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|Full name||ERDAS IMAGINE File Format (IMG)|
The format we are calling ERDAS_IMG to distinguish from other uses of the .img file extension is a proprietary, partially documented format for multi-layer geo-referenced raster images developed originally for use with ERDAS IMAGINE software. This format is used widely for processing remote sensing data, since it provides a framework for integrating sensor data and imagery from many sources. This description covers all chronological versions of the format because the compilers of this resource have been unable to find documentation that clearly distinguishes between formats as produced by different versions of the software. Comments welcome. This format is one of the formats used for data delivery via the National Map as of early 2015, in particular for the National Elevation Dataset (NED).
A key capability of ERDAS_IMG is that it distinguishes between two types of raster layer:
Another key feature of the ERDAS_IMG format is that it is designed to store technical and georeferencing/geocoding metadata from the source data imported as layers. This supplies a record of provenance. See External File Format Header Object Types.
ERDAS IMAGINE software uses a tiled format to store raster layers. The tiled format allows raster layers to be displayed and resampled quickly. The default tile size used to be 64 pixels by 64 pixels, but now appears to be 512 pixels by 512 pixels. Within a tile, raster data uses the BSQ_enc ordering for pixels. Optionally, a large raster layer may be organized in a "pyramid" for faster processing. In pyramid layers, reduced, subsampled raster layers are created from the original raster layer.
ERDAS_IMG is based on a Hierarchical File Format (HFA) structure. Many of the items that can be included are optional. In addition, because of the open nature of the HFA format, developers may create and add new types of items to the file. Raster layers may be compressed using a variant of Run Length Encoding known as Dynamic Range Run Length Encoding.
For images requiring more than 2 Gbytes of disk space (although some sources indicate that the limit is now 4 Gbytes), a two file dataset is required. The .img file contains the usual structure of headers and structural metadata but the actual image data is kept in a separate non-HFA file format, known as a Large Raster Spill File, with the extension .ige.
|Production phase||Primarily a middle-state format, used to hold a collection of raster layers derived from differing sources, and aligned in preparation for flattening to a single layer for printing or display, for example as GeoTIFF. See Notes below for an example workflow. An ERDAS_IMG file is often archived or distributed as a version that supports the ability to revise the final image, for example, by adjusting coloring for thematic raster layers or merge with additional data.|
|LC experience or existing holdings|
|Disclosure||Proprietary, partially documented. Originally developed by ERDAS, Inc., now part of Hexagon Geospatial.|
|Documentation||Partial documentation in the help system for ERDAS IMAGINE software at http://doc.hexagongeospatial.com/ERDAS%20IMAGINE/ERDAS_IMAGINE_Help/#ERDASIMAGINEFileFormatConcept.htm. See all links in the IMG HFA MIF section of the help files.|
Various government agencies and projects distribute data in ERDAS_IMG format, including: the Multi-Resolution Land Characteristics (MRLC) consortium; the Coastal Change Analysis Program, and the National Elevation Dataset. Accompanying metadata sometimes mentions a particular version, presumed to refer to the version of ERDAS IMAGINE used to create the files. For example, the National Land Cover Database from MRLC, uses version 9.3.
Viewing the ERDAS_IMG multi-layered images requires specialized software. A free desktop viewer, ERDAS ER Viewer is available from Hexagon Geospatial. ERDAS_IMG files can be viewed in GeoViewer, freely downloadable from LizardTech. ERDAS_IMG files can be read and written by ArcGIS Desktop and other ESRI applications. See Supported raster dataset file formats from ArcGIS help. It is also supported by Safe Software's FME engine for format conversion.
There appear to be two software libraries that support the reading and writing of ERDAS_IMG files:
|Licensing and patents||TBD|
|Transparency||Header information and definitions for layers and tables are in ASCII. However, the raster data itself is in binary and usually highly compressed.|
|Self-documentation||TBD. Incorporates header information from imported source files.|
|External dependencies||None beyond the need for specialized software.|
|Technical protection considerations||No support for encryption within the format is apparent from the documentation.|
|Normal rendering||ERDAS_IMG is designed for building raster images from layers from different sources. It is not typically used for digital photography or other images from a single source. The technical structure of an ERDAS_IMG file supports panning, zooming, and printing given a choice as to which raster layers to display.|
|Clarity (high image resolution)||The format is designed to use different number of bits per pixel for each band in each layer depending on the source data. Values up to 64 bits per pixel are supported. Spatial resolution (e.g., in pixels per inch) is not relevant for remote sensing images. In terms of pixel dimensions, the format is used for very large images mosaiced from many sub-images.|
|Color maintenance||Strict color maintenance is not seen as a requirement for remote sensing data and there is no support for ICC profiles in ERDAS_IMG.|
|Support for vector graphics, including graphic effects and typography||No support for vector graphics.|
|Support for multispectral bands||Good support. A raster layer may have any number of bands.|
|Functionality beyond normal rendering||Can incorporate and align many raster layers in a way that permits users to choose which layers to present on a display or print.|
|GIS images and datasets|
|Normal functionality||Georeferencing/geocoding metadata can be stored for each source layer in an ERDAS_IMG file. This allows layers to be transformed to a single projection for display or printing.|
|Support for GIS metadata||Richer contextual metadata for files distributed in this format is typically stored in an accompanying file. USGS files have accompanying FGDC metadata, often in several formats (e.g., XML, HTML and plain text).|
|Support for grids||The raster data in layers in an ERDAS_IMG file is inherently griddable.|
||All ERDAS_IMG resources include a .img file. But large images over 4 Gbytes (and sometimes those between 2 and 4 Gbytes as well), also have a .ige file. See below.|
|Magic numbers||ASCII: EHFA_HEADER_TAG
||This magic number is not unique to the .img file from ERDAS. It applies to other ERDAS files based on the HFA structure.|
||Used for a Large Raster Spill File if needed.|
|Magic numbers||ASCII: ERDAS_IMG_EXTERNAL_RASTER
||Applies to files with .ige extension.|
As of January 2015, ERDAS IMAGINE software is distributed by Hexagon Geospatial, which is part of the Intergraph Corporation. Intergraph is in turn part of the Hexagon conglomerate, which acquired Intergraph in 2010.
The Landsat team recommend a workflow using an ERDAS_IMG file as an intermediate file in order to fill in pixels that are missing in source images from Landsat 7. The final output of the workflow is a GeoTIFF image designed to be aesthetic rather than scientifically precise. Preparing Image Base Maps for the Digital Aerial Sketchmapping (DASM) System, from the U.S. Forestry Service, provides instructions for another map-making workflow. This workflow outputs the ERDAS_IMG file as well as files in JFIF_1_02 (JPEG File Interchange Format) and MrSID formats.
See Hexagon: Our History for the complex chain of corporate acquisitions. The Internet Archive reveals a view of the complex branding context for ERDAS IMAGINE over the years. A website for ERDAS, www.erdas.com, was first captured by the Internet Archive in 1997. The company described itself as a "mapping software company specializing in Geographic Imaging solutions since 1978." The version of ERDAS IMAGINE was 8.3. The company description in August 2008 indicates that ERDAS was acquired by Leica Geosystems in 2001 and that Hexagon had acquired Leica Geosystems in 2005. ERDAS IMAGINE 2009 was version 9.3. By June 2012, www.erdas.com redirected to geospatial.intergraph.com. As of January 2015, the earlier URLs redirect to www.hexagongeospatial.com, a site first crawled by the Internet Archive in May 2014.