Changes between Version 26 and Version 27 of udg/ecoms

Mar 14, 2013 1:30:34 PM (9 years ago)



  • udg/ecoms

    v26 v27  
    5454= Accesing the Data Portal via Web = #s.web.access
     56The ''SPECS-EUPORIAS Data Portal'' can be accessed through the '''Data Portal URL''' provided in the abstract. First of all, an authentication dialog will request a valid user name and password.
     61\includegraphics[width = 0.8 \linewidth]{loginTHREDDS.png}
     62\caption{Authentication dialog}
     68Afterwards, the different datasets described in Sec. \ref{s.thredds} are listed as links in the web browser window (Fig.~\ref{fig:dir}).  By clicking in any of the datasets, a new window will appear providing information on the variables and geospatial and time coverages, and offering different options for data access and/or visualization (Fig.~\ref{fig:mainwin}). Currently, only the OPeNDAP access service is fully operative in the portal. Therefore, in this example, we will illustrate the use of this service, which allows selecting time/spatial data slices from the OPeNDAP data access form shown in Fig. \ref{fig:opendapwin} and downloading the resulting data in both ASCII and Binary formats.
     73\includegraphics[width = 0.85 \linewidth]{fig01.png}
     74\caption{Catalog of the EUPORIAS-SPECS System4 datasets. Note that although they only include a few variables, their size range from one to four Terabytes.
     84\includegraphics[width=0.8 \linewidth]{fig02.png}
     85\caption{Detail of a particular dataset with information on the included variables and geospatial and time coverages. The different options for data access and visualization are also shown.}
     91Note that, as explained before, the variables provided by the data portal (e.g. minimum temperature) are stored as gridsets. Thus, in addition to these variables, also auxiliary coordinate variables (lat, lon, run, time, member) should be handled for geo-temporal data referencing (see Fig. \ref{fig:opendapwin}). Moreover, three time coordinates are included as referece for different grid variables because they are defined for different forecast times (one extra time for precipitation and different temporal resolution for mean sea level pressure). Note that this highly complicates the direct analysis of the data and, hence, this options is only recommend for data exploration. In the following we show how to use this service to explore the structure of the datasets and to obtain simple pieces of information in `ASCII format`.
     96\includegraphics[width=0.8 \linewidth]{openDAPwindow.png}
     97\caption{Detail of the OPeNDAP dataset access form for a particular dataset.}
     103By default, if no specifications are given in the different subsetting boxes of the OpenDAP form, the whole data on the whole spatio/temporal and member ranges of the dataset would be accessed. However, this option will raise an error due to the large size of the request (the maximum size of a single request has been set to 100 Mbytes in the ''SPECS-EUPORIAS data portal'' for the sake of multi-connection efficiency). The basic steps to retrieve subsets of data are the following:
     105\item To select a variable click on the checkbox to its left.
     106\item To constrain the variable, edit the information that appears in the text boxes below the variable. This is a vector of integers indicating index positions of length three, with the following order: \texttt{[start:stride:end]}.
     107\item To get ASCII or binary values for the selected variables, click on the ''Get ASCII'' or ''Get Binary'' buttons of the ''Action'' field. Note that the URL displayed in the ''Data URL'' field is updated as you select and/or constrain variables. The URL in this field can be cut and pasted in various `OPeNDAP` clients.
     111The main disadvantage of the OpenDAP service from the end-user point of view is that the specifications for subsetting dimensions are not given in their original magnitudes (i.e., latitudes and longitudes are not given in decimal degrees), but by the indexes of their position along their respective axes (note that first index value is always 0). Thus, to find out the indexes for the desired selection, we need to dump and analyze the particular values defined in the coordinate variable. For instance, Fig.~\ref{fig:latlonDump} shows the 241 values defined for the `lat` (latitude) coordinate, as provided by the ''Get ASCII'' option (selecting the corresponding check-box).
     117\caption{Text file displaying the values for the \texttt{lat} (latitude) coordinate variable.}
     123Using these facilities it can be obtained after some calculations that the closest `lat` and `lon` coordinates for a particular location of interest (e.g. Madrid) are 66 and 475, respectively. Thus, the time series for Madrid corresponding to the example described in the previous section (minimum temperature forecasts for January with one-month lead time, i.e. from the simulations started the first of December) could be requested as shown in Fig. \ref{fig:opendapquery}. Note that the indices selected for the run coordinate correspond to the December initilizations (index positions 11, 23,...; note that indexes start in 0) and for the time coordinate correspond to January (positions, 31 to 62, in days after the run time). Note that the proper use of this service requires a full understanding of the data structure and, therefore, it is only advised for data exploration.
     128\includegraphics[width= 0.95 \linewidth]{opendapquery.png}
     129\caption{Detail of the query from the OPeNDAP dataset access form to retrieve a subset (a time series for a single gridbox) of minimum temperature.}
    56135= Example of Data Analysis with `R` = #app1
    57136= References =