NAME

tmatch -- Find closest match between rows in two tables

USAGE

tmatch (input1, input2, output, match1, match2, maxnorm)

DESCRIPTION

This task combines rows from two tables into one. Rows are combined by looking at each row in the first table and finding the row in the second table whose match columns are closest to those in the first. The distance between two rows is defined in the usual way to be the square root of the sum of the squares of the difference between the corresponding match columns. Rows are only written to the output table if the distance between them is less than the distance specified by maxnorm. This task converts match column units to degrees for the purpose of computing the distance if the column units are recognized angular units (seconds, minutes, degrees, hours, or radians or any abbreviation of these). If the column units are blank or some unrecognized string, no conversion is done. Thus, if the match column is in some recognized angular units, maxnorm should be specified in degrees.

Columns named by the parameters incol1 and incol2 will be copied to the output table. If these parameters are left blank, all columns will be copied to the output table. Columns will have the same name in the output table as in the input table, except that columns with the same name in both input tables will have the suffix "_1" and "_2" added to indicate which table they were copied from.

This task optionally allows you to supply a set of weighting factors that are multiplied by the difference between corresponding match columns when computing the distance. These factors can be used to supply units conversion when column units information is missing from the input table or as a way to weight information from columns containing distinct information. If factors are supplied, any column units information is ignored. If the factors are left bank, they are ignored and column units information is used to convert to degrees when possible.

If the match columns contain spherical coordinates, parameter sphere should be set to yes so that the distance on a sphere can be computed. If the match columns do contain spherical units, the first column should be the longitude column (such as right ascension) and the second column should be the latitude column (such as declination). Columns should also be in some recognized angular units, or a conversion factor should be supplied explicitly.

The task optionally produces a diagnostic output file if a file name is supplied to parameter diagfile. The diagnostic file contains the rows from the first table that were not matched, the cases where more than one row in the first table matched a single row in the second table, and the ten matched rows in the with the largest distance between them. Rows in the diagnostic output are identfied by their table number and row number and optionally by the contents of the columns specified by the parameters nmcol1 and nmcol2.

This task differs from tjoin in two respects. First tjoin combines tables on the basis of a single column, while this task can combine tables based on multiple columns. Second, tjoin places every combination of rows matching within the specified tolerance in the output table, while this task only puts the closest match in the output table.

PARAMETERS

input[string]

First input table name.

input[string]

Second input table name.

output [string]

Output table name.

match[string]

A column template describing columns from the first table used to match the two tables. A column name template is a comma or whitespace list of strings. Each string may either be a column name a pattern containg wildcard characters which matches several column names. This parameter will also accept the name of a list file (preceded by the "@" character) containing column names to be matched. If the first non-white character in the template is the negation character (either "~" or "!"), all columns NOT appearing in the list will be matched.

match[string]

A column name template describing columns from the second table used to match the two tables. This parameter follows the same format rules as match1. The number of columns must equal those in match1.

maxnorm min= 0.0, max=INDEF [real]

The distance between two rows must be less than maxnorm in order for them to match. Recognized angular units are converted to degrees before computing the distance. The recognized units are seconds, minutes, degrees, hours, radians, or any abbreviation of these.

(incol= " ") [string]

A column name template describing the columns to be copied from the first input table to the output table. If this parameter is left blank (the default) all columns in the first input table will be copied to the output.

(incol= " ") [string]

A column name template describing the columns to be copied from the second input table to the output table. If this parameter is left blank (the default) all columns in the second input table will be copied to the output.

(factor = " ") [string]

A comma or white space separated list of numeric factors multiplied by the individual column differences when computing the distance between rows in the first and second tables. If this parameter is left blank (the default) conversion of angular units to degrees will be performed, but not other weighting will be performed. If a list of values is supplied, units conversion will NOT be performed, the supplied numeric factors will be used instead.

(diagfile = " ") [string]

The name of the diagnostic output file. If the name is left blank (the default) no dignostic output is produced. Diagnostic output can be sent to the terminal by setting this parameter to STDOUT or STDERR. The diagnostic output contains a list of rows that were not matched, cases where more than one row in the first table matched a single row in the second table, and the ten pairs of rows with the largest distance between them.

(nmcol= " ") [string]

A column template describing the columns from the first table that are printed in the diagnostic output. The table and row number are always printed, if this parameter is not blank, the specified columns are also printed.

(nmcol= " ") [string]

A column template describing the columns from the second table that are printed in the diagnostic output.

(sphere = no) [bool]

If this parameter is set to yes, a correction appropriate for spherical coordinates will be applied to the first column difference. The correction is the cosine of the average of the two second column values. In order for this correction to be valid, the first column must contain the longitude component and the second column the latitude component. Units should be convertable to degrees or an explicit conversion factor should be supplied.

EXAMPLES

1. Two star catalogs are being matched. They both have the following columns:

Name             CH*12      %12s ""
RA               D        %10.1h hours
Dec              D        %10.0h degrees
V                R         %7.2f ""
B-V              R         %7.2f ""
U-B              R         %7.2f ""

To find the best match between the catalogs within a ten arcsecond radius one would use the following command:

tt> tmatch catalog1.tab catalog2.tab match.tab \
>>> ra,dec ra,dec 0:00:10 sphere+

The search radius can either be supplied in sexgesimal notation, as above, or in decimal degrees.

2. Suppose the input catalogs did not contain units information, as would be the case if they were text files. The units conversion could then be supplied explicitly through the factor parameter:

tt> tmatch catalog1.tab catalog2.tab match.tab \
>>> ra,dec ra,dec 0:00:10 factor=15,1 sphere+

3. Suppose we want the output table to only contain the name from the first catalog and get the rest of its information from the second catalog. This could be done with the following command:

tt> tmatch catalog1.tab catalog2.tab match.tab \
>>> ra,dec ra,dec 0:00:10 incol1=name sphere+

4. To get diagnostic output from the task, use the following command:

tt> tmatch catalog1.tab catalog2.tab match.tab ra,dec ra,dec \
>>> diag=diag.txt nmcol1=name nmcol2=name 0:00:10 sphere+

The following is a subset of the diagnostic output produced:

The following objects matched the same object:
1:163 6601  GEM
1:164 6601  GEM
2:163 6601  GEM


The following objects have the largest norms:
Norm = 0.00253
1:371 2319  SCO
2:371 2319  SCO

Norm = 0.00247
1:368 2101  SCO
2:368 2101  SCO

The number before the colon is the table number, the number after the colon is the row number, and the rest of the line is from the name column.

REFERENCES

Written by Bernie Simon

SEE ALSO

tjoin