Developer’s Corner: Customizable arrow in GeoServer

GeoServer

Dear Reader,

Today we want to share a recent development in  GeoServer: an arrow mark whose proportions can be customized to get the type of arrow you’re looking for.
The general syntax for this arrow mark is the following:
<WellKnownName>extshape://arrow?hr=[hrValue]&t=[tValue]&ab=[abValue]</WellKnownName>
All parameters are ratios, and they are all optional. By default the arrow is the usual 1 by 1 mark, whose actual size you can control in SLD via the Mark size attribute. In particular, the parameters and their default values are:
  • t: thickness of the arrow base, between 0 and 1, default is 0.2
  • hr: height over width ratio, a positive number, default is 2
  • ab: arrow head base ratio, between 0 and 1, default is 0.5
The values can be modified to change the proportions of the arrow, for example, varying t between 0 and 1 results in the following arrows:
arrow1
Changing the height to width ratio will generate arrows that are wider or thinner:
arrow2
Finally, changing the arrow head base will move the position where the head of the arrow starts:
arrow3
Playing with all the three parameters will allow you to get the arrow of the desired proportions. Now, what if your desired characteristic is just having a long arrow? Since size is controlled by the mark size attribute, you’ll have to make a big arrow whose height ratio makes for a thin one, and maybe push up a bit the arrow base, like in this example:
<?xml version=”1.0″ encoding=”ISO-8859-1″?>
<StyledLayerDescriptor version=”1.0.0″
 xsi:schemaLocation=”http://www.opengis.net/sld StyledLayerDescriptor.xsd”
 xmlns:ogc=”http://www.opengis.net/ogc
 xmlns:xlink=”http://www.w3.org/1999/xlink
    <Name>ThinArrow</Name>
      <Title>Thin long arrow</Title>
        <Rule>
          <Name>r1</Name>
            <PointSymbolizer>
              <Graphic>
                <Mark>
                  <WellKnownName>extshape://arrow?hr=4&amp;ab=0.8</WellKnownName>
                  <Fill>
                    <CssParameter name=”fill”>#000000</CssParameter>
                  </Fill>
                </Mark>
              <Size>40</Size>
            </Graphic>
          </PointSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>
Which will result in the following arrows:

arrow4

We would like to thanks the GEOS Fugro for funding this work under a GeoSolutions Enterprise Services support contract.

The GeoSolutions team,

http://www.geo-solutions.it

 

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