GetConvergenceAngle Method

In This Topic

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

Syntax

C#
Visual Basic (Declaration)

public double GetConvergenceAngle( 
   Coordinate2D coordinate
)

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

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, Windows 10, Windows 8.1

Reference

SpatialReference Class
SpatialReference Members

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