Raster GIS File Formats

Raster GIS File Formats

Raster datasets are a matrix of cells or pixels where each cell contains a value representing information, eg temperature. Rasters examples include aerial photographs, satellites imagery, digital pictures and scanned maps.

Extension File Type Description
ERDAS Imagine (IMG) .IMG

ERDAS Imagine IMG is a proprietary format that was developed by Hexagon Geospatial. IMG files are frequently  sed for raster data in storing  single and multiple bands of satellite data.

IMG files use an hierarchical format HFA, that can be used  to store basic information about the file. This data and file type is non-mandatory and may include file property information, ground control points and sensor type.

Each raster layer as part of an IMG file contains information about its data values. For example, this includes projection, statistics, attributes, pyramids and whether or not it’s a continuous or discrete type of raster.

American Standard Code for Information Interchange ASCII Grid .ASC

ASCII uses a set of number,  including floats, between 0 and 255 for information storage and processing. They also contain header information with a set of keywords.

In their native form, ASCII text files store GIS data in a delimited format. This could be comma, space or tab-delimited format. Going from non-spatial to spatial data, you can run a conversion process tool like ASCII to raster.

The GeoTIFF has become an industry image standard file for GIS and satellite remote sensing applications. GeoTIFFs may be accompanied by other files:
  • TFW - this is the world file that is required to give your raster geolocation
  • XML - these files optionally accompany GeoTIFFs to include all metadata
  • AUX - these are auxiliary files that store projections and other information
  • OVR - these are pyramid files that improve performance for raster display

IDRISI assigns RST extensions to all raster layers, which consist of numeric grid cell values as integers, real numbers, bytes and RGB24.

The associated raster documentation file RDC is a companion text TXT file for RST files. This assigns the number of columns and rows to RST files and further records the file type, coordinate system, reference units and positional error.

Envi RAW Raster .BIL
Band Interleaved BIL files are a raster storage extension option for single or multi-band aerial and satellite imagery.
  • Band Interleaved for Line, which is where the acronym BIL originates, stores pixel information based on rows for all bands in an image
  • Band interleaved by pixel BIP assigns pixel values for each band by rows
  • The Band sequential format BSQ stores separate bands by rows

BIL files consist of a header file HDR that describes the number of columns, rows, bands, bit depth and layout in an image.

PCI Geomatics Database File (PCIDSK) .PIX

PIX files are raster storage layers developed as a proprietary format system by PCI Geomatics. It’s a flexible type that stores all image and auxiliary data called 'segments' in the platform, as self-contained files. For example, segments can include image channels, training site and histogram information.

As a database file, PIX files can hold raster channels with varying bit depths. They can also store projections, attribute information, metadata and imagery or vectors.

Esri Grid  

Grid files are a proprietary format developed by Esri. Grids have no extension and are unique because they can hold attribute data in a raster file. But the catch is that you can only add attributes to integer grids.

Attributes are stored in a value attribute table VAT – one record for each unique value in the grid, and the count representing the number of cells.

The two types of Esri Grid files are integer and floating point grids. Land cover would be an example of a discrete grid. Each class has a unique integer cell value. Elevation data is an example of a floating point grid. Each cell represents an elevation floating value.

Compressed files .ZIP

ZIP files are compressed collections of files of any type (not exclusively raster files), and are commonly used to store and share collections of raster GIS files.

This compression format is appended to this list as Asseti consumes ZIP collections of raster GIS files.