RegCoords: Spatial Region Coordinates


This document describes the specification of coordinate systems, and the interpretation of coordinate values, for spatial region filtering.

Pixel coordinate systems

The default coordinate system for regions is PHYSICAL, which means that region position and size values are taken from the original data. (Note that this is a change from the original IRAF/PROS implementation, in which the IMAGE coordinate system was the default.) PHYSICAL coordinates always refer to pixel positions on the original image (using IRAF LTM and LTV keywords). With PHYSICAL coordinates, if a set of coordinates specifies the position of an object in an original FITS file, the same coordinates will specify the same object in any FITS derived from the original. Physical coordinates are invariant with blocking of FITS files or taking sections of images, even when a blocked section is written to a new file.

Thus, although a value in pixels refers, by default, to the PHYSICAL coordinate system, you may specify that position values refer to the image coordinate system using the global or local properties commands:

  global coordsys image
  circle 512 512 100
The global command changes the coordinate system for all regions that follow, while the local command changes the coordinate system only for the region immediately following:
  local coordsys image
  circle 512 512 100
  circle 1024 1024 200
This changes the coordinate system only for the region that follows. In the above example, the second region uses the global coordinate system (PHYSICAL by default).

World Coordinate Systems

If World Coordinate System information is contained in the data file being filtered, it also is possible to define regions using a sky coordinate system. Supported systems include:
  name                  description
  ----                  -----------
  PHYSICAL              pixel coords of original file using LTM/LTV
  IMAGE                 pixel coords of current file
  FK4, B1950            sky coordinate systems
  FK5, J2000            sky coordinate systems
  GALACTIC              sky coordinate systems
  ECLIPTIC              sky coordinate systems
  ICRS                  currently same as J2000
  LINEAR                linear wcs as defined in file
In addition, two mosaic coordinate systems have been defined that utilize the (evolving) IRAF mosaic keywords:
  name                  description
  ----                  -----------
  AMPLIFIER             mosaic coords of original file using ATM/ATV
  DETECTOR              mosaic coords of original file using DTM/DTV
Again, to use one of these coordinate systems, the global or local properties commands are used:
  global coordsys galactic

WCS Positions and Sizes

In addition to pixels, positional values in a WCS-enabled region can be specified using sexagesimal or degrees format:
  position arguments    description
  ------------------    -----------
  [num]                 context-dependent (see below)
  [num]d                degrees
  [num]r                radians
  [num]p                physical pixels
  [num]i                image pixels
  [num]:[num]:[num]     hms for 'odd' position arguments
  [num]:[num]:[num]     dms for 'even' position arguments
  [num]h[num]m[num]s    explicit hms
  [num]d[num]m[num]s    explicit dms
If ':' is used as sexagesimal separator, the value is considered to be specifying hours/minutes/seconds if it is the first argument of a positional pair, and degrees/minutes/seconds for the second argument of a pair (except for galactic coordinates, which always use degrees):
  argument      description
  -----------   -----------
  10:20:30.0    10 hours, 20 minutes, 30 seconds for 1st positional argument
                10 degrees, 20 minutes, 30 seconds for 2nd positional argument
  10h20m30.0    10 hours, 20 minutes, 30 seconds
  10d20m30.0    10 degrees, 20 minutes, 30 seconds
  10.20d        10.2 degrees
Similarly, the units of size values are defined by the formating character(s) attached to a number:
  size arguments        description
  --------------        -----------
  [num]                 context-dependent (see below)
  [num]"                arc seconds
  [num]'                arc minutes
  [num]d                degrees
  [num]r                radians
  [num]p                physical pixels
  [num]i                image pixels
For example:
  argument      description
  -----------   -----------
  10            ten pixels
  10'           ten minutes of arc
  10"           ten seconds of arc
  10d           ten degrees
  10p           ten pixels
  0.5r          half of a radian

An example of using sky coordinate systems follows:

  global coordsys B1950
  -box 175.54d 20.01156d 10' 10'
  local coordsys J2000
  pie 179.57d 22.4d 0 360 n=4 && annulus 179.57d 22.4d 3' 24' n=5
At the FK4 1950 coordinates 175.54d RA, 20.01156d DEC exclude a 10 minute by 10 minute box. Then at the FK5 2000 coordinates 179.57d RA 22.4d DEC draw a radial profile regions pattern with 4 quadrants and 5 annuli ranging from 3 minutes to 24 minutes in diameter. In this example, the default coordinate system is overridden by the commands in the regions spec.

NB: The Meaning of Pure Numbers Are Context Sensitive

When a "pure number" (i.e. one without a format directive such as 'd' for 'degrees') is specified as a position or size, its interpretation depends on the context defined by the 'coordsys' keyword. In general, the rule is:

All pure numbers have implied units corresponding to the current coordinate system.

If no coordinate system is explicitly specified, the default system is implicitly assumed to be PHYSICAL. In practice this means that for IMAGE and PHYSICAL systems, pure numbers are pixels. Otherwise, for all systems other than LINEAR, pure numbers are degrees. For LINEAR systems, pure numbers are in the units of the linear system. This rule covers both positions and sizes.

As a corollary, when a sky-formatted number is used with the IMAGE or PHYSICAL coordinate system (which includes the default case of no coordsys being specified), the formatted number is assumed to be in the units of the WCS contained in the current file. If no sky WCS is specified, an error results.


  ellipse 202.44382d 47.181656d 0.01d 0.02d

In the absence of a specified coordinate system, the circle uses the default PHYSICAL units of pixels, while the ellipse explicitly uses degrees, presumably to go with the WCS in the current file.

 global coordsys=fk5 
 global color=green font="system 10 normal"
 circle 202.44382 47.181656 0.01
 circle 202.44382 47.181656 10p

Here, the circles use the FK5 units of degrees (except for the explicit use of pixels in the second radius), while the ellipse explicitly specifies pixels. The ellipse angle is in degrees.

Note that Chandra data format appears to use "coordsys=physical" implicitly. Therefore, for most Chandra applications, valid regions can be generated safely by asking ds9 to save/display regions in pixels using the PHYSICAL coordsys.

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Last updated: November 17, 2005