Creating a custom geographic coordinate system

Summary This topic shows two methods to create a user-defined geographic coordinate system (GCS).

About creating a custom geographic coordinate system

A GCS includes a name, angular unit of measure, datum (which includes a spheroid), and a prime meridian. It is a model of the earth in a three-dimensional (3D) coordinate system. Latitude-longitude or lat/lon data is in a GCS. You can access the majority of the properties and methods through the IGeographicCoordinateSystem interface with additional properties that are available in IGeographicCoordinateSystem2. Although most developers will not need to create a custom GCS, IGeographicCoordinateSystemEdit contains the Define and DefineEx methods.
The following code example shows how to use the Define method to create a user-defined GCS. The ISpatialReferenceFactory interface allows you to create the Datum, PrimeMeridian, and AngularUnit component parts. These components can also be created using a similar Define method available on their classes.
[C#] 
private IGeographicCoordinateSystem CreateGeographicCoordinateSystem()
{


    // Set up the SpatialReferenceEnvironment.
    // SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    Type factoryType = Type.GetTypeFromProgID(
        "esriGeometry.SpatialReferenceEnvironment");
    System.Object obj = Activator.CreateInstance(factoryType);
    ISpatialReferenceFactory3 spatialReferenceFactory = obj as
        ISpatialReferenceFactory3;

    // Create the datum, prime meridian, and angular unit from existing definitions.
    IDatum datum = spatialReferenceFactory.CreateDatum((int)
        esriSRDatumType.esriSRDatum_OSGB1936);
    IPrimeMeridian primeMeridian = spatialReferenceFactory.CreatePrimeMeridian((int)
        esriSRPrimeMType.esriSRPrimeM_Greenwich);
    IUnit unit = spatialReferenceFactory.CreateUnit((int)
        esriSRUnitType.esriSRUnit_Degree);

    IGeographicCoordinateSystemEdit geographicCoordinateSystemEdit = new
        GeographicCoordinateSystemClass();

    object name = "UserDefined Geographic Coordinate System";
    object alias = "UserDefined GCS";
    object abbreviation = "UserDefined";
    object remarks = "User Defined Geographic Coordinate System based on OSGB1936";
    object usage = "Suitable for the UK";
    object datumObject = datum as object;
    object primeMeridianObject = primeMeridian as object;
    object unitObject = unit as object;

    geographicCoordinateSystemEdit.Define(ref name, ref alias, ref abbreviation, ref
        remarks, ref usage, ref datumObject, ref primeMeridianObject, ref unitObject)
        ;

    IGeographicCoordinateSystem userDefinedGeographicCoordinateSystem =
        geographicCoordinateSystemEdit as IGeographicCoordinateSystem;
    return userDefinedGeographicCoordinateSystem;

}
[VB.NET] 
Private Function CreateGeographicCoordinateSystem() As IGeographicCoordinateSystem
    
    
    ' Set up the SpatialReferenceEnvironment.
    ' SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    
    Dim factoryType As Type = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment")
    Dim spatialReferenceFactory As ISpatialReferenceFactory3 = CType(Activator.CreateInstance(factoryType), ISpatialReferenceFactory3)
    
    ' Create the datum, prime meridian, and angular unit from existing definitions.
    Dim datum As IDatum = spatialReferenceFactory.CreateDatum(CInt(esriSRDatumType.esriSRDatum_OSGB1936))
    Dim primeMeridian As IPrimeMeridian = spatialReferenceFactory.CreatePrimeMeridian(CInt(esriSRPrimeMType.esriSRPrimeM_Greenwich))
    Dim unit As IUnit = spatialReferenceFactory.CreateUnit(CInt(esriSRUnitType.esriSRUnit_Degree))
    
    Dim geographicCoordinateSystemEdit As IGeographicCoordinateSystemEdit = New GeographicCoordinateSystemClass()
    
    Dim Name As Object = "UserDefined Geographic Coordinate System"
    
    ' Note: Alias is a reserved keyword in VB .NET. To use it as a variable name, encase it in brackets [ ].
    
    Dim [Alias] As Object = "UserDefined GCS"
    Dim abbreviation As Object = "UserDefined"
    Dim remarks As Object = "User Defined Geographic Coordinate System based on OSGB1936"
    Dim usage As Object = "Suitable for the UK"
    Dim datumObject As Object = TryCast(datum, Object)
    Dim primeMeridianObject As Object = TryCast(primeMeridian, Object)
    Dim unitObject As Object = TryCast(unit, Object)
    
    geographicCoordinateSystemEdit.Define(Name, [Alias], abbreviation, remarks, usage, datumObject, primeMeridianObject, unitObject)
    
    Dim userDefinedGeographicCoordinateSystem As IGeographicCoordinateSystem = TryCast(geographicCoordinateSystemEdit, IGeographicCoordinateSystem)
    
    Return userDefinedGeographicCoordinateSystem
    
End Function
The following code example shows how the DefineEx method can be used. It uses SpatialReferenceEnvironment to create the Datum, PrimeMeridian, and Unit components:
[C#] 
private IGeographicCoordinateSystem CreateGeographicCoordinateSystemEx()
{

    // Set up the SpatialReferenceEnvironment.
    // SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    Type t = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
    System.Object obj = Activator.CreateInstance(t);
    ISpatialReferenceFactory3 spatialReferenceFactory = obj as
        ISpatialReferenceFactory3;

    // Create the datum, prime meridian, and angular unit from existing definitions.

    IDatum datum = spatialReferenceFactory.CreateDatum((int)
        esriSRDatumType.esriSRDatum_OSGB1936);
    IPrimeMeridian primeMeridian = spatialReferenceFactory.CreatePrimeMeridian((int)
        esriSRPrimeMType.esriSRPrimeM_Greenwich);
    IUnit unit = spatialReferenceFactory.CreateUnit((int)
        esriSRUnitType.esriSRUnit_Degree);

    IGeographicCoordinateSystemEdit geographicCoordinateSystemEdit = new
        GeographicCoordinateSystemClass();

    String name = "UserDefined Geographic Coordinate System";
    String alias = "UserDefined GCS";
    String abbreviation = "UserDefined";
    String remarks = "User Defined Geographic Coordinate System based on OSGB1936";
    String usage = "Suitable for the UK";

    geographicCoordinateSystemEdit.DefineEx(name, alias, abbreviation, remarks,
        usage, datum, primeMeridian, unit as IAngularUnit);

    IGeographicCoordinateSystem userDefinedGeographicCoordinateSystem =
        geographicCoordinateSystemEdit as IGeographicCoordinateSystem;

    return userDefinedGeographicCoordinateSystem;

}
[VB.NET] 
Private Function CreateGeographicCoordinateSystemEx() As IGeographicCoordinateSystem
    
    ' Set up the SpatialReferenceEnvironment.
    ' SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    
    Dim t As Type = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment")
    Dim obj As System.Object = Activator.CreateInstance(t)
    Dim spatialReferenceFactory As ESRI.ArcGIS.Geometry.ISpatialReferenceFactory3 = obj
    
    ' Create the datum, prime meridian, and angular unit from existing definitions.
    
    Dim datum As IDatum = spatialReferenceFactory.CreateDatum(CInt(esriSRDatumType.esriSRDatum_OSGB1936))
    Dim primeMeridian As IPrimeMeridian = spatialReferenceFactory.CreatePrimeMeridian(CInt(esriSRPrimeMType.esriSRPrimeM_Greenwich))
    Dim unit As IUnit = spatialReferenceFactory.CreateUnit(CInt(Fix(esriSRUnitType.esriSRUnit_Degree)))
    
    Dim geographicCoordinateSystemEdit As IGeographicCoordinateSystemEdit = New GeographicCoordinateSystemClass()
    
    Dim Name As String = "UserDefined Geographic Coordinate System"
    
    ' Note: Alias is a reserved keyword in VB .NET. To use it as a variable name, encase it in brackets [ ].
    
    Dim [Alias] As String = "UserDefined GCS"
    Dim abbreviation As String = "UserDefined"
    Dim remarks As String = "User Defined Geographic Coordinate System based on OSGB1936"
    Dim usage As String = "Suitable for the UK"
    
    geographicCoordinateSystemEdit.DefineEx(Name, [Alias], abbreviation, remarks, usage, datum, primeMeridian, TryCast(unit, IAngularUnit))
    
    Dim userDefinedGeographicCoordinateSystem As IGeographicCoordinateSystem = TryCast(geographicCoordinateSystemEdit, IGeographicCoordinateSystem)
    
    Return userDefinedGeographicCoordinateSystem
    
End Function





To use the code in this topic, reference the following assemblies in your Visual Studio project. In the code files, you will need using (C#) or Imports (VB .NET) directives for the corresponding namespaces (given in parenthesis below if different from the assembly name):
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ArcView ArcView
ArcEditor ArcEditor
ArcInfo ArcInfo
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