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Implementing Web GIS Applications using Open Source Software

RDBMS Component
RDBMS enables the fast storage and retrieval of large amounts of information. The interface that is used to insert, manipulate, and extract data is called the Structured Query Language (SQL). In case of the prototype system, the PostgreSQL database server has been used to manage the attribute data and multi-media content (e.g. image files). PostgreSQL is an Object-Relational DBMS, supporting almost all SQL constructs, including sub-selects, transactions, and user-defined types and functions. The database consists of tables that hold all the available attribute data. The attribute table includes fields that can be used to store binary data (images, multi-media contents etc.). Remote update of the database by permitted users is also made possible. An authentication mechanism to check which users can be allowed to enter new data or update the existing attribute data in the relational database is also provided.

Web Component
The system includes of two kinds of web interfaces that enable online access to the GIS layers and attribute information. Access to spatial data (GRASS database) is provided using the GRASSLinks interface (HUSE 1995) and PHP (PHP Hypertext Preprocessor) is used as an interface to access the attribute data (PostgreSQL). Since a majority of GRASS commands can be executed in the command mode, it is quite easy to enhance the GRASSLinks interface to provide added functionality to the system. In prototype system, several additional features were incorporated into the GIS web-interface to tailor the system for its present requirements. PHP is a server-side scripting language. PHP scripts are included within an HTML document to confer it the capacity of generating web contents on demand. Both GRASSLinks and PHP are distributed under the GNU General Public License and can be deployed on variety hardware platforms. PHP can be used to connect to several database engines such as PostgreSQL, MySQL etc.

System Features
An online demonstration of the basic features is available at http://gisws.media.osaka -cu.ac.jp/slink/. “Spatial Query” option allows the user to retrieve attribute data from the RDBMS table by selecting a location on the raster image displayed on the web browser. The user selects the GRASS data layers from an interactive menu based on which the GRASS raster layer is displayed on the web-browser. The user can also select vector maps and site data as overlays for raster map layer (Figure 2). Interactive zoom/pan capability allows the user to view the displayed maps in greater details or to choose different areas for display. Once the desired area is displayed on the web-browser, the user is allowed to view the attribute table by “clicking” on respective site. The relation database is queried based on the geographical location (Figure 3a) of the “clicked” site. Attribute data is displayed in two stages. Firstly, a brief summary (Figure 3b) of the attribute information is presented. The summary table also includes a hypertext link, which can be followed to view more detailed information including figures and field photographs is also provided (Figure 4a and 4b).

Figure 2 Query basemap

Figure 3 (a) Location (b) Query results.

    
Figure 4 Detailed attribute information on individual landslide. (a-left, b-right)
“Database Search” option allows the user to retrieve attribute information by keyword searching (Figure 5). Search fields include name, location and date. In addition full-text searching is also provided. The results of the text based searching are the same as those shown in Figure 3b and Figure 4.