Inside Datasets

Generally the user does not need to understand much about the internal structure of a dataset. A dataset contains several kinds of data, called `items', . Items can be quite small (e.g. a single number), intermediate in size (e.g. the history item, use to store history information), or very large (e.g. the image item in an image dataset, which is used to store the pixel data, or the visdata item in a visibility dataset, which contains the correlations). Large items are stored as a host system file, whereas all small items are stored in a common file which is rather inappropriately called header. Indeed, there are a number of instances where the word ``header'' is used where ``item'' or ``small item'' would be a more appropriate description.

The implementation of datasets as directories does complicate some manipulations of your datasets, since your favourite image, etc, is not just a file anymore. On the other hand, as the host system sees a Miriad dataset as a normal directory, all the usual host commands to manipulate directories can be used. On UNIX a -r switch often has to be used with the command, to indicate that the operation is to be applied `recursively' (i.e. to all files in the directory). For example, to delete a dataset, use

% rm -r dataset

If you have aliased rm to prompt you before deleting a file (as is common in a number of the standard login scripts at Epping), you will be prompted before deleting each individual file within a dataset. This can become somewhat tedious, so you might want to make another alias to delete without prompting. For example, insert

     alias rrm 'rm'

in your .cshrc file. Similarly to copy a dataset, you would use

% cp -r dataset1 dataset2

Generally the user is insulated from this internal organisation of a dataset and can always think of them as a whole. However there are a few Miriad utilities to manipulate at the item level. These tasks do not contain any astronomical knowledge. Consequently they may seem somewhat crude. These tasks include:

• itemize: List the items in a dataset.
• delhd: Delete an item from a dataset.
• puthd: Add an item which consists of a single-value to a dataset.
• copyhd: Copy an item from one dataset to another.
• gethd: Print the value of an item. This is most useful in scripts.

Note the rather inappropriate use of `hd' in the preceding names, where something suggesting items (rather than header) would have been more appropriate.

As an example, consider the itemize task, which lists the items in a dataset. For the test image dataset, gauss (created with imgen in Chapter 2) itemize will tell us of the following items:

   % itemize in=gauss
   Itemize: version 1.1 4-mar-91
   naxis    = 2
   naxis1   = 256
   naxis2   = 256
   crpix1   = 129
   crpix2   = 129
   cdelt1   = -4.848137e-06
   cdelt2   = 4.848137e-06
   crval1   = 0
   crval2   = 0
   history    (text data, 38 elements)
   image      (real data, 65536 elements)

Here the item naxis consists of a single integer, having the value of 2. The item image is a larger item (being the pixel data) consisting of 65536 (256 by 256) real numbers.

Note for images that many items have FITS-like names (although they are lower case, and the units can be different from the FITS standard). A list of the items in an image and visibility dataset are given in Appendices B and C.