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Web-based GIS

Simple maps in web pages

The Internet and the world of computerised maps are tailor-made for each other. Maps are all about the visualisation of information; the Internet is all about the accessibility of information.

The World Wide Web (WWW) is founded on the exchange of simple files carrying a mixture of text and images. The form and content of these pages are encoded in Hypertext Markup Language (HTML), which can impart certain elements of behaviour. There are literally millions of web pages out there containing map images. Although this on its own does not really constitute GIS, there are some features of HTML that, when used in conjunction with map-based image content, can replicate some simple GIS-like functions within a standard web page.

HTML provides several different ways of presenting information and embedding links to further related pages of information. The simplest form is the textual hyperlink. Images can also be used as a hyperlink. The HTML image map tag enables the attachment of hyperlinks to specific portions of an image. HTML image maps are very popular as they can work with any image showing a set of objects, each with its own link to further information. These are very commonly used with maps and give different results depending where you click on the map.

The use of image maps is fairly crude and they can be confusing with too many hotspots.

Another rather more precise way is to use an image input element on an HTML form. This acts to submit the form, with the pixel location of the mouse click being passed to the next page as a pair of variables (this requires some server-side scripting). If you know the grid coordinates of the real-world extent of the map, and the dimensions of the image itself in pixels, you can generate GIS-like events based on the click location.

Internet mapping sites

Web authoring techniques are very useful because they can be built using standard web technology and do not require any specialist mapping software. They have one major drawback, however, as you are restricted to a single static map image; you cannot change the view of the map. It is much more exciting to be able to generate a new map dynamically based on choices made by the user.

There has been a remarkable explosion in the last few years of web sites offering the creation of maps for anywhere in the world, to any level of detail, based on user-defined parameters. The basic function of these sites is fairly standard: the user has a range of options for selecting a location, including place name, postcode, full address and grid location. There will be a zoom function and an ability to move around at a given scale in any direction. The key thing is that the map image delivered to the page is generated dynamically on the server; it is not a pre-prepared static image.

The range of Internet mapping sites is very diverse: some use purpose-built software, some use off-the-shelf Internet GIS software, some use raster imagery, some generate custom maps from vector data, and some provide printer-friendly versions.

Ordnance Survey web services such as OS OpenSpace and OS OnDemand provide raster imagery for customers to consume either using an API or directly via a GIS.

Internet GIS software

The Internet mapping sites described on the previous page primarily exist to generate user-defined maps. A large amount of map data is stored on the web server, a request for a map is received from a client terminal, a custom image is then generated by software on the server and this image is delivered back to the client within a web page. The software may differ but the end result is a map to look at or print.

The emerge of cloud technology and open source software has transformed the delivery and accessibility of Internet mapping and location services. It is standard practice for a web mapping site to cope with over 10m transactions per hour, with global server farms managed by Amazon for example.

The world of the GIS user is also being revolutionised by the Internet. All of the leading GIS software vendors now have products that adopt the client-server architecture. This means the software and data resides on a web server and multiple-client applications access the GIS processing functions across a network. This model is overtaking the use of multiple desktop software installations, especially in large organisations. It can even work across the WWW, and there are many sites that do more than just generate location maps. You can have GIS tools on the web page accessing the full range of GIS functions on the server: selection by attribute, spatial query, thematic mapping, data editing or 3-D visualisation . A major advantage of this model is that the centralised map data only has to be stored and maintained in a single location, meaning users are always viewing the most up-to-date records.

With Internet GIS there is always a trade-off between the sophistication of user tools and response times. Any system that uses the Internet is constrained by the download speed of the connection. The smaller the amounts of data being used and the simpler the client user interface, the faster the application. Internet GIS products differ in the way this balance is approached. Some systems use a very simplistic user front end and display the results of the server-side process by delivering a simple raster image. This means that the applications tend to be fast and robust and will work within a standard browser. Other systems require a client-side plug-in to be downloaded to give richer functionality to the user. Larger amounts of data can be downloaded from the server to the plug-in, which can make the system work more slowly; the benefit, however, is a more sophisticated set of user tools and greater interaction with the data. The choice needs to be made based on the specific requirements of the application and the expertise of the user community.

Web GIS futures

One of the ultimate goals of the GI industry is to have full interoperability between web-based geographic datasets enabling information stored at different locations on the web to be viewed together in single applications (see section 6.3 Standards). With many of the major current GIS products, not only can you access web-based client server versions of the software but the standard desktop software can also load files stored centrally on the Internet. So you could be looking at your locally held map files and then overlay a layer read from a universal resource location (URL) on the WWW. The full vision of interoperability will have web-based applications that can read all data files from any location on the Internet, irrespective of data formats or the software being used.

As well as reading a data layer from a URL it is also possible to submit specific queries (requests) and receive back one or more individual elements. This is known as feature serving. For example, a GIS application could submit a query to a web server requesting information about a district boundaries layer. The response could be a list of the district names held in a particular table. If the client then submits a request for a particular district, the boundary polygon geometry can be returned in response and the individual feature is served to the client application for display, analysis or download. The OGC is very active in establishing standards for map and feature serving on the web.

The model by which information is exchanged between systems across the Internet, with small packets of data being returned in response to specific queries, is becoming more pervasive in all areas of computing. Information providers can establish web services in which a single data store is created and standard open programming interfaces are published and made available to system developers.

This means that a system can be developed that can call in exactly the required pieces of information at exactly the time they are needed, rather than having to maintain many multiple copies of the same data, which can soon become out of date and degraded. Fuelled by increased bandwidth, better security and adoption of standards, the Internet has moved from the periphery to become a fundamental component of real-time IT architectures. The world of GIS is no exception to this trend and many GI web services are being developed and made available, replacing the situation of organisations having to obtain large volumes of map data to manage themselves.

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