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<H2><A NAME="SECTION00075000000000000000"></A><A NAME="ss:frameattributes"></A>
<BR>
Frame Attributes
</H2>

<P>
We will now briefly outline the various attributes associated with a
FrameFrame (this is, of course, in addition to those inherited from the
MappingMapping class). We will not delve too deeply into the details of each
attribute, for which you should consult the appropriate description in
Appendix&nbsp;<A HREF="node216.html#ss:attributedescriptions">C</A>. Instead, we aim simply to sketch
the range of facilities available:

<P><DL>
<DT><STRONG>NaxesNaxes</STRONG></DT>
<DD>
<BR>
A read-only integer giving the number of Frame axes.

<P>
</DD>
<DT><STRONG>TitleTitle</STRONG></DT>
<DD>
<BR>
A string describing the coordinate system which the Frame represents.

<P>
</DD>
<DT><STRONG>Label(axis)Label(axis)</STRONG></DT>
<DD>
<BR>
A label string for each axis.

<P>
</DD>
<DT><STRONG>Unit(axis)Unit(axis)</STRONG></DT>
<DD>
<BR>
A string describing the physical units on each axis. You can choose
whether to make this attribute ``active'' or ``passive'' (using
astSetActiveUnitastSetActiveUnit
). If active, its value will be taken into account when finding the
Mapping between two Frames (<SPAN  CLASS="textit">e.g.</SPAN> a scaling of 0.001 would be used
to connect two axis with units of ``km'' and ``m''). If passive, its value
is ignored. Its use is described in more detail in &#167;<A HREF="node85.html#ss:frameunits">7.14</A>.

<P>
</DD>
<DT><STRONG>Symbol(axis)Symbol(axis)</STRONG></DT>
<DD>
<BR>
A string containing a ``short form'' symbol (<SPAN  CLASS="textit">e.g.</SPAN> like ``X''
or ``Y'') used to represent the quantity plotted on each axis.

<P>
</DD>
<DT><STRONG>Digits/Digits(axis)Digits/Digits(axis)</STRONG></DT>
<DD>
<BR>
The preferred number of digits of precision to be used when formatting
values for display on each axis.

<P>
</DD>
<DT><STRONG>Format(axis)Format(axis)</STRONG></DT>
<DD>
<BR>
A string containing a <SPAN  CLASS="textit">format specifier</SPAN> which determines exactly
how values should be formatted for display on each axis
(&#167;<A HREF="node77.html#ss:formattingaxisvalues">7.6</A>). If this attribute is un-set, the
formatting is based on the Digits value, otherwise the Format string
over-rides the Digits value.

<P>
</DD>
<DT><STRONG>Direction(axis)Direction(axis)</STRONG></DT>
<DD>
<BR>
A boolean (integer) value which indicates in which direction each axis
should be plotted. If it is non-zero (the default), the axis should be
plotted in the conventional direction--<SPAN  CLASS="textit">i.e.</SPAN> increasing to the
right for the abscissa and increasing upwards for the ordinate. If it
is zero, the axis should be plotted in reverse.  This attribute is
provided as a hint only and programs are free to ignore it if they
wish.

<P>
</DD>
<DT><STRONG>DomainDomain</STRONG></DT>
<DD>
<BR>
A character string which identifies the <SPAN  CLASS="textit">physical domain</SPAN> to
which the Frame's coordinate system applies. The primary purpose of
this attribute is to prevent unwanted conversions from occurring
between coordinate systems which are not related. Its use is described
in more detail in &#167;<A HREF="node83.html#ss:framedomains">7.12</A>.

<P>
</DD>
<DT><STRONG>SystemSystem</STRONG></DT>
<DD>
<BR>
A character string which identifies the specific coordinate system used
to describe positions within the physical domain represented by the Frame.
For a simple Frame, this attribute currently has a fixed value of
``Cartesian'', but could in principle be extended to include options such
as ``Polar'', ``Cylindrical'', <SPAN  CLASS="textit">etc</SPAN>. More specialised Frames such
as the SkyFrameSkyFrame, TimeFrameTimeFrame  and SpecFrameSpecFrame, re-define the allowed values to be
appropriate to the domain which they describe. For instance, the SkyFrame
allows values such as ``FK4'' and ``Galactic'', and the SpecFrame allows
values such as ``frequency'' and ``wavelength''.

<P>
</DD>
<DT><STRONG>EpochEpoch</STRONG></DT>
<DD>
<BR>
This value is used to qualify a coordinate system by giving the moment in
time when the coordinates are correct. Usually, this will be the date of
observation. The Epoch value is important in cases where coordinates
systems move with respect to each other over time. An example of two such
coordinate systems are the FK4 and FK5 celestial coordinate systems.

<P>
</DD>
<DT><STRONG>ObsLonObsLon</STRONG></DT>
<DD>
<BR>
Specifies the longitude of the observer (assumed to be on the surface of
the earth). The basic Frame class does not use this value, but
specialised sub-classes may. For instance, the SpecFrame class uses it to
calculate the relative velocity of the observer and the centre of the
earth for use in converting between standards of rest.

<P>
</DD>
<DT><STRONG>ObsLatObsLat</STRONG></DT>
<DD>
<BR>
Specifies the latitude of the observer. Use in conjunction with ObsLon.

<P>
</DD>
</DL>

<P>
There are also some further Frame attributes, not described above,
which are important when Frames are used as templates to search for
other Frames. Their use goes beyond the present discussion.
<P>

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