Start ArcMap with a new empty map and add the data with the unknown coordinate system.If you set the data frame's coordinate system and the data with a known coordinate system lines up with the unknown data, the data frame's coordinate system is that of the unknown data's. If you change the data frame's coordinate system, all layers that have coordinate systems will be projected on the fly to the new coordinate system. If data does not have a defined coordinate system, ArcMap cannot project it on the fly. If a data source has a defined coordinate system, ArcMap can project it on the fly to a different coordinate system. The term projection, or PRJ, is an older term that is also used, but it is not as precise. The term coordinate system can refer to data expressed in decimal degrees, or a projected coordinate system expressed in meters or feet. This data can be drawn in ArcMap, but cannot be projected. If the coordinate system is unknown, you will receive this warning message when trying to add the layer to ArcMap: The following data sources you added are missing spatial reference information. The technique described below helps identify the correct coordinate system. Coordinate system information is usually obtained from the data source, but not always, as with legacy data.
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