EDIT

          case 'stamen_terrain':

            // see http://developer.mapquest.com/web/products/open/map

            return new OpenLayers.Layer.XYZ(

              "Stamen Terrain",

              [

                "https://stamen-tiles.a.ssl.fastly.net/terrain/${z}/${x}/${y}.jpg"

              ],

              {

                // If using the MapQuest-OSM tiles, OpenStreetMap must be given credit for the data

                attribution: 'Map tiles by <a href="http://stamen.com">Stamen Design</a>, under <a href="http://creativecommons.org/licenses/by/3.0">CC BY 3.0</a>. Data by <a href="http://openstreetmap.org">OpenStreetMap</a>, under <a href="http://www.openstreetmap.org/copyright">ODbL</a>.',

                transitionEffect: "resize",

                sphericalMercator: true

              }

            );

          case 'open_topomap':

            // see http://developer.mapquest.com/web/products/open/map

            return new OpenLayers.Layer.XYZ(

              "OpenTopoMap",

              [

                "http://a.tile.opentopomap.org/${z}/${x}/${y}.png",

                "http://b.tile.opentopomap.org/${z}/${x}/${y}.png",

                "http://c.tile.opentopomap.org/${z}/${x}/${y}.png"

              ],

              {

                // If using the MapQuest-OSM tiles, OpenStreetMap must be given credit for the data

                attribution: 'Kartendaten: © <a href="https://openstreetmap.org/copyright">OpenStreetMap</a>-Mitwirkende, SRTM | Kartendarstellung: © <a href="http://opentopomap.org">OpenTopoMap</a> (<a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA</a>)',

                transitionEffect: "resize",

                sphericalMercator: true

              }

            );

            //