODY parameter in the GetMap request.
  • The other major item is the generation of an SLD document from MapServer classes. These functions are currently available through MapServer/MapScript interface. Here are the functions available:

    • on a map object: generatesld
    • on a layer object: generatesld

    Additional MapScript functions have been added or will be added to complement these functions:

    • on a map object: applysld
    • on a layer object: applysld

Note

When generating an SLD from MapServer classes, if there is a pixmap symbol you need to have this symbol available through a URL so it can be converted as an ExternalGraphic symbol in the SLD. To do this, you need to define the URL through a web object level metadata called WMS_SLD_SYMBOL_URL in your map file. The SLD generated uses this URL and concatenates the name of the pixmap symbol file to get the value that is generated as the ExternaGraphic URL.

PHP/MapScript Example that Generates an SLD from a Mapfile

The following is a small script that calls the generateSLD() function to create an SLD for a specific layer in a mapfile:

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<?php

// define variables
define( "MAPFILE", "D:/ms4w/apps/cadastra/map/cadastra.map" );
define( "MODULE", "php_mapscript.dll" );

// load the mapscript module
if (!extension_loaded("MapScript")) dl(MODULE);

// open map
$oMap = ms_newMapObj( MAPFILE );

// get the parcel layer
$oLayer = $oMap->getLayerByName("parcel");

// force visibilty of the layer
$oLayer->set('status', MS_ON);

// generate the sld for that layer
$SLD = $oLayer->generateSLD();

// save sld to a file
$fp = fopen("parcel-sld.xml", "a");
fputs( $fp, $SLD );
fclose($fp);

?>

Named Styles support

Named styles support are introduced in MapServer 5.2. The support is base on MS RFC 39: Support of WMS/SLD Named Styles.

MapServer 5.2 introduces the possibility to assign a group to a series of classes defined on a layer object using two new non-mandatory keywords CLASSGROUP (at the layer level) and GROUP at the class level:

LAYER
    ...
    CLASSGROUP "group1"
    ...
    CLASS
        NAME "name1"
        GROUP "group1"
        ...
    END
    CLASS
        NAME "name2"
        GROUP "group2"
        ...
    END
    CLASS
        NAME "name3"
        GROUP "group1"
        ...
    END
    ...

At rendering time, if the CLASSGROUP is defined, only classes that have the same group name would be used. Based on this concept, WMS/SLD support uses the class groups as named styles. Each group of classes is considered equivalent to a named style:

  • The GetCapbilities request will output all the styles that are available
  • The GetMap request can use the STYLES parameter to specify a named style
  • The GetLegendGraphic can use the STYLES parameter to specify a named style

Other Items Implemented

  • Support of filled polygons with Mark and ExternalGraphic symbols.
  • MapScript functions to parse and apply SLD.
  • SLD_BODY request support on client and server side.

Issues Found During Implementation

  • Limitation of the FilterEncoding to comparison and logical filters. The spatial filters were not made available since it required major changes in MapServer WMS support.