Position Class Reference

The Position class allows us to represent positions on the celestial sphere (or on Earth, at a push). More...

#include <position.h>

Public Types
enum	System {   _Natural, TopoPolar, AzAlt, EqPolar,   HADec, RADec, Geog, Velocity,   _Null }
	The known coordinate systems. More...

Public Member Functions
	Position (double lon, double lat, System system, System into_system=_Natural)
	Create a new pair of angles. More...
double	lon (void) const
	The azimuthal coordinate. More...
double	lat (void) const
	The latitudinal coordinate.
System	system (void) const
	The coordinate system in which this position is expressed.
bool	is_null_p (void) const
	Test if the Position is a 'null' one.
void	set_observatory (const Position &)
	Set the observatory position, for a subsequent call to to_system. More...
void	set_reftime (const AstroTime &)
	Set the reference time, for a subsequent call to to_system. More...
Position	to_system (System into_system) const
	Return another Position, in the given system. More...

Static Public Member Functions
static Position	get_null (void)
	Return a 'null' Position, which can be used as a sentinel value.

Detailed Description

The Position class allows us to represent positions on the celestial sphere (or on Earth, at a push).

We're able to support a subset of coordinate conversions.

Member Enumeration Documentation

enum Position::System

The known coordinate systems.

enum	explanation
TopoPolar	horizontal coordinate system: longitude is azimuthal coordinate of spherical polar coordinates (east of south, in range [0,2pi]); latitude is north of horizontal
AzAlt	horizontal coordinate system: azimuth (azimuth is east of north, in range [0,2pi]) and altitude (north of horizontal)
HADec	equatorial: LHA (west-positive) and Declination
EqPolar	equatorial: same as HADec, but longitude is negative HA, thus the azimuthal coordinate of spherical polars, but in a right-handed equatorial frame with zero longitude on the meridian
RADec	equatorial: Right Ascension and Declination
Geog	geographic: latitude and east longitude
Velocity	the coordinates are velocities in a right-handed system, in rad/sec

(the _Natural and _Null systems are placemarkers, and should not be used externally to this class implementation).

Constructor & Destructor Documentation

Position::Position	(	double	lon,
		double	lat,
		System	system,
		System	into_system = _Natural
	)

Create a new pair of angles.

The lon and lat attributes are a pair of angles giving longitude and latitude, in radians.

There is limited support for converting between systems, the set of possible 'conversions' is rather ad-hoc, and any impermissible ones call fatal().

The system argument to this constructor indicates the coordinate system in which the two angle arguments are expressed, and the into_system argument indicates the system of the resulting object. If the into_system argument is not provided, then both TopoPolar and AzAlt coordinates are turned into TopoPolar, and both HADec and EqPolar arguments into HADec. If the fourth argument is present, then TopoPolar and AzAlt coordinates are interconvertible, as are HADec and EqPolar. For conversions between those pairs, see to_system.

After these conversions, the .lon and .lat values are initialised in their appropriately adjusted form. Thus (writing degrees instead of radians, for convenience): after Position(10, 30, AzAlt), the .lon is +170 and .lat is +30; after Position(200, 30, AzAlt), the .lon is +340; after Position(-10, 30), the .lon is +350; after Position(10, 30, HADec), the .lon is +350 and .lat is +30.

In all cases, the longitude is normalised into [0,2pi), and the latitude is forced to be in [-pi/2,pi/2].

FIXME: Tidyups/refactoring: the Position class is now over-complicated, with far too much fiddly branching. Or is it? Things like TrackingSchedule need specific documentation concerning which coordinate system the arguments are in, with the result that rotation-by-colatitude happens in multiple places (and I always get the signs wrong first time). I can't help feeling these should be in a single place, but every time I conclude that single place is 'in the Position class', I end up with a mess in here.

Parameters

lon	the longitudinal angle in the specified system
lat	the latitudinal angle in the specified system
system	the system in which the above two arguments are expressed
into_system	the system in which the resulting Position object holds its coordinates

Member Function Documentation

double Position::lon ( void  ) const

inline

The azimuthal coordinate.

This is east-positive, from the reference position, which has zero degrees at south.

void Position::set_observatory

(

const Position &

obs

)

Set the observatory position, for a subsequent call to to_system.

It is a fatal error if this is not set before a subsequent conversion to AzAlt coordinates.

The position must be in the Geog system.

See also

Position::System

void Position::set_reftime

(

const AstroTime &

t

)

Set the reference time, for a subsequent call to to_system.

It is a fatal error if this is not set before a subsequent conversion from RA-Dec coordinates.

Position Position::to_system

(

Position::System

into_system

)

const

Return another Position, in the given system.

Depending on the conversion, it may be necessary to have called set_reftime (for conversions involving the RADec system) or set_observatory (for conversion to equatorial systems). If such required information is needed but has not been set, this results in a fatal error.

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The documentation for this class was generated from the following files:

• position.h
• position.cpp