DChart - Development of an interface for the conversion of geodata in a NetCDF data model and p

The Dapper Data Viewer – called DChart – is a browser-based user interface of PMEL that is supported by Dapper to interactively browse, select, visu- alize and download datasets and metadata from local or remote sources, ac- cording to Cornillon et al. (2009), Sirott (2002) and Sirott (n.d.c). The development of DChart was funded by NOAA’s High Performance and Computing (HPCC) programs (Sirott, 2002) and is part of the Dapper server distribution as interactive web client (Sirott, n.d.a). It was created due to the availability of a huge amount of oceanographic in-situ observations through the OPeN- DAP protocol, but the lack of suitable web applications that support the in-situ OPeNDAP Sequence protocol, as Sirott (2006b, n.d.c) signifies.

The DChart-servlet was written by Joe Sirott of NOAA/PMEL (Sirott, 2009, n.d.b). It can be accessed by default under the URL http://localhost: 8080/dchart/, assuming that the default settings of the installation were not modified (Sirott, 2009, n.d.b). OPeNDAP clients in general – such as DChart– are discussed in subsection 2.2.3 on page 47.

2.4.1 Design goals

Sirott (2002) describes that the design goals of DChart were to create an highly responsive and easy to learn user interface based on a web browser technology without the need of installing any Java applets or proprietary plug-ins. The interface should be designed for previewing and downloading Dapper datasets, with the possibility of selecting specific desired areas by the use of a highly interactive map (Sirott, 2002).

2.4.2 Functionality

DChart is interactive and contains a dragable interactive map, a layer for selecting categorized data stations, and a plot window for visualizing data from one or multiple stations as well as from gridded data, as explained by Sirott (n.d.c). Several plot types that allow direct zoom and pan interaction with the chart are also supported by DChart as Sirott (n.d.c) notes. A feature to center the interactive map at a specific location is realized by the GeoNames geographical database3_{, available as a web service for DChart}

(Sirott, 2009).

The name of a variable in DChart corresponds to the variable name of an in-situ file served by Dapper via the CDP-service. It is possible to change this name for such data by modifying the DChart configuration file. In case that data is served by the Dapper NetCDF-service, the variable name in DChart corresponds to the variable attribute entry long_name in a NetCDF file.

Data streamed to DChart from Dapper via OPeNDAP is defined by URLs that are accepting the schemes for local file access, DODS and HTTP. OPeN- DAP Sequence data streamed to DChart by Dappers CDP-service is read from the Climate Data Portal server. Requested in-situ data can be downloaded – from Dapper version 2.1.0 – as uncompressed or compressed TSV or CSV file, as well as Java Ocean Atlas JOS file, and gridded data can be downloaded in DChart as NetCDF file. However there is no function implemented to download in-situ data as NetCDF file (Sirott, 2009, n.d.b). Beside

3_{GeoNames. Retrieved 29 March 2011, from GeoNames Inc.: http://www.geonames.}

the possibility of downloading data, a functionality allowing the animation of data in Google Earth is furthermore implemented from version 1.0.0RC1 (Sirott, 2009).

Sirott (2002) specifies that DChart requires actual browsers to run , but that it does not need any applets or plug-ins, as Sirott (n.d.c) also states. Occasionally incompatibility problems can occur with some browsers due to different architectures that may not be always supported. Sirott (2002) mentions that the libraries and frameworks of DChart are immature and that debugging, maintenance and testing is difficult, also due to the fact of many different technologies that are used within DChart.

Figure 2.10 on page 65 shows a capture image of the DChart user interface. A larger capture image of this CEOP-AEGIS Data Portal can also be found in the appendix at section A.3 on page 124. Data visualized in this capture image was derived from the numerical model output data of GRAPES. This is a non-hydrostatic numerical weather prediction model and stands for Global/Regional Assimilation and Prediction S ystem, developed by the China Meteorological Administration (Menenti et al., 2010; CEOP-AEGIS Inc. 2010; Zhang & Shen, 2008). GRAPES data is used within the CEOP- AEGIS project by algorithms to calculate the surface energy balance index (see Faivre, Colin, Jia, & Menenti, 2010)

2.4.3 System architecture

By the reason of the continuing development of web browser technologies such as JavaScript, DOM, XML or XSLT that are used within DChart, Sirott (2006b, n.d.c) states that it was possible to create a web interface with richer functionalities as traditional technologies would permit. DChart is accessible trough a web browser, but the functionality is similar to a desktop application (Sirott, n.d.c).

The DChart technology is – like for example also Google Earth – based on an AJAX (Asynchronous J ava and X ML) user interface – a JavaScript remote scripting technology that is using asynchronous requests to commu- nicate with the web server (Sirott, 2002, n.d.c). By the use of this remote scripting technology, DChart obtains web access to the requested in-situ or gridded data collections (Sirott, n.d.c). Sirott (2009, n.d.b) explains that the AJAX user interface implemented in DChart uses portions of Yahoo’s UI JavaScript Library as well as of the Prototype JavaScript library for gener- ating a dynamic web application.

The web browser user interface DChart (that was designed by the use of the JavaScript technology) communicates via XML with the DChart servlet. So called XmlHttpRequest objects are used by JavaScript to request data from

Figure 2.10: Capture image of the browser-based OPeNDAP user interface DChart for accessing data from the Dapper OPeNDAP data server. A larger image can be found in the appendix at section A.3 on page 124

the server asynchronously via XML. In return, when the browser is receiving the requested data, these requests are transformed to HTML fragments via the XSLT (Extensible S tylesheet Language T ransformations) technology, a XML-based language for the transformation of XML documents. In this regard, a HTML Document Object Model (DOM) is used to insert these HTML fragments in the HTML page, as detailed by Sirott (2002).

The open-source Java chart library JFreeChart in combination with the SGT library developed by NOAA allows the visualization of charts within DChart. JFreeChart serves for station plots, and a modified version of SGT for gridded plots. The Java library GeoTools is used by DChart to process shapefiles (Sirott, 2009, n.d.b).

Figure 2.11 on page 66 illustrates the architecture of DChart (Sirott, 2002), as it was explained before.

Figure 2.11: System architecture of the OPeNDAP client DChart (Sirott, 2002)

In document Development of an interface for the conversion of geodata in a NetCDF data model and publication of this data by the use of the web application DChart, related to the CEOP-AEGIS project (Page 92-96)