THREDDS catalogs collect, organize, and describe accessible datasets. They provide a hierarchical structure for organizing the datasets as well as an access method (URL), and a human-understandable name for each dataset. Further descriptive information about each dataset can also be added.

Here’s an example of a very simple catalog:

<?xml version="1.0" ?>  <!-- 1 -->
<catalog xmlns="http://www.unidata.ucar.edu/namespaces/thredds/InvCatalog/v1.0" > <!-- 2 -->
  <service name="odap" serviceType="OpenDAP" base="/thredds/dodsC/" /> <!-- 3 -->
  <dataset name="SAGE III Ozone 2006-10-31" serviceName="odap" urlPath="sage/20061031.nc" ID="20061031.nc"/> <!-- 4 -->
</catalog> <!-- 5 -->

with this line-by-line explanation:

  1. The first line indicates that this is an XML document.
  2. This is the root element of the XML, the catalog element. It must declare the THREDDS catalog namespace with the xmlns attribute exactly as shown.
  3. This declares a service named odap that will serve data via the OpenDAP protocol. Many other data access services come bundled with THREDDS.
  4. This declares a dataset named SAGE III Ozone 2006-10-31. It references the odap service, meaning it will be served via OpenDAP. The URL to access the dataset is discussed next.
  5. This closes the catalog element.

Constructing An Access URL

Using the catalog directly above, here are the steps for client software to construct a dataset access URL:

  1. Find the service referenced by the dataset:

    <service name="odap" serviceType="OpenDAP" base="/thredds/dodsC/" />
<dataset name="SAGE III Ozone 2006-10-31" serviceName="odap" urlPath="sage/20061031.nc" ID="20061031.nc"/>
  2. Append the service base path to the server root to construct the service base URL:
    • serverRoot = http://hostname:port
    • serviceBasePath = /thredds/dodsC/
    • serviceBaseUrl = serverRoot + serviceBasePath = http://hostname:port/thredds/dodsC/`
  3. Find the urlPath referenced by the dataset: 
    <dataset name="SAGE III Ozone 2006-10-31" serviceName="odap" urlPath="sage/20061031.nc" ID="20061031.nc"/>
  4. Append the dataset urlPath to the service base URL to get the dataset access URL:
    • serviceBaseUrl = http://hostname:port/thredds/dodsC/
    • datasetUrlPath = sage/20061031.nc
    • datasetAccessUrl = serviceBaseUrl + datasetUrlPath = http://hostname:port/thredds/dodsC/sage/20061031.nc

In summary, construct a URL from a client catalog with these 3 pieces:

http://hostname:port/thredds/dodsC/sage/20061031.nc
<------------------><------------><--------------->
     server            service         dataset