If you want to manually edit the SLD for the selected Layer then click on the Advanced tab.
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Here you can manually configure the style of your OGC Layer. Astun supply some default SLD files in the folder specified in the iShare Settings General tab SldSnippetDefinitionPath. For more information on SLD please refer to the GeoServer documentation which has plenty of examples http://docs.geoserver.org/stable/en/user/styling/sld-cookbook/index.html.
Toolbar
There is a Toolbar at the top of the Editing pane for common functions:
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If you wish to apply different styles to a Layer then you can do this using Multiple Rules. Any field used as a SLD property must be made available as a displayed field in the layer options. The following SLD is an example of how we have styled the Councils layer to display different colours for different types of authority and it appears in iShare GIS like this:
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