转载来自：http://zhihu.esrichina.com.cn/?/question/6858

arcgis知乎是一个arcgis方面知识的专业问答平台。

解决办法】：
首先创建自定义geotransformation，然后用IGeometry.ProjectEx进行投影转换。参考代码（以wgs1984转到Xian80为例）：
 private void func1ToolStripMenuItem_Click(object sender, EventArgs e) 
{ 
Type t = Type.GetTypeFromProgID(esriGeometry.SpatialReferenceEnvironment); 
System.Object obj = Activator.CreateInstance(t); 
ISpatialReferenceFactory srFact = obj as ISpatialReferenceFactory; 
IGeographicCoordinateSystem wgs84GCS = srFact.CreateGeographicCoordinateSystem((int)esriSRGeoCSType.esriSRGeoCS_WGS1984); 
IProjectedCoordinateSystem xian80PCS = srFact.CreateProjectedCoordinateSystem((int)esriSRProjCS4Type.esriSRProjCS_Xian1980_3_Degree_GK_CM_102E); 

IGeoTransformation geoTrans = CreateGeoTransformation((ISpatialReference)wgs84GCS, (ISpatialReference)xian80PCS); 

IWorkspaceFactory wksfac = new FileGDBWorkspaceFactoryClass(); 
IFeatureWorkspace feawks = wksfac.OpenFromFile(@E:TEST.gdb,0) as IFeatureWorkspace; 
IFeatureClass fc = feawks.OpenFeatureClass(POINT); 
IFeature fea = fc.Search(null, false).NextFeature(); 
IGeometry5 geometry; 
IPoint point = fea.ShapeCopy as IPoint; 
geometry = point as IGeometry5; 
geometry.SpatialReference = wgs84GCS; 

geometry.ProjectEx(xian80PCS, esriTransformDirection.esriTransformForward, geoTrans, false, 0.0, 0.0); 
point = geometry as IPoint; 
MessageBox.Show(point.X.ToString(),point.Y.ToString()); 
} 

private IGeoTransformation CreateGeoTransformation(ISpatialReference pInputSR, ISpatialReference pOutputSR) 
{ 
IGeoTransformation pGeoTrans; 
pGeoTrans = new CoordinateFrameTransformationClass(); 

((ICoordinateFrameTransformation)pGeoTrans).PutSpatialReferences(pInputSR, pOutputSR); 
((ICoordinateFrameTransformation)pGeoTrans).PutParameters(p1, p2, p3, p4, p5, p6, p7); 

return pGeoTrans; 
}