WebGIS client functionalities & OGC interfaces

The DRB GIS will provide two WebGIS clients (public and expert WebGIS) which are based on the same technology and are presented in the same look-and-feel. The toolbox of the expert WebGIS includes more functionality than the public version. In both cases, the WebGIS client’s main task is to serve as viewing tool for geographic data.

The graphical user interface includes functions like the following:

Figure 31: WebGIS – functions

> "Identify" for retrieving database information on geographic objects > "Zoom in" for enlargement of the current map extent

> "Zoom out" for reduction of the current map extent > "Zoom to full extent" of Danube river basin

> "Go back" to previous extent > "Pan" for moving the map extent > "Search" is the link to the query tool > "Help" for support

By clicking on the search button, a query mask is provided. The available geodatasets and their attribute data can be limited to those the user is interested in. Depending on the users'

privileges, only pre-defined queries are available or the user may additionally define his/her own.

5.5.1. Public WebGIS

Any analysis produced by experts needs specialised knowledge for correct interpretation. Raw data is not suitable for public users who lack factual know-how to understand possible errors or anomalies. Therefore, these users need geoinformation products that minimize the possibility of misinterpretation while offering a maximum of information from soundly pre-processed data. Apart from the main purpose of the public WebGIS, the viewing of geodatasets in the form of maps, retrieving information from the attribute information is required. Basically, there are two ways to provide the public users with attribute information:

> use the "identify" tool

> use pre-defined queries available in the query tool

The identify tool corresponds to a tool included in every GIS software to get information connected to one specific geographic object (e.g. to click on the symbol of a city and thus retrieve e.g. the city name and the number of inhabitants).

Pre-defined queries have to be devised by the system administrator. The result shows information that the public users can easily understand. By using appropriate symbolization and classification, differences and similarities are visible at first sight.

5.5.2. Expert WebGIS

The expert WebGIS looks similar to the public WebGIS but includes more tools to examine the underlying data. The expert user's focus is not on only viewing geographic information and on the visual interpretation of values, but rather on the extraction of information as a basis and illustration e.g. for the creation of reports. The query tool supports the expert user in retrieving specific data in a chosen spatial extent and allows showing the query results as tabular data or

in the map window. The possibility of downloading data allows further processing in an extra- DRB GIS-environment (e.g. for statistical analysis).

The extension of the public WebGIS towards the expert version results from the expansion/addition of the following functionalities:

> Expert WebGIS client > (extended) query tool > Data download tool

5.5.3. OGC Interfaces

The Open Geospatial Consortium (OGC) is an international organization that is leading the development of standards for geospatial and location based services to facilitate interoperability of GIS data. OGC works with private enterprises and governmental and academic institutions to create open and extensible software application programming interfaces for GIS and other mainstream technologies. The specifications are adopted by ISO for the development of international standards (e.g. Web Mapping Service ISO/DIS 19128).

Two OGC services have to be considered in the system definition phase of the DRB GIS: > Web Mapping Service (WMS)

http://portal.opengis.org/files/?artifact_id=5316 (31. 1. 2005) > Web Feature Service (WFS)

https://portal.opengeospatial.org/files/?artifact_id=7176 (31. 1. 2005)

A Web Map Service (WMS) produces an image (e.g. GIF, JPG) of GIS data. The WMS is a protocol that gives clients access to an image (not the features themselves) of geodata via a specially structured http request. In that way, the data can be called into an (GIS) application without being stored on a local hard-disk or network.

If showing georeferenced images is not sufficient, the Web Feature Service provides a useful possibility. This service is necessary if the client does not only want to view the images of data, but rather wants to use the actual features (e.g. for analysis and query).

WFS harmonizes the access to data by using a standardized access method: > Create a new feature instance

> Delete a feature instance > Update a feature instance

> Get or Query features based on spatial or non-spatial constraints. Furthermore, the following WFS operations can be carried out:

> GetCapabilities (obligatory) > GetFeature (obligatory)

> DescribeFeatureType (obligatory)

> Transaction (Insert, Update, Delete) (optional) > LockFeature (optional)

> GetFeatureWithLock (optional).

Queries are not stated in SQL, but in an OGC XML-based query language called Filter Encoding Implementation Specification.

One of the main advantages of integrating standard interfaces is that they are developed independently of a specific software. If the centralized DRB GIS server incorporates, for example, WMS, the web mapping servers of the participating countries can simply include

georeferenced images of these geodatasets in their national webmapping application. In a further DRB GIS development phase, the centralized DRB GIS server can work the other way round: the countries provide their data as WMS service, and the central DRB GIS web mapping server merges these data to one map of the Danube River Basin. If the DRB GIS should be turned into a decentralized system in the future, both OGC interfaces (WMS and WFS) together have to be supported (at least) in a forthcoming DRB GIS project phase.