File Format (GIS)
GIS data can be simply described as geographical data to which attributes have been linked. Geographical data is all information that relates to space, for example, a road and a lake. Geographical features can be described with a matrix (raster data), or points, lines and polygons (vector data).
The format of the file describes in which way the geographical features have been saved. It is very important to consider the file format of GIS data because software programs rarely support all file types. If you want to use GIS data that was saved in a particular format not supported by your GIS program, you must either find a way to transform the data or simply use another GIS program.
Nearly all GIS programs have their own file format. These file formats were created to optimise the efficiency of the program itself, and were not designed to be used in other external programs. Nevertheless, most GIS programs support other formats by having functions to import and export datasets. These functions are usually well documented and standardised.
Below is a list of some of the most common GIS file formats. It should be noted that this list is far from being complete!
Many GIS programs are based on vector technology. These are the most complex methods because there are many ways to save coordinates, attributes, data structure and visualisation information.
ARC, Esri generate line;
Simple ASCII format which can handle point and line data.
DGN, MicroStation Design Files:
DGN is an intern format for MicroStation, a CAD program (CAD means “Computer-Aided Design”). This format is well documented and standardised, which makes it possible to use it an import/export format. DGN files contain detailed visualisation information (display).
DLG, Digital Line Graphs
DLG is used by the US Geological Survey (USGS) for handling vector information from printed paper maps. It contains very precise coordinate information and sophisticated information about object classification, but no other attributes. DLG does not contain any visualisation information (display). This format is mainly used by the USGS and other American agencies, which have used it for publication of many digital maps.
DWG, Autodesk Drawing Files
DWG is an intern format for AutoCAD. AutoCAD can convert DWG files to DXF files without loosing graphic information. There are many possibilities for saving attribute data in DWG files. A common standard method uses Extended Entity Data (EED) to link attributes, but other methods are possible. Because of the lack of standards for linking attributes, problems may occur while converting this format between systems.
DXF is a common transfer format for vector data. It contains visualisation information and is supported by nearly all graphic programs. There are many ways for saving attribute data in DXF format and to link DXF objects to external attributes (see DWG above). Nearly all programs can successfully import this format because of high standards.
E00 is a transfer format available both as ASCII and binary form. It is mainly used to exchange files between different versions of ARC/INFO, but can also be read by many other GIS programs. It is a common format for GIS data found on the Internet.
XML-standard for exchanging and saving geographical vector data. It is used in the Open GIS Consortium.
KF85, Kommunförbundets transfereringsformat (ISOK)
Is a format, which can handle point, line and polygon data, as well as text and symbols. But it is not possible to convert and exchange attribute data.
MIF/MID, MapInfo Interchange Format
MIF/MID is MapInfo’s standard format, but most other GIS programs can also read it. The format handles three types of information: geometry attributes and visualisation.
SDTS is a transfer format developed in the USA and is designed for handling all types of geographical data. SDTS can be saves as ASCII or binary. In principle, all geographical objects can be saved as SDTS, including coordinates, complex attributes and visualisation information. These advantages nevertheless increase complexity. To simplify it, many standards have been developed as “co-projects” to SDTS. The first of these standards is Topological Vector Profile (TVP), used to save some types of vector data.
SHP, ESRI shapefile
Shape is ArcView’s internal format for vector data. Associated to the Shape file (*.shp), there is a file to handle attributes (*.dbf) and an index file (*.shx). Nearly all other GIS programs can import this format.
XML-standard for presentation of vector on the Internet. It is approved in the World Wide Web Consortium.
TIGER is an ASCII transfer format made by the US Census Bureau to save road maps. It contains complete geographic coordinates and is line-based. The most important attributes include road names and address information. TIGER has its own visualisation information.
VPF, Vector Product Format
VPF is a binary format made by the US Defense Mapping Agency. It is well documented and can easily be used internally or as a transfer format It contains geometry and attribute information, but no visualisation information. VPF files are also named VMAP product. The Digital Chart of the World (DCW) is published in this form.
VXP, Idrisi32 ASCII vector export format
IDRISI 32’s vector export format (ASCII).
WMF, Microsoft Windows Metafile
WMF is a vector file format for Microsoft Windows Operation Systems. WMF files are actually a compilation of GDI (Graphics Device Interface).
Raster files are used generally to save images, like for example, a scanned paper map, digital photographs or satellite images, but also to save variables which continuously vary in space, like topography and temperature. Images from satellites, or other aircrafts, are known as remote sensing data. The resolution of the remote sensing data and scanned maps refers to the area on the ground covered by one pixel. This differs to other image data, where the resolution is given in dots per inch (dpi).
ADRG, Arc Digitized Raster Graphics
ADRG is a format created by the US military to save paper maps in raster format.
BIL, Band Interleaved by Line
BIL is a computer compatible tape (CCT) format that stores all bands of remotely sensed data in one image file. Scanlines are sequenced by interleaving all image bands. The CCT header appears once in a set.
BIP, Band Interleaved by Pixel
When using the BIP image format, each line of an image is stored sequentially, line 1 all bands, line 2 all bands, etc. For example, the first line of a three-band image would be stored as p1b1, p1b2, p1b3, p2b1, p2b2, p2b3, where p1b1 indicates pixel one, band one, p1b2 indicates pixel one, band two, etc.
BSQ, Band Sequential
BSQ is a computer compatible tape (CCT) format that stores each band of satellite data in one image file for all scanlines in the imagery array. The CCT headers are recorded on each band.
DEM is a raster format created by the USGS (US Geological Survey) for saving elevation data. In contrast to other raster formats where the cell values represent the colour intensity, the cell values in DEM represent the elevation for that position on the Earth’s surface.
*.dem, *.hdr, DEM ArcInfo
ArcINFO:s (ESRI) elevation data format.
GTOPO30, Global 30 Arc Second Elevation Data Set
GTOPO30 is a global, digital elevation model with a horizontal cell size of approx. 1km (30 seconds). GTOPO30 was created from different raster and vector sources.
GeoTIFF is a form of TIFF (Tag Image File Format) format for georeferenced raster data.
GRIB, GRid In Binary
GRIB is the World Meteorological Organisation’s (WMO) standard for grid-based meteorological data.
PCX, PC Paintbrush Exchange
PCX is a common raster format found in many scanners and graphic programs.
SDTS, Spatial Data Transfer Standard
SDTS is a format for transferring geographical information. A SDTS variant is specifically made for transferring raster data.
TIFF, Tagged Image File Format
Like PCX, TIFF is a common raster format produced by drawing programs and scanners. TIFF format gives a relatively big data file, but compresses the data without loss of information.