next up previous
Next: Plotting Generalized Curves
Up: Producing Graphical Output (Plots)
Previous: Plotting Geodesic Curves

Plotting Curves Parallel to Axes

As there is no Plot function to draw a ``straight line'', drawing axes and grid lines to represent coordinate systems requires a slightly different approach. The problem is that for some coordinate systems, these grid lines will not be geodesics, so astCurve and astPolyCurve ([*]) cannot easily be used (you would have to resort to approximating grid lines by many small elements). Lines of constant celestial latitude provide an example of this, with the exception of the equator which is a geodesic.

The astGridLine function allows these curves to be drawn, as follows:

int axis;
double length;

...

astGridLine( plot, axis, start, length );

Here, ``axis'' specifies which physical coordinate axis we wish to draw parallel to. The ``start'' array contains the coordinates of the start of the curve and ``length'' specifies the distance to draw along the axis in physical coordinate space.



next up previous
Next: Plotting Generalized Curves
Up: Producing Graphical Output (Plots)
Previous: Plotting Geodesic Curves

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils