Ordnance Survey – Great Britain's national mapping agency
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6.5: Web GIS (1)
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.
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.
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.