3.1. File types and extensions
Often, GIS software has its own file types for vector and raster data. For example, the Environmental Systems Research Institute’s (ESRI) commercial ArcGIS software most commonly uses shapefiles for vector data and grids for raster data. Shapefiles and grids have come to be some of the most commonly known GIS formats and can often be read by and incorporated into other GIS programs without the need for conversion, as is the case for Quantum GIS (QGIS). It is important to note that most layers are made up of multiple files with different file extensions. For example, a shapefile may look like a single file in the GIS, but actually, it is composed of multiple file extensions. It is detrimental to the data layer if one of these extensions gets lost or deleted. Some of the extensions often associated with shapefiles are: *.shp (required file that stores the feature geometry), *.shx (required file that stores the index of the feature geometry), *.dbf (database table that stores the attribute information) or *.prj (file that stores the coordinate system information). The entire list of extensions can be found by searching “shapefile file extensions” in the ArcGIS help menu (http://resources.arcgis.com/en/help/main/10.1/).
Multiple raster dataset file formats including ASCII (*.asc), GeoTiff (*.tif), JPEG (*.jpg), and GIFs (*.gif), to name only a few, can be supported in most GIS software. Again it is important to keep all files when copying and moving data. For example, a Geotiff file often comes with two files, *.tif and *.tfw. The *.tif holds the image, and the *.tfw holds the information about the georeference.