Types of provider
Printed maps are, by definition, rasters, but in the online world whether you provide your mapping data 'raw' or in some rasterised form depends on your objectives, so online map providers can broadly be divided into different categories:
- in most countries, a national or regional map agency has traditionally been responsible for producing maps. These are normally government bodies, and were often originally created for military purposes, as names such as Britain's Ordnance Survey make clear. They vary in their attitude to copyright, what data they provide, and how commercially oriented they are, i.e. whether they charge for the data or the licence to use it. In the EU, the INSPIRE directive is opening up online mapping from government bodies, particularly for non-commercial purposes
- these map agencies were however generally slow to implement online mapping, which enabled general online service providers to fill the gap. Of these, Google is by far the best known, though Yahoo! and Microsoft provide similar services (the latter used to be known as Virtual Earth, but was recently renamed Bing for reasons best known to Microsoft). Generally speaking, these do not actually produce maps themselves, but license them from commercial producers. Because of this, they do not provide any vector data. They do however provide their services for free, reckoning to recoup their costs from related advertising and the like
- other companies and individuals also provide map data; the latter do not have the resources to create full-blown topographical maps, but as any GPS device can record any point or line, for example, a walking or cycling route, there is now a large amount of vector data online provided by individuals
- individuals can also co-operate in online ventures for creating maps. Of these, OpenStreetMap is the best known, a collaborative venture using GPS devices and similar technologies to produce copyright-free maps which can be used by anyone without restriction. Inevitably, this sort of volunteer-based project is patchy in coverage, but some parts of the world are now well-mapped
In the paper world, the country or region is split into different sheets dependent on the scale, and an overview sheet shows which sheet corresponds to which geographical area. The online world is similar, with the total area being divided into so-called tiles, the area covered by each tile depending on the scale or zoom level. How these tiles are indexed varies, but the principle is much the same; see, for example, descriptions of the tiling techniques used by Google and Microsoft. JPEG and PNG are the bitmap formats most widely used for these raster tiles.
Besides the tiling schemes above, many servers use the Web Map Service protocol, a standard defined by the Open Geospatial Consortium (OGC).
Whichever access scheme is used, a provider can restrict access to registered users; some of these registration schemes are payable, some are free of charge.
An image provided in a tiling scheme is not necessarily a map in the traditional topographical sense. Any geographically referenced image can be split into tiles and served in the same way. This is commonly used for aerial or satellite imagery, ortho-rectified so that the images are fully compatible with projected maps.
There are an enormous number of different formats for storing vector data, and unfortunately little standardisation, though there are a number of different attempts to provide standards in XML, such as GML and KML, and JSON (GeoJSON). Commercial GIS tend to use their own proprietory formats, such as ESRI shapefiles or DGN; every GPS manufacturer seems to use its own format (GPS Babel will convert them), though to some extent GPX provides a standard here; spatial databases tend to use WKT (Well Known Text) and its binary equivalent WKB. OGR is an open-source program which enables conversion between the most widely-used formats.
The OGC equivalent of WMS for serving vectors over the web is the Web Feature Service. WFS servers provide geographical features in a standardised GML format, with standardised requests for spatial and/or attribute selection.
See the IT section of this website for further info on what software is used on this site.