MRT Explanation Fields

Introduction to Explanation

The explanation field provides two functions in the meta-data header. The obvious one is to provide further descriptive information about the column. Due to the size of the meta-data table the explanation should be brief. If the explanation can not fit on one line in the meta-data table then it may be broken into multiple lines where the additional lines are indented to start of the explanation column plus one character space to the right. For lengthy explanations a Note flag can be appended at the end. The other function of the explanation column is to allow for additional data checking. These feature can be use to check various patterns in the data including increasing or decreasing columns, conformation of listed data flags, and the presences of NULL values.

Additional limit checking

The first set of characters (followed by a blank) in the explanation column of the meta-data table can specify validity checks on the range, the possibility of having unspecified or NULL values, or the (increasing or decreasing order) order of data in the column.


Limits in a column can be specified in the explanation column, with values enclosed within square brackets [ or ]; the square bracket must be the first character of the explanation if present. The only exception arises when an asterisk is present before the first square bracket to indicate the presence of extended notes (see Notes).

  1. For an alphabetic column (A-format), the limits describe the valid character set, i.e. the list of valid characters in the column. As an example, an uncertainty column labelled u_lab with an A1format may only contain a blank, a question mark or a colon (which is different from the default; this feature can be specified as follows:
          3   A1  ---  u_lab   [ :?] Uncertainty flag on parameter lab

    The dash sign (-) may be used to specify consecutive characters, e.g. [A-F] for any character of the set {A,B,C,D,E,F}.

  2. For a numeric column, the limits can be specified with two numbers separated by a comma or slash and enclosed in square brackets. The inclusion or non-inclusion of the limits as acceptable values follow the standard mathematical conventions, i.e. an opening ] bracket means that the lower value is excluded, a closing ] bracket that the upper value is included.

As an example, the following specifies a parameter 350 < {lambda}< 650 :

 75- 80  F6.2  nm  lambda  ]350,650[ Wavelength

Both limits are not required: to express that a value has to be strictly positive, use the expression ]0,]; the expression [,0] specifies a negative or null value. Writing [] is acceptable when no range checking applies; this writing is required if a not-NULL or a sorting order has to be specified.

When specified, limiting numbers should represent actual limits, and not the range of all possible values which can be inferred from the format (e.g. [-999,9999] for an I4 number).

Note that some labels have implicit limits, listed in the column Limits of the tables in the label standards. These defaults are overridden (for numeric columns only) by the limits specified within square brackets in the description file : writing e.g.[] as the first word of the explanation of a column labelled GLON removes the condition 0 <=GLON < 360.

NULL values

A NULL or unspecified value is always indicated by setting all bytes of the column to blanks.

  • [!] (exclamation mark) indicates that a NULL value is forbidden (i.e.the column can never be blank)
  • [?] (question mark) indicates that a NULL value is allowed (i.e.the column may be blank). This is required for numeric columns with NULL values.


Following the range, it is also possible to specify that this column is increasing or decreasing throughout the table. If n is the row number, the conventions are:

+ the value in the column is strictly increasing, val(n+1) > val(n)
+= the value in the column is increasing: val(n+1) >=val(n)
the value in the column is strictly decreasing: val(n+1) < val(n)
-= the value in the column is decreasing: val(n+1) <=val(n)

Lengthy explanations

If more space is required to fully document a column (e.g. a long reference list) then the prefered method of expressing the data is with a Note. Each column that requires further information is appended with a number surrounded by “()”s. These numbers give the references to the Note section that follows the meta-data table. An example is given below. Notes should be added by hand after the program has successfully finished writing the final machine readable table.

   Bytes Format  Units   Label    Explanations
   1-  5  A5     ---     Cluster  Abell cluster designation
   7- 22  A16    ---     Name     Galaxy name (1)
  24- 25  I2     h       RAh      Right Ascension 1950 (hours)
  27- 28  I2     min     RAm      Right Ascension 1950 (minutes)
  30- 33  F4.1   s       RAs      Right Ascension 1950 (seconds)
      35  A1     -       DE-      Declination 1950 (sign)
  36- 37  I2     deg     DEd      Declination 1950 (degrees)
  39- 40  I2     arcmin  DEm      Declination 1950 (minutes)
  42- 43  I2     arcsec  DEs      Declination 1950 (seconds)
  47- 51  F5.2   mag     B(0)     []? Integrated magnitude
      52  A1     ---     n_B(0)   [RMCPHK] Origin of B(0) (2)
  55- 59  I5     km/s    RV       Velocity
  62- 64  I3     km/s    e_RV     Mean error on RV
Note (1): Names with RA given with five digits are normally from
    Zwicky catalogues; the other (with four digits in RA, starting or
    not with the letter A) are anonymous galaxies.
Note (2):
    C = CCD photometry (CfA redshift survey 1992)
    H = Zwicky magnitude split by observers
    K = Markarian catalogue of galaxies (1967)
    M = MCG (Vorontsov-Velyaminov et al. 1962-68)
    P = observer's eye estimate
    R = RC1 (de Vaucouleurs & de Vaucouleurs 1964)