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Display descriptors and Display instance descriptors are obsolete since FEWS version 2013.02. You can remove them both from your configuration. Explorer.xml can be optimized by replacing the java class names by config file references but this is not strictly necessary.
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Figure 22 Elements in the DisplayDescriptors configuration
displayDescriptor
Root element of the display descriptor configuration. One entry is required for each display defined.
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- Id: Id or Name of the display
description
Optional description of the display. This is used for reference only.
className
Java class called when running the display as referenced by its Id. NOTE; this class must implement the DELFT-FEWS display plug-in interface.