Specifying symbolizer sizes in ground units

The SLD 1.0 specification allows giving symbolizer sizes in a single unit of measure: pixels. This means that the size of symbolizers is the same at all zoom levels (which is usually the desired behaviour).

The Symbology Encoding 1.1 specification provides a uom attribute on Symbolizer elements. This allows specifying styling parameter sizes in ground units of metres or feet, as well as the default which is screen pixels. When ground units are used, the screen size of styled elements increases as the map is zoomed in to larger scales. GeoServer supports the SE 1.1 uom attribute in its extended SLD 1.0 support.

Note

This extended feature is officially supported in GeoServer 2.1.0. It is available in GeoServer 2.0.3 if the -DenableDpiUomRescaling=true system variable is specified for the JVM.

The value of the uom attribute is a URI indicating the desired unit. The units of measure supported are those given in the SE 1.1 specification:

http://www.opengeospatial.org/se/units/metre
http://www.opengeospatial.org/se/units/foot
http://www.opengeospatial.org/se/units/pixel

Note

The px override modifier for parameters values is not currently supported.

Example

The following SLD shows the uom attribute used to specify the width of a LineSymbolizer in metres:

<?xml version="1.0" encoding="ISO-8859-1"?>
<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">
  <NamedLayer>
    <Name>5m blue line</Name>
    <UserStyle>
      <Title>tm blue line</Title>
      <Abstract>Default line style, 5m wide blue</Abstract>

      <FeatureTypeStyle>
        <Rule>
          <Title>Blue Line, 5m large</Title>
          <LineSymbolizer uom="http://www.opengeospatial.org/se/units/metre">
            <Stroke>
              <CssParameter name="stroke">#0000FF</CssParameter>
              <CssParameter name="stroke-width">5</CssParameter>
            </Stroke>
          </LineSymbolizer>
        </Rule>

      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

Applying the style to the tiger:tiger_roads dataset shows how the line widths increase as the map is zoomed in:

../../../_images/roads_uom1.png
../../../_images/roads_uom2.png
../../../_images/roads_uom3.png
Previous: Variable substitution in SLD
Next: Label Obstacles