“ArcGIS Unknown Spatial Reference”问题：
shp文件在Arcgis打开后经常因为原有坐标系无法识别而丢失信息，出现以下提示信息：
"Unknown Spatial Reference
The following data sources you added are missing spatial reference information. This data can be drawn in ArcMap, but cannot be projected."
解决办法：
ArcToolBox > Data Management Tools > Projections and Transformations > Define Projection，这里要给文件加入的坐标系统应当是地理坐标系统（Geographyical Coordinate System），下一步再将投影转换到你所要的投影系统下。
ESRI Supporter Center关于这个问题给出的解决方法：
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Error: Unknown Spatial Reference
Article ID: 31999
Software: ArcGIS - ArcEditor 9.0, 9.1, 9.2 ArcGIS - ArcInfo 9.0, 9.1, 9.2 ArcGIS - ArcView 9.0, 9.1, 9.2
Platforms: N/A
Error Message
Adding data to ArcMap which does not have the coordinate system or projection defined displays the following error message:
"Unknown Spatial Reference
The following data sources you added are missing spatial reference information. This data can be drawn in ArcMap, but cannot be projected."
Cause
The data does not have a coordinate system or projection defined.
This message is returned for all data types which can be added to ArcMap including geodatabase feature classes, shapefiles, coverages and CAD data, as well as supported raster or image formats which do not have a coordinate system definition.
Solution or Workaround
Identify and define the coordinate system for the data, using the Define Projection Tool in ArcToolBox > Data Management Tools > Projections and Transformations or the data‘s property page in ArcCatalog.
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原地址：http://miss4813.iteye.com/blog/772423