Each external layer object should have the following properties:

• id: short string identifying the layer;
• label: layer label to be displayed at the interface;
• type: layer type, should be equal to raster or vector;
• label: short string to describe the layers at the interface;
• url: URL of tilejson, if type=vector;
• vectorLayers: array vector layer ids, if type=vector;
• tiles: array of tile URLs. Can be used as a mutually exclusive alternative to url if type is vector. Required if type is raster;
• source: object describing layer source:
  • label: short string describing the source;
  • url: URL to source.