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All basemaps added to the gallery need to have the same spatial reference. If the default ArcGIS.com basemaps are used then all additional items added to the gallery need to be in Web Mercator (wkids: 102100, 102113 and 3857). If the default basemaps are not used you can add basemaps in any spatial reference as long as all the items added to the gallery share that spatial reference. To achieve the best performance, it is recommended that all basemaps added to the gallery are cached (tiled) layers.
All basemaps added to the gallery need to have the same spatial reference. If the default ArcGIS.com basemaps are used then all additional items added to the gallery need to be in Web Mercator (wkids: 102100, 102113 and 3857). If the default basemaps are not used you can add basemaps in any spatial reference as long as all the items added to the gallery share that spatial reference. To achieve the best performance, it is recommended that all basemaps added to the gallery are cached (tiled) layers.