### astOffset2

Calculate an offset along a geodesic curve in a 2D Frame

#### Description:

This function finds the Frame coordinate values of a point which is offset a specified distance along the geodesic curve at a given angle from a specified starting point. It can only be used with 2-dimensional Frames.

For example, in a basic Frame, this offset will be along the straight line joining two points. For a more specialised Frame describing a sky coordinate system, however, it would be along the great circle passing through two sky positions.

#### Synopsis

double astOffset2( AstFrame $\ast$this, const double point1, double angle, double offset, double point2 );

#### Parameters:

##### this
Pointer to the Frame.
##### point1
An array of double, with one element for each Frame axis (Naxes attribute). This should contain the coordinates of the point marking the start of the geodesic curve.
##### angle
The angle (in radians) from the positive direction of the second axis, to the direction of the required position, as seen from the starting position. Positive rotation is in the sense of rotation from the positive direction of axis 2 to the positive direction of axis 1.
##### offset
The required offset from the first point along the geodesic curve. If this is positive, it will be in the direction of the given angle. If it is negative, it will be in the opposite direction.
##### point2
An array of double, with one element for each Frame axis in which the coordinates of the required point will be returned.

#### Returned Value

##### astOffset2
The direction of the geodesic curve at the end point. That is, the angle (in radians) between the positive direction of the second axis and the continuation of the geodesic curve at the requested end point. Positive rotation is in the sense of rotation from the positive direction of axis 2 to the positive direction of axis 1.

#### Notes:

• The geodesic curve used by this function is the path of shortest distance between two points, as defined by the astDistance function.

• An error will be reported if the Frame is not 2-dimensional.

• This function will return " bad" coordinate values (AST__BAD) if any of the input coordinates has this value.