2.2. Style

The design choices made to represent content is a key aspect of cartography. The style used when rendering data into a visualisation is the result of these choices.

The Open Geospatial Consortium standard for recording style is divided into two parts:

  • Symbology Encoding (SE): Records a “feature type style” documenting how individual features are drawn using a series of rules.

  • Style Layer Descriptor (SLD): Records which “feature type styles” may be used with a layer.

The Symbology Encoding standard provides the terms we will be using to describe style:

  • Stroke: borders and outlines of shapes

  • Fill: interior of shapes

2.2.1. Line symbolizer

A line symbolizer documents how individual strokes are used to draw a line string, including color and line width.

../../../_images/LineSymbolizer.svg

The SLD specification provides a default stroke used when drawing line strings. These values for color and width will be used if needed.

<LineSymbolizer>
  <Stroke/>
</LineSymbolizer>

GeoServer includes a default line.sld file providing a blue stroke. This file is used when you initially set up a linestring layer.

From GeoServer’s line.sld style:

<LineSymbolizer>
  <Stroke>
    <CssParameter name="stroke">#0000FF</CssParameter>
  </Stroke>
</LineSymbolizer>

2.2.2. Polygon symbolizer

A polygon symbolizer documents both the the stroke in addition to the fill used to draw a polygon. A fill can consist of a color, pattern, or other texture:

The SLD specification provides a default gray fill, but does not supply a stroke. These values will be used if you do not provide an alternative.

../../../_images/PolygonSymbolizer.svg

GeoServer includes a default polygon.sld file providing a gray fill and a black outline. This file will be used when you initially create a polygon layer.

From GeoServer’s polygon.sld style:

<PolygonSymbolizer>
  <Fill>
    <CssParameter name="fill">#AAAAAA</CssParameter>
  </Fill>
  <Stroke>
    <CssParameter name="stroke">#000000</CssParameter>
    <CssParameter name="stroke-width">1</CssParameter>
  </Stroke>
</PolygonSymbolizer>

2.2.3. Point symbolizer

A point symbolizer documents the “mark” used to represent a point. A mark may be defined by a glyph (icon) or a common mark (circle, square, etc.). The point symbolizer records the stroke and fill used to draw the mark.

../../../_images/PointSymbolizer.svg

From GeoServer’s default point.sld style:

<PointSymbolizer>
  <Graphic>
    <Mark>
      <WellKnownName>square</WellKnownName>
      <Fill>
        <CssParameter name="fill">#FF0000</CssParameter>
      </Fill>
    </Mark>
    <Size>6</Size>
  </Graphic>
</PointSymbolizer>

2.2.4. Text symbolizer

A text symbolizer provides details on how labels are to be drawn, including font, size, and color information.

../../../_images/TextSymbolizer.svg

From the populated_places.sld style:

<sld:TextSymbolizer>
    <sld:Label>
        <ogc:PropertyName>NAME</ogc:PropertyName>
    </sld:Label>
    <sld:Font>
        <sld:CssParameter name="font-family">Arial</sld:CssParameter>
        <sld:CssParameter name="font-size">10.0</sld:CssParameter>
        <sld:CssParameter name="font-style">normal</sld:CssParameter>
        <sld:CssParameter name="font-weight">bold</sld:CssParameter>
    </sld:Font>
    <sld:Halo>
        <sld:Radius>1</sld:Radius>
        <sld:Fill>
            <sld:CssParameter name="fill">#FFFFFF</sld:CssParameter>
        </sld:Fill>
    </sld:Halo>
    <sld:Fill>
        <sld:CssParameter name="fill">#000000</sld:CssParameter>
    </sld:Fill>
</sld:TextSymbolizer>

Note

The Style Layer Descriptor standard makes use of the Filter Encoding specification to create small expressions as shown above to access the NAME of each city:

<ogc:PropertyName>NAME</ogc:PropertyName>

This same approach can be used to dynamically generate any values needed for styling.

2.2.5. Raster symbolizer

A raster symbolizer provides a mapping from raster values to colors displayed. This can be provided by a color table, function, or directly mapping bands of data to use for the display channels.

From GeoServer’s dem.sld style:

<RasterSymbolizer>
  <Opacity>1.0</Opacity>
  <ColorMap>
    <ColorMapEntry color="#000000" quantity="-500" label="nodata" opacity="0.0" />
    <ColorMapEntry color="#AAFFAA" quantity="0" label="values" />
    <ColorMapEntry color="#00FF00" quantity="1000"/>
    <ColorMapEntry color="#FFFF00" quantity="1200" label="values" />
    <ColorMapEntry color="#FF7F00" quantity="1400" label="values" />
    <ColorMapEntry color="#BF7F3F" quantity="1600" label="values" />
    <ColorMapEntry color="#000000" quantity="2000" label="values" />
  </ColorMap>
</RasterSymbolizer>
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