From Ka-Map Wiki

[edit] Layers Setup

• Edit your mapfile like this
• Add transparency to the MAP section

  MAP
    TRANSPARENT ON
    ...

• Add GROUP "Name" to the layer section, and this will add the layer group in the legend

  LAYER 
    GROUP "Layer Name"
  END

• Assign a CLASS NAME for each class to be displayed into the layer group:

  LAYER
    GROUP "Roads"
    ...
    CLASS
      NAME "Highways"
      ...

• For example

  LAYER 
    GROUP "Roads"
  END

  LAYER 
    GROUP "Rivers and Streams"
  END