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GIS Files 6: Expert GIS concepts

6.1: Data formats (2)

Proprietary file types

Every software product is designed to work with a specific set of file types. In essence that is what the software does: it reads the particular binary code to extract the stored information and then does something with it, for example, displays it on screen, sends it to a printer or performs calculations. Commercial software products usually have their own specific binary formats.

It would be impossible to describe the full range of different file types used in GIS as there are so many. However, it is important to recognise that each product handles the storage of information in different ways and, to fully understand what is happening to the data, it is useful to consider the files involved for your own system – what's going on under the bonnet, so to speak. Here are a few examples:

Tab files example

  • single file for each layer – in some systems all information for a given layer is stored in a single file (for example, .dxf files in AutoCAD®);
  • multiple files for each layer – some systems use a series of files for each layer (for example, MapInfo® has a .tab file for each table of information, but this is just a pointer to a set of files containing the geometry, attributes, identifiers and indexes separately); and
  • a folder of files for each layer – more complex systems can use a more complicated hierarchy of files held in a specific folder to store the information (for example, ArcInfo® coverages).

In these last two examples you only ever interact with the data through the GIS software interface. The individual files are not designed to be edited outside the GIS as this will almost certainly corrupt the data.

GIS can read image data from standard graphics file formats but often need an additional file to register the image in space. Examples of such files are MapInfo .tab and ESRI® .tfw. These are in fact simple ASCII files. If you have any examples on your own computer try viewing the contents in a text editor; this can be useful to understand how they work. On the next page we'll see how ASCII files can be important in the transfer of data between systems.

< 6.1: Data formats (1) | 6.1: Data formats (3) >

ArcInfo and ESRI are trademarks of Environmental Systems Research Institute, Inc. AutoCAD is a registered trademark and DXF a trademark of Autodesk Incorporated. IBM is a registered trademark of International Business Machines Corporation. Ingres is a registered trademark of Computer Associates International, Inc. MapInfo is a registered trademark of MapInfo Corporation. Microsoft is a registered trademark of Microsoft Corporation. OpenGIS is a registered trademark of Open GIS Consortium, Inc. Oracle is a registered trademark of Oracle Corporation. Sybase is a registered trademark of Sybase Inc.

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