The *Miriad* uv data structure required a more general structure. Unfortunately
this is more complicated and somewhat cumbersome for simple cases.
A uv data set can be viewed as an ordered (generally time ordered) stream of
named records or ``variables''. There are markers in this data stream, to
indicate when
several variables change ``simultaneously'' (i.e. they correspond to the
same time). Each variable
consists of an array of values, the type of which can be either integer, real
or double precision, etc. Correlation data, u and v coordinates,
time and baseline numbers are specific examples of variables.
Because of the special nature of these variables, special routines are
available to simplify accessing them. A list of the variables
that may be present in a uv data set is given in Appendix I.

In addition to this variable stream, a uv file will contain a file giving flagging information.

It should be noted that ``variables'' and ``items''
are quite distinct. For a particular data set, variables vary, or at least may
vary,
whereas data items are fixed. The notion of variables is unique to uv data sets,
whereas all *Miriad* data sets are composed of data items. The stream of
uv variables is implemented as three data items, called `visdata`,
`vartable` and `flags`.

There is a ``miriad'' of uv routines.
The routines used to access and manipulate a uv data set are given in the
following table.

2011-08-19