投影转经纬度

private IPoint PRJtoGCS( double x, double y)
{
    IPoint pPoint = new PointClass();
    pPoint.PutCoords(x, y);
    ISpatialReferenceFactory pSRF = new SpatialReferenceEnvironmentClass();
    pPoint.SpatialReference = pSRF.CreateProjectedCoordinateSystem( 2414 );
pPoint.Project(pSRF.CreateGeographicCoordinateSystem(( int )esriSRGeoCSType.esriSRGeoCS_Beijing1954));
    return pPoint;
}

其中，pPoint.SpatialReference = pSRF.CreateProjectedCoordinateSystem( 2414 );

这行代码是设置pPoint 的空间参考，也就是要转化的点的投影坐标。如果不知道投影坐标的话，转化会报异常。

2414 为该投影的enum 值

pPoint.Project(pSRF.CreateGeographicCoordinateSystem(( int )esriSRGeoCSType.esriSRGeoCS_Beijing1954));
将该点的投影坐标转化为经纬度。

经纬度到投影：

private IPoint GCStoPRJ(IPoint pPoint, int GCSType, int PRJType)
{
    ISpatialReferenceFactory pSRF = new SpatialReferenceEnvironmentClass();
    pPoint.SpatialReference = pSRF.CreateGeographicCoordinateSystem(GCSType);
    pPoint.Project(pSRF.CreateProjectedCoordinateSystem(PRJType));
    return pPoint;
}