GetConvergenceAngle Method

In This Topic

Gets the grid convergence for this spatial reference at the given point.

Syntax

Visual Basic (Declaration)
C#

Public Function GetConvergenceAngle( _
   ByVal coordinate As Coordinate2D _
) As Double

public double GetConvergenceAngle( 
   Coordinate2D coordinate
)

Parameters

coordinate: The point

Return Value

The grid convergence in radians. Returns 0 if the spatial reference is a geographic coordinate system. Returns NaN if the point is outside the projection's horizon or on error.

Remarks

The grid convergence is the angle between True North and Grid North at a point on the map. The grid convergence can be used to convert a horizontal direction expressed as an aziumth in a geographic coordinate system (relative to True North) to a direction expressed as a bearing in a projected coordinate system (relative to Grid North) and vice versa.

The formula to obtain a bearing, b, from an azimuth, a, using the grid convergence, c, returned from this is b = a - c.

The grid convergence returned by this method is positive when Grid North lies east of True North. This sign convention is sometimes named the Gauss-Bomford convention.

Example

Determine grid convergence for a SpatialReference at a given point

{
  Coordinate2D coordinate = new Coordinate2D(10, 30);
  double angle = SpatialReferences.WGS84.GetConvergenceAngle(coordinate);
  // angle = 0

  SpatialReference srUTM30N = SpatialReferenceBuilder.CreateSpatialReference(32630);
  coordinate.X = 500000;
  coordinate.Y = 550000;
  angle = srUTM30N.GetConvergenceAngle(coordinate);
  // angle = 0

  MapPoint pointWGS84 = MapPointBuilderEx.CreateMapPoint(10, 50, SpatialReferences.WGS84);
  MapPoint pointUTM30N = GeometryEngine.Instance.Project(
    pointWGS84, srUTM30N) as MapPoint;

  coordinate = (Coordinate2D)pointUTM30N;
  // get convergence angle and convert to degrees
  angle = srUTM30N.GetConvergenceAngle(coordinate) * 180 / Math.PI;
  // angle = 10.03
}

Requirements

Target Platforms: Windows 11 Home, Pro, Enterprise (64 bit)

ArcGIS Pro version: 3.0 or higher.

Reference

SpatialReference Class
SpatialReference Members

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