Layer instances are sufficent to produce displayed the end product of 'get map'
requests. They are produced in RasterSourcesModel from a combination of a WMS 'GetMap'
(or whatever the analogous request in whatever OGC service is being produced) and an instance
of OgcSource
Layer instances are sufficent to produce displayed the end product of 'get map' requests. They are produced in RasterSourcesModel from a combination of a WMS 'GetMap' (or whatever the analogous request in whatever OGC service is being produced) and an instance of OgcSource