Here's some revised code that I tested successfully:
x -165.997618623476, y 72.0016465340226
Here's the prj file contents:
PROJCS["NAD27_BLM_3N_ftUS",GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-165],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",1640416.67],PARAMETER["false_northing",0],UNIT["Foot_US",0.30480060960121924]]
Here's the code:
public static void TestProjectToGCS()
{
string prjFile = @"D:\projects\blm3n.prj";
var t = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
var srf = Activator.CreateInstance(t) as ISpatialReferenceFactory2;
var sr = srf.CreateESRISpatialReferenceFromPRJFile(prjFile);
IPoint p = new PointClass();
p.PutCoords(1527538.32, 26211344.31);
p.SpatialReference = sr;
var p2 = (IPoint)ProjectToGCS(p);
Debug.Print("x {0}, y {1}", p2.X, p2.Y);
}
public static IGeometry ProjectToGCS(IGeometry inGeom)
{
if (!(inGeom.SpatialReference is IProjectedCoordinateSystem5))
throw new Exception("geometry is not in a projected coordinate system");
var outGeom = ((IClone)inGeom).Clone() as IGeometry;
var outSR = ((IProjectedCoordinateSystem5)outGeom.SpatialReference).GeographicCoordinateSystem as ISpatialReference;
outGeom.Project(outSR);
return outGeom;
}
It seems you've a problem with the coordinate system. Now you've the same CS for both layers but probably one of them has the wrong one. You should try to check this first, then, if this is the error, you need to define the right projection before reproject for the system you want.
If this is not the error, as the shift position is consistent, you can fix it in editing mode. You should select all the yellow points and then, with the snapping on, you move one yellow point (if they are all selected they will all move) for the correspondent green point. This way you will fix the problem.
Best Answer
You will need to supply a transformation. This link should get you a PDF from ESRI listing the appropriate transformation for your area of interest. Here is a link to some ESRI documentation on transformations.