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match(1)			 USER COMMANDS			      match(1)

name
       match - match two lists of stars

synopsis
       match fileA xcolA ycolA magcolA fileB xcolB ycolB magcolB
	      [id1=] [id2=] [max_iter=]	[halt_sigma=]
	      [outfile=] [trirad=] [nobj=] [matchrad=]
	      [scale=] [min_scale=] [max_scale=] [transonly]
	      [recalc] [linear|quadratic|cubic]
	      [medtf] [medsigclip=]  [intrans=]	[identity [xsh=] [ysh=]]
	      [--version] [--help] [help]

DESCRIPTION
       This  program  is  designed  to	match up items in two different	lists,
       which may have two different systems of coordinates.  The  program  al-
       lows  the two sets of coordinates to be related by a linear, quadratic,
       or cubic	transformation.	 It is an implementation of the	algorithm  de-
       scribed	in  Valdes et al., Publications	of the Astronomical Society of
       the Pacific, vol	107, page 1119 (1995); see also	Droege et al.,	Publi-
       cations	of the Astronomical Society of the Pacific, vol	118, page 1666
       (2006).

       The basic idea is to select the brightest nobj objects from each	 list,
       then  to	create a (very large) set of triangles using the stars in each
       list.  By finding similar triangles in the two sets, one	 can  identify
       matching	 stars	from  the  two	lists.	 This method is	insensitive to
       translation, rotation, scaling, and inversion, but it can take  a  long
       time.   After  finding  candidate  matching pairs, the program enters a
       loop in which it	computes a geometric  transformation,  determines  the
       residuals  between the actual and transformed coordinates, and discards
       the the most discrepant items; the user can set parameters  controlling
       the  iteration  of  this	loop using command-line	arguments max_iter and
       halt_sigma.

OPTIONS
       id1=N  column in	fileA containing "ID" value of items

       id2=N  column in	fileB containing "ID" value of items (the values  must
	      be  unique  within  each	list;  the program will	not check, and
	      strange behavior will result if duplicate	IDs occur)

       outfile=abc
	      place output in files with base name "abc"
		  abc.mtA    items from	list A which had matches in list B
		  abc.mtB    items from	list B which had matches in list A
		  abc.unA    items from	list A which had no matches in list B
		  abc.unB    items from	list B which had no matches in list A

       trirad=0.001
	      critical value for counting triangles as a match (default	 value
	      0.002; see below)

       nobj=30
	      use the brightest	'nobj' values in each list during the matching
	      process (default value 20)

       matchrad=4
	      after applying transformation to listA, so that both  lists  are
	      in  coordinate system of listB, items from the two lists must be
	      within this distance to count as 'matched' (default value	5)

       scale=2.0
	      if present, only triangles  with	the  given  scale  factor  are
	      counted  as  matches  (in	 this example, only triangles in coord
	      system A which are about 2.0 times larger	than triangles in  co-
	      ord system B are counted as matches)

       min_scale=1.5
	      if  present,  only  triangles with ratios	greater	than the given
	      scale factor are counted as matches

       max_scale=2.5
	      if present, only triangles with ratios less than the given scale
	      factor are counted as matches

       max_iter=3
	      during  the  solution process, there is a	loop in	which the code
	      matches stars in the two lists, finds  the  residuals  for  each
	      pair, and	discards pairs with big	residuals.  This argument sets
	      a	limit on the number of times this loop can  be	entered.   De-
	      fault  value  is	given  by  AT_MATCH_MAXITER  parameter in atp-
	      match.h, currently 3.

       halt_sigma=1.0e-12
	      during the solution process, if  the  typical  residual  between
	      pairs  of	stars in the two lists drops to	this level, stop iter-
	      ating and	declare	success.  The units  are  the  square  of  the
	      units  in	the second list	of stars; often, radians squared.  De-
	      fault value is given by  AT_MATCH_HALTSIGMA  parameter  in  atp-
	      match.h, currently 1.0e-12.

       transonly
	      if  present,  stop  after	 having	derived	a transformation which
	      best takes the 'nobj' objects from list A	into objects  in  list
	      B.  Don't	go on to find the matching objects explicitly.

       recalc if  present,  after  having  derived  a  transformation with the
	      brightest	'nobj' objects,	and having applied that	transformation
	      to  ALL  the  objects  in	list A,	derive a transformation	again,
	      this time	using ALL the objects in list A	and list B

       linear use a linear transformation (the default)

       quadratic
	      use a quadratic transformation

       cubic  use a cubic transformation

       medtf  calculate	the MEDTF statistics (see below); assumes that a  sim-
	      ple translation connects the two lists

       medsigclip=N
	      discard  matched	pairs more than	N stdev	from the median	offset
	      before calculating MEDTF statistics

       identity
	      as an initial guess, use a linear	TRANS in which there is	no ro-
	      tation,  change  in  scale, or translation (unless xsh= and ysh=
	      are specified)

       xsh=   in initial guess,	use linear TRANS with the given	translation in
	      the 'x' direction

       ysh=   in initial guess,	use linear TRANS with the given	translation in
	      the 'x' direction

       intrans=file
	      as an initial guess, use the TRANS which is given	in  the	 ASCII
	      text file	'file' (see below for the format)

       --version
	      print the	current	version	number and exit

       --help print list of command-line options

       help   print list of command-line options

       The  argument "trirad" may need some extra explanation.	For a full de-
       sciption, see the paper by Valdes et al.	 In short, stars  are  grouped
       into triangles, the sides of which are labelled "a", "b"	and "c"	in or-
       der of decreasing size.	The normalized ratios b/a and c/a
       are formed for each triangle.  In order for a triangle,	t1,  from  one
       list  to	 be considered identical to a triangle,	t2, in the other list,
       it must satisfy the equation

	    sqrt[ (t1.ba - t2.ba)^2 + (t1.ca - t2.ca)^2	] <= trirad

       So the value of "trirad"	has units of neither list; it exists in	a two-
       dimensional  "triangle-space"  formed  by  the  coordinates b/a and c/a
       (each of	which ranges from 0.0 to 1.0).

       One way the user	can control the	matching of triangles is to specify  a
       "scale  factor".	  Only	triangles  with	a ratio	of size	falling	with a
       particular window will be counted as matches.  The user can control the
       "scale factor window" in	two ways:

	    using  the	scale= option alone; for example, scale=1.5 The	window
	    is	centered  on  the  given  factor,  and	extends	 a  percentage
	    (AT_MATCH_PERCENT) above it	and below it.

	    using   both   min_scale  and  max_scale  together;	 for  example,
	    min_scale=1.3 max_scale=1.7	The window extends from	the  min_scale
	    to max_scale, including both boundaries.

       If  the	two  lists  are	known to have the same scale and rotation, the
       user may	use the	identity keyword to force  an  initial	guess  at  the
       TRANS: a	linear model with
	    a=0.0   b=1.0   c=0.0
	    d=0.0   e=0.0   f=1.0
       The program will	use this initial TRANS structure to match up the items
       in the two lists, rather	than trying to figure out  the	transformation
       itself.	 After	looking	 for  matched items with this fixed model, the
       program will use	the matched pairs  it  finds  to  define  an  improved
       TRANS.

       In a similar vein, if the user knows that the scale and rotation	of the
       two sets	of objects are identical, but  there  is  a  fixed  and	 known
       translation of XX units in the x-direction and YY units in the y-direc-
       tion, he	can use	the identity keyword with additional arguments
	    xsh=XX  ysh=YY
       which will create an initial TRANS with
	    a=XX    b=1.0   c=0.0
	    d=YY    e=0.0   f=1.0

       Finally,	for ultimate control, the user may specify the exact values of
       the coefficients	for the	initial	TRANS via the intrans=filename option.
       The program will	look for an ASCII text file with the  given  filename,
       which must have the following format:
	 a)  blank lines are skipped
	 b)  lines starting with the '#' character are skipped
	 c)  the first non-skipped line	must describe the order	of the model,
		either by one of the following keywords

		    linear quadratic cubic

		or by a	line like norder=NN, in	which
		NN has value 1 for linear, 2 for quadratic, or 3 for cubic.
	 d) all	following non-blank lines must give values for the coefficients
		of the specified TRANS model, in the general form

		    a=33.34
		    b=1.3e-4
		    c 99.01
		    d -5.323e1
	       Note that the equals sign between the coefficient and the value
		 is optional

       In  any	case,  if the user specifies the initial TRANS,	either via the
       identity	or intrans method, that	initial	model is used to  convert  the
       coordinates  of	objects	in list	A into the system of list B.  The pro-
       gram then looks for matches within the matchrad units.	It  then  uses
       only the	matched	pairs to generate an improved TRANS model.

       On the other hand, if the user does NOT provide an initial TRANS	model,
       the program searches for	one itself.  After its first guess,  the  code
       matches up candidate pairs of stars from	the two	lists.	It then	enters
       a loop in which it calculates a plate solution, applies the solution to
       the  stars in list A, then compares the predicted positions from	list A
       to the actual positions of stars	in list	B.  Based on those  residuals,
       the code	marks some of the pairs	as "bad", discards them, and goes back
       to the top of the loop.	The max_iter and halt_sigma arguments  can  be
       used  to	control	the number of times the	code enters this loop and dis-
       cards possible matches.	Note that, if one uses the project_coords rou-
       tine to turn (RA, Dec) positions	from a catalog into an input list, the
       units will of this list will be radians;	hence, the  residuals  from  a
       solution	 against  this	list  will  be	in radians squared.  A typical
       residual	of 1 arcsec  corresponds  to  halt_sigma  of  2.4e-11  radians
       squared.

SEE ALSO
       The  web	 pages	for the	match program contain additional documentation
       and a description of the	associated project_coords and apply_match pro-
       grams.  Read them at http://spiff.rit.edu/match/

AUTHOR
       Michael Richmond	< mwrsps at rit	dot edu	>

version	0.13		       November	30, 2010		      match(1)

name | synopsis | DESCRIPTION | OPTIONS | SEE ALSO | AUTHOR

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