idlastro / Astronomical Utilities: ARROWS

[Source code]

NAME
ARROWS
PURPOSE
To display "weathervane" directional arrows on an astronomical image 
EXPLANATION
Overlays a graphic showing orientation of North and East.
CALLING SEQUENCE
ARROWS,h, [ xcen, ycen, ARROWLEN= , CHARSIZE=  COLOR= , /DATA
                        FONT=, /NORMAL, /NOTVERTEX, THICK=  ]
INPUTS
h - FITS header array, must include astrometry
OPTIONAL INPUTS
xcen,ycen - numeric scalars, specifying the center position of
         arrows.   Position in device units unless the /NORMALIZED 
         keyword is specified.   If not supplied, then ARROWS
         will prompt for xcen and ycen
OPTIONAL KEYWORD INPUTS
arrowlen  - length of arrows in terms of normal Y size of vector-drawn
              character,  default  = 3.5, floating point scalar
charsize  - character size, default = 2.0, floating point scalar
color     -  color name or number for the arrows and NE letters.  See
          cgCOLOR() for a list of color names.                    
Data - if this keyword is set and nonzero, the input center (xcen,
          ycen) is understood to be in data coordinates
font - IDL vector font number (1-20) to use to display NE letters.
          For example, set font=13 to use complex italic font.
NotVertex - Normally (historically) the specified xcen,ycen indicated
            the position of the vertex of the figure.  If this
            keyword is set, the xcen,ycen coordinates refer to a sort
            of 'center of mass' of the figure.  This allows the
            figure to always appear with the area irregardless of
            the rotation angle.
Normal - if this keyword is set and nonzero, the input center 
         (xcen,ycen) is taken to be in normalized coordinates.   The
         default is device coordinates.
thick     - line thickness, default = 2.0, floating point scalar
OUTPUTS
none
EXAMPLE
Draw a weathervane at (400,100) on the currently active window, 
showing the orientation of the image associated with a FITS header, hdr
IDL> arrows, hdr, 400, 100
METHOD
Uses EXTAST to EXTract ASTrometry from the FITS header.   The 
directions of North and East are computed and the procedure
ONE_ARROW called to create the "weathervane".
PROCEDURES USED
GETROT - Computes rotation from the FITS header
ONE_ARROW - Draw a labeled arrow 
ZPARCHECK
REVISON HISTORY
written by B. Boothman 2/5/86 
Recoded with new procedures ONE_ARROW, ONE_RAY.  R.S.Hill,HSTX,5/20/92
Added separate determination for N and E arrow to properly display
  arrows irregardless of handedness or other peculiarities and added
  /NotVertex keyword to improve positioning of figure. E.Deutsch 1/10/93
Added /DATA and /NORMAL keywords W. Landsman      July 1993
Recognize GSSS header    W. Landsman       June 1993
Added /FONT keyword W. Landsman           April 1995
Modified to work correctly for COLOR=0  J.Wm.Parker, HITC   1995 May 25
Work correctly for negative CDELT values   W. Landsman   Feb. 1996
Use GETROT to compute rotation   W. Landsman    June 2003
Restored /NotVertex keyword which was not working after June 2003 change
           W. Landsman  January 2004