Engine中如何进行七参数投影转换？

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

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;
}