How to convert PCS based point X/Y into GCS Lat/Lon using ArcObjects

Question

There are so many permutations of questions I've tried to find the answer to my question from various sources, but I have yet to actually find an answer the way that makes sense to me (and works).

I'm using ArcObjects in a ArcMap add-in, and I have shape file loaded with sample data that comes in a Projected Coordinate System.

I want to convert a feature's points (IPoint) which are X/Y coordinates, into it's BASE GCS Lat/Lon values.

For example:

The Data Source in the Shape File shows (Custom since Factory Code shows ZERO):

Projected Coordinate System: NAD27_BLM_3N_ftUS Projection: Transverse_Mercator false_easting: 1640416.67000000 false_northing: 0.00000000 central_meridian: -165.00000000 scale_factor: 0.99960000 latitude_of_origin: 0.00000000 Linear Unit: Foot_US

Geographic Coordinate System: GCS_North_American_1927 Datum: D_North_American_1927 Prime Meridian: Greenwich Angular Unit: Degree

And I want to convert the X/Y coordinates to be Lat/Lon instead with GCS NAD 27.

There is no transformation, it's the same Datum.

I have tried to use the following code:

IPoint geographicPoint = new Point();
geographicPoint.X = _pcsX;
geographicPoint.Y = _pcsY;
geographicPoint.Z = _z;
geographicPoint.M = _m;
geographicPoint.SpatialReference = _inputSpatialReference; // PCS

ISpatialReference outputSpatialReference = SpatialReference.GenerateSpatialReference(outputCoordinateSystemWKID, false); // GCS

((IGeometry2)geographicPoint).ProjectEx(outputSpatialReference, esriTransformDirection.esriTransformReverse, null, false, 0, 0);

_gcsLat = geographicPoint.Y;
_gcsLong = geographicPoint.X;

I have also tried to use the Project() method as well instead of ProjectEx:

((IGeometry2)geographicPoint).Project(outputSpatialReference);

however, in both cases, I just get the exact same X/Y coordinates in the X/Y values of the geographicPoint, when I am expecting the X/Y values to now be the Long/Lat as I have requested the output coordinate system that is a Geographic Coordinate System.

Source Point:

X: 2440016.02472701 Y: 25980973.0785536

After Projection to 4267:

LAT: 25980973.0785536 LON: 2440016.02472701

Can anyone help?

As Requested: the PRJ file of the input document: 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["false_easting",1640416.67],PARAMETER["false_northing",0],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-165],PARAMETER["scale_factor",0.9996],UNIT["Foot_US",0.30480060960121924]]

Answer 1

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;            
}

Answer 2

This is how I've done it previously (freqently) and had no problems whatsoever:

int NAD27_UTMzone4N = 26704; //http://spatialreference.org/ref/epsg/nad27-utm-zone-4n/
int NAD27_Geograpic = 4267;  //http://spatialreference.org/ref/epsg/nad27/

ISpatialReferenceFactory2 srFactory = new SpatialReferenceEnvironmentClass();
ISpatialReference ProjectedSR = srFactory.CreateSpatialReference(NAD27_UTMzone4N);
ISpatialReference GeographicSR = srFactory.CreateSpatialReference(NAD27_Geograpic);

IPoint geographicPoint = new PointClass();
geographicPoint.X = _pcsX;
geographicPoint.Y = _pcsY;
geographicPoint.Z = _z;
geographicPoint.M = _m;
geographicPoint.SpatialReference = ProjectedSR;

geograhpicPoint.Project(GeographicSR);

_gcsLat = geographicPoint.Y;
_gcsLong = geographicPoint.X;

I use the Project() method of the IGeometry which, provided no transformation need be supplied, works well. Refer IGeomertry.Project Method.

I use the ISpatialReferenceFactory2 interface to generate the spatial reference from the EPSG code for well known coordinate systems, this works better than supplying text or prj file as there's no room for mistakes. Refer ISpatialReferenceFactory2.CreateSpatialReference Method.

It is possible that Z and M values are throwing the projection off, if these values aren't important don't set them. In this case a bad value is far worse than no value.

Answer 3

Unfortunately StackExchange broke my account causing me to not be able to comment on answers etc.

This is for Kirk Kuykendall, I appreciate your assistance with this very much. It's great to have people like you to come to when things just don't work right. I am excited to try the solution from the PRJ file, however, I will need to come up with some way to do this without the use of the actual PRJ file. Hopefully I can figure out how to get that projection definition from the ArcObjects in memory and somehow create the spatial reference that way without needing to have the user provide the PRJ file. Otherwise this will cause issues in the workflow being requested.

But I appreciate your help with this. I am having a ton of other issues I am now finding converting from known projections (not custom ones). Where I thought I was getting good values before, but now realize I am not getting good values at all.

For example, when I try the same code to convert from 32065 (Projected) to its base GCS (4267) it works, and then when I try and convert as well to WGS84 (4326) using a transformation NAD_1927_To_WGS_1984_1 I get invalid values! Which really surprizes me.

Projecting without transformation (From: 32065 PCS) (To: 4267 GCS)

Input X: 1545657.33157618, Y: 11654131.4567511

Output LAT: 32.1073377480588, LON: -93.3061315609481

Projecting using transformation NAD_1927_To_WGS_1984_1 (To: 4326)

Input X: 1545657.33157618, Y: 11654131.4567511

Output LAT: 32.1073377480588, LON: 91.2102941689212

Well that Output LAT LON is wrong, 91 is way off, if anything it should be -91, and even then if I check where that coordinate is, it's totally wrong!

I'm starting to get confused why you can't just choose input/output/transformation and use ProjectEx and get valid results :P