Understanding the XML

Unfortunately, metadata in netCDF files is frequently missing or inadequate. As long as you are using only COARDS compliant netCDF files, you can still use the automatic XML generation technique described above to generate a template configuration file, which you can then edit. If you are using other types of datasets, you have to manually insert the metadata for the dataset.

You will have to modify the XML configuration file if your metadata is missing or insufficient; to do this, you will have to understand the syntax and semantics of the LAS XML configuration file. A sample XML file is below, followed by definitions for LAS XML elements and attributes.

Sample XML file

<?xml version='1.0' ?>
<lasdata>
  <institution name="Pacific Marine Environmental Lab"
   url="http://www.pmel.noaa.gov"/>
<!-- Define operations -->
    <operations url="http://foo.com/las-bin/LASserver.pl" >
      <shade name="Shade a variable" class="LAS::Server::Ferret" method="draw">
        <arg type="variable"/>
         <arg type="region"/>
       </shade>
    </operations>
<!-- Define datasets/variables -->
   <datasets>
    <coads_climatology_cdf name="COADS Climatology" url="file:coads_climatology" doc="doc/coads_climatology.html">
     <variables>
      <airt name="Air Temperature" units="DEG C">
       <link match="/lasdata/grids/coads_climatology_cdf_grid"/>
      </airt>
      <sst name="Sea Surface Temperature" units="Deg C">
       <link match="/lasdata/grids/coads_climatology_cdf_grid"/>
      </SST>
     </variables>
     <composite>
      <wind_vectors name="Wind vectors" js="VecVariable">
       <link match="../../variables/uwnd"/>
       <link match="../../variables/vwnd"/>
      </wind_vectors>
     </composite>
    </coads_climatology_cdf>
   </datasets>
<!-- Define grids -->
   <grids>
    <coads_climatology_cdf_grid>
     <link match="/lasdata/axes/coads_climatology_cdf_COADSX"/>
     <link match="/lasdata/axes/coads_climatology_cdf_COADSY"/>
     <link match="/lasdata/axes/coads_climatology_cdf_TIME"/>
    </coads_climatology_cdf_grid>
   </grids>
<!-- Define axes -->
   <axes>
    <coads_climatology_cdf_COADSX type="x" units="degrees_east">
     <arange start="21" step="2" size="180"/>
    </coads_climatology_cdf_COADSX>
    <coads_climatology_cdf_COADSY type="y" units="degrees_north">
     <arange start="-89" step="2" size="90"/>
    </coads_climatology_cdf_COADSY>
    <coads_climatology_cdf_TIME type="t" units="month">
     <arange start="1-1-16" step="1" size="12"/>
    </coads_climatology_cdf_TIME>
   </axes>
  </lasdata>

XML Element and Attribute Definitions

The elements and attributes that make up a LAS XML specification are defined below. Note the following:

• When <*> is used for a element tag, it implies that the tag can be any string compliant with the XML specification, as long as the tag is unique.
• Regular expressions are used for describing the children of an element. For example, dataset* implies that 0 or more child dataset elements are required; dataset+ implies that 1 or more child dataset elements are required.

Lasdata element

The lasdata element is the root of the LAS XML hierarchy.

Link element

The link element is used to associate one part of the XML specification with another. It is analogous to the anchor tag in HTML, with the exception that the contents that are linked to are substituted in place. The following XML:

<a>
  <b>
    <link match="/a/d"/>
  </b>
  <d name="foo"/>
</a>

is equivalent to:

<a>
  <b>
    <d name="foo"/>
  </b>
  <d name="foo/>
</a>

Datasets element

The datasets section contains information on all of the datasets supported by the server. Here you will find metadata such as variable names, variable units, or the file or URL to be associated with a variable or dataset.

Dataset element

Each dataset is described by a dataset element. The name of the element can be anything but properties or institution; the name must be unique. It is coads_climatology_cdf in the sample file.

Variables element

Composite element

Composite elements are used to describe "virtual" variables that are composed of one or more dataset variables.

Comp_tag element

Variable element

Grids element

Grid element

Axes element

Axis element

Arange element

V element

Properties element

Properties are general purpose tags that can be associated with a LAS XML element. LAS currently uses properties to associate visualization "hints" for Ferret with datasets or variables. A set of Ferret properties might look like:

<properties>
  <ferret>
    <size>0.25</size>
    <format>netCDF</format>
  </ferret>
</properties>

The child elements of the <properties> tag can be anything; it is up to the server application to decode their semantics.

Institution element