GRID - Draw a coordinate grid at specified intervals

Description:

Draw a grid in the current projection at specified intervals in spherical coordinates. The intervals, start and end values should all be specified in degrees. The defaults for the grid separations normally produce desirable effects for all sky plots. In specifying the grid intervals it is sometimes necessary to take account of rounding errors that might occur, and to bear in mind that in some geometries a single point on the celestial sphere maps onto two points on the projected coordinates - some care is needed to ensure that the whole grid is drawn.

Usage:

grid [phimin] [phimax] [phistep] [themin] [themax] [thestep]

Parameters:

PHIMIN = _DOUBLE (Read and Write)

The start longitude in degrees for the coordinate grid.

If no value is specified on the command line, the current value is used. The current value is initially set to 0.0.

PHIMAX = _DOUBLE (Read and Write)

The end longitude in degrees for the coordinate grid.

If no value is specified on the command line, the current value is used. The current value is initially set to 360.0.

PHISTEP = _DOUBLE (Read and Write)

The spacing between longitude grid lines in degrees.

If no value is specified on the command line, the current value is used. The current value is initially set to 30.0.

THEMIN = _DOUBLE (Read and Write)

The start latitude in degrees for the coordinate grid.

If no value is specified on the command line, the current value is used. The current value is initially set to -90.0.

THEMAX = _DOUBLE (Read and Write)

The end latitude in degrees for the coordinate grid.

If no value is specified on the command line, the current value is used. The current value is initially set to 90.0.

THESTEP = _DOUBLE (Read and Write)

The spacing between latitude grid lines in degrees.

If no value is specified on the command line, the current value is used. The current value is initially set to 10.0.

PROJECTION = _LITERAL (Read)

The geometry to be used to plot the grid. This is explained in more detail in the section on projections. Allowed values: "NONE", "TAN", "SIN", "ARC", "GLS", "AITOFF", "MERCATOR" and "STG".

This parameter is not specified on the command line. The value of the global parameter PONGO_PROJECTN is used. If PONGO_PROJECTN is not defined, the default value "NONE" is used.

RACENTRE = _LITERAL (Read)

The centre of the projection in RA (i.e. the angle must be specified as hh:mm:ss.sss). This parameter is only required for PROJECTION values other than "NONE".

This parameter is not specified on the command line. The value of the global parameter PONGO_RACENTRE is used. If PONGO_RACENTRE is not defined, the default value "0" is used.

DECCENTRE = _LITERAL (Read)

The centre of the projection in declination (i.e. the angle must be specified as dd:mm:ss.sss). This parameter is only required for PROJECTION values other than "NONE".

This parameter is not specified on the command line. The value of the global parameter PONGO_DECCENTRE is used. If PONGO_DECCENTRE is not defined, the default value "0" is used.

Notes:

• It is sometimes necessary to specify the grid intervals in a manner which avoids rounding errors to obtain the last grid line. e.g 9.9999 instead of 10. You may also need to sometimes draw the grid twice using say 30.00001 as well as 29.99999 to get this final line.