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<title>Coordinate system in TAP metadata</title>
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<h1>How to declare coordinate system(s) in TAP metadata?</h1>
<p>
The TAP-1.0 standard does not define any way to convey the
coordinate system used by coordinate columns. However, this is
more and more required by recent catalogs using recent epoch
(e.g. 2015 for Gaia-DR1) or coordinate systems not expected by
default by most of the VO applications (e.g. something different
from ICRS J2000).
</p>
<p>
In order to provide most accurate metadata as possible, the
TAPLibrary gives a way to specify the coordinate system of your
coordinates. Here is how to proceed in function of how you provide
your TAP metadata:
</p>
<h3>In <code>TAP_SCHEMA</code></h3>
<p>
You must add a table in <code>TAP_SCHEMA</code>:
<code>coosys</code>. This table must have the following columns:
</p>
<ul>
<li><code>id</code>: VARCHAR <i>(ID of the coordinate system
definition as it must be in the VOTable)</i></li>
<li><code>system</code>: VARCHAR <i>(The coordinate system among:
<code>ICRS</code>, <code>eq_FK5</code>, <code>eq_FK4</code>,
<code>ecl_FK4</code>, <code>ecl_FK5</code>,
<code>galactic</code>, <code>supergalactic</code>,
<code>xy</code>, <code>barycentric</code>, <code>geo_app</code>)
</i></li>
<li><code>equinox</code>: VARCHAR <i>(Required to fix the equatorial
or ecliptic systems (as e.g. <code>J2000</code> as the default
for <code>eq_FK5</code> or <code>B1950</code> as the default
for <code>eq_FK4</code>))</i></li>
<li><code>epoch</code>: VARCHAR <i>(Epoch of the positions
(if necessary))</i></li>
</ul>
<p>
Then, you need an additional column in
<code>TAP_SCHEMA.columns</code>:
</p>
<ul>
<li><code>coosys_id</code>: VARCHAR <i>(ID of the used coordinate
systems (if any))</i></li>
</ul>
<p>
Now, all you have to do, is add any coordinate system you need in
<code>TAP_SCHEMA.coosys</code> and set the appropriate coordinate
system ID in the column <code>coosys_id</code> of all columns using
one. Refresh the metadata of your TAP service and now a
<code>COOSSYS</code> item should be in the metadata part of your
VOTable results (only when there are coordinates with a specified
coordinate system).
</p>
<h3>In an XML file</h3>
<p>
Unfortunately, the XML definition of the node <code>column</code>
does not provide any standard way to provide something as complex
as a coordinate system. So, either you keep not providing coordinate
systems in your TAP service, or you choose a different method of
metadata declaration. </p>
<p><i>Tip:
Starting your TAP service with an XML file as way to provide
metadata will immediately create and initialize correctly the
<code>TAP_SCHEMA</code> schema in your database. So, if you want
to provide coordinate system information, all you have to do, is to
switch the metadata declaration method in order to use this newly
created <code>TAP_SCHEMA</code>. Then, just follow the steps listed
above for this method.
</i></p>
<h3>Programmatically</h3>
<p>
To add a coordinate system to a column, you just have to use the
new function of <code>TAPColumn</code>:
<code>setCoosys(TAPCoosys)</code>. As in the
<code>TAP_SCHEMA</code> method, you can use the same
<code>TAPCoosys</code> object for several columns.
</p>
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