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Building a Alpha-by-Value MapΒΆ

The idea behind alpha-by-value maps is very simple (even if the paper runs to 11 pages) we just make a polygon more transparent in proportion to the population that is contained in it. Here is the same map as before but with the transparency adjusted in proportion to population (again click to explore it).

../_images/alpha.png

Now this is exactly the same data set as in the previous map but now our eye is drawn to California and Texas as they are more populous while Nebraska and friends have faded into the background (quite literally). Even Michigan is less obvious now.

If you look at the SLD below you can see there is one key difference between this file and the previous one. I am now calculating the opacity using a function.

Lines 39-44 (and in the other classes) divides the population of the state (the PERSONS attribute) by 30 million (just higher than California’s population) and then uses this value (between 0 and 1) as the opacity value when rendering the polygon.

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<?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" 
  xmlns:gml="http://www.opengis.net/gml"
  xsi:schemaLocation="http://www.opengis.net/sld 
  http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
  <NamedLayer>
    <Name>USA states unemployment</Name>
    <UserStyle>
      <Name>unemployment</Name>
      <Title>unemployment in the United States</Title>
      <Abstract>A sample filter that filters the United States into three
        categories of unemployment, drawn in different colors</Abstract>
      <FeatureTypeStyle>
        <Rule>
          <Title>&lt; 5%</Title>
          <ogc:Filter>
            <ogc:PropertyIsLessThan>
              <ogc:Mul>           
                <ogc:Div>
                  <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  <ogc:Add>
                    <ogc:PropertyName>EMPLOYED</ogc:PropertyName>
                    <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  </ogc:Add>
                </ogc:Div>
                <ogc:Literal>100</ogc:Literal>
              </ogc:Mul>  
              <ogc:Literal>5</ogc:Literal>
            </ogc:PropertyIsLessThan>
          </ogc:Filter>
          <PolygonSymbolizer>
            <Fill>
              <!-- CssParameters allowed are fill (the color) and fill-opacity -->
              <CssParameter name="fill">#4DFF4D</CssParameter>
              <CssParameter name="fill-opacity">
                <ogc:Div>
                  <ogc:PropertyName>PERSONS</ogc:PropertyName>
                  <ogc:Literal>30e6</ogc:Literal>
                </ogc:Div>
              </CssParameter>
            </Fill>     
          </PolygonSymbolizer>
        </Rule>
        <Rule>
          <Title>5-7%</Title>
          <ogc:Filter>
            <ogc:PropertyIsBetween>
              <ogc:Mul>           
                <ogc:Div>
                  <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  <ogc:Add>
                    <ogc:PropertyName>EMPLOYED</ogc:PropertyName>
                    <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  </ogc:Add>
                </ogc:Div>
                <ogc:Literal>100</ogc:Literal>
              </ogc:Mul>  
              <ogc:LowerBoundary>
                <ogc:Literal>5</ogc:Literal>
              </ogc:LowerBoundary>
              <ogc:UpperBoundary>
                <ogc:Literal>7</ogc:Literal>
              </ogc:UpperBoundary>
            </ogc:PropertyIsBetween>
          </ogc:Filter>
          <PolygonSymbolizer>
            <Fill>
              <!-- CssParameters allowed are fill (the color) and fill-opacity -->
              <CssParameter name="fill">#FF4D4D</CssParameter>
              <CssParameter name="fill-opacity">
                <ogc:Div>
                  <ogc:PropertyName>PERSONS</ogc:PropertyName>
                  <ogc:Literal>30e6</ogc:Literal>
                </ogc:Div>
              </CssParameter>
            </Fill>     
          </PolygonSymbolizer>
        </Rule>
        <Rule>
          <Title>&gt; 7%</Title>
          <!-- like a linesymbolizer but with a fill too -->
          <ogc:Filter>
            <ogc:PropertyIsGreaterThan>
              <ogc:Mul>           
                <ogc:Div>
                  <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  <ogc:Add>
                    <ogc:PropertyName>EMPLOYED</ogc:PropertyName>
                    <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                  </ogc:Add>
                </ogc:Div>
                <ogc:Literal>100</ogc:Literal>
              </ogc:Mul>  
              <ogc:Literal>7</ogc:Literal>
            </ogc:PropertyIsGreaterThan>
          </ogc:Filter>
          <PolygonSymbolizer>
            <Fill>
              <!-- CssParameters allowed are fill (the color) and fill-opacity -->
              <CssParameter name="fill">#4D4DFF</CssParameter>
              <CssParameter name="fill-opacity">
                <ogc:Div>
                  <ogc:PropertyName>PERSONS</ogc:PropertyName>
                  <ogc:Literal>30e6</ogc:Literal>
                </ogc:Div>
              </CssParameter>
            </Fill>     
          </PolygonSymbolizer>
        </Rule>
        <Rule>
          <Title>Boundary</Title>
          <LineSymbolizer>
            <Stroke>
              <CssParameter name="stroke-width">0.2</CssParameter>
            </Stroke>
          </LineSymbolizer>
          <TextSymbolizer>
            <Label>

              <ogc:PropertyName>STATE_ABBR</ogc:PropertyName><![CDATA[
                ]]><ogc:Function name="numberFormat">
                <ogc:Literal>#.##</ogc:Literal>
                <ogc:Mul>           
                  <ogc:Div>
                    <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                    <ogc:Add>
                      <ogc:PropertyName>EMPLOYED</ogc:PropertyName>
                      <ogc:PropertyName>UNEMPLOY</ogc:PropertyName>
                    </ogc:Add>
                  </ogc:Div>
                  <ogc:Literal>100</ogc:Literal>
                </ogc:Mul>  
              </ogc:Function>

            </Label>
            <Font>
              <CssParameter name="font-family">Times New Roman</CssParameter>
              <CssParameter name="font-style">Normal</CssParameter>
              <CssParameter name="font-size">14</CssParameter>
            </Font>
            <LabelPlacement>
              <PointPlacement>
                <AnchorPoint>
                  <AnchorPointX>0.5</AnchorPointX>
                  <AnchorPointY>0.5</AnchorPointY>
                </AnchorPoint>
              </PointPlacement>
            </LabelPlacement>
          </TextSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

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© Copyright 2010, Ian Turton. Last updated on Nov 01, 2010. Created using Sphinx 1.0b2.