option_layer_maplint: OSM - Maplint (errors)
option_layer_mapnik: OSM - Mapnik
option_layer_nearmap: "Australia: NearMap"
+ option_layer_geoeye_haiti: "Haiti: GeoEye"
option_layer_ooc_25k: "UK historic: 1:25k"
option_layer_ooc_7th: "UK historic: 7th"
option_layer_ooc_npe: "UK historic: NPE"