uk.ac.starlink.ttools.func

Class Sky

java.lang.Object

uk.ac.starlink.ttools.func.Sky

public class Sky
extends Object

Functions useful for working with shapes on a sphere. All angles are expressed in degrees.

Since:

13 Feb 2019

Author:

Mark Taylor

Method Summary

Modifier and Type	Method and Description
static boolean	inSkyPolygon(double lon0, double lat0, double... lonLats) Tests whether a given sky position is inside the polygon defined by a given set of vertices.
static double	midLat(double lat1, double lat2) Determines the latitude midway between two given latitudes.
static double	midLon(double lon1, double lon2) Determines the longitude mid-way between two given longitudes.
static double	skyDistance(double lon1, double lat1, double lon2, double lat2) Calculates the separation (distance around a great circle) of two points on the sky in degrees.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

skyDistance

public static double skyDistance(double lon1,
                                 double lat1,
                                 double lon2,
                                 double lat2)

Calculates the separation (distance around a great circle) of two points on the sky in degrees.

This function is identical to skyDistanceDegrees in class CoordsDegrees.

Parameters:

lon1 - point 1 longitude in degrees

lat1 - point 1 latitude in degrees

lon2 - point 2 longitude in degrees

lat2 - point 2 latitude in degrees

Returns:

angular distance between point 1 and point 2 in degrees

midLon

public static double midLon(double lon1,
                            double lon2)

Determines the longitude mid-way between two given longitudes. In most cases this is just the mean of the two values, but this function copes correctly with the case where the given values straddle the lon=0 line.

Parameters:

lon1 - first longitude in degrees

lon2 - second longitude in degrees

Returns:

longitude midway between the given values

Examples:

midLon(204.0, 203.5) = 203.75, midLon(2, 359) = 0.5

midLat

public static double midLat(double lat1,
                            double lat2)

Determines the latitude midway between two given latitudes. This simply returns the mean of the two values, but is supplied for convenience to use alongside the midLon function.

Parameters:

lat1 - first latitude in degrees

lat2 - second latitude in degrees

Returns:

latitude midway between the given values

Examples:

midLat(23.5, 24.0) = 23.75

inSkyPolygon

public static boolean inSkyPolygon(double lon0,
                                   double lat0,
                                   double... lonLats)

Tests whether a given sky position is inside the polygon defined by a given set of vertices. The bounding lines of the polygon are the minor arcs of great circles between adjacent vertices, with an extra line assumed between the first and last supplied vertex. The interior of the polygon is defined to be the smaller of the two regions separated by the boundary. The vertices are specified as a sequence of lon_i, lat_i pairs.

The implementation of this point-in-polygon function is mostly correct, but may not be bulletproof. It ought to work for relatively small regions anywhere on the sky, but for instance it may get the sense wrong for regions that extend to cover both poles.

Parameters:

lon0 - test point latitude in degrees

lat0 - test point longitude in degrees

lonLats - 2N arguments (lon1, lat1, lon2, lat2, ..., lonN, latN) giving (longitude, latitude) vertices of an N-sided polygon in degrees

Returns:

true iff test point is inside, or on the border of, the polygon