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Space Topics: Space Imaging

Image Formats and Metadata

[EDITORIAL NOTE: A discussion of FITS, JPEG, GIF, PNG, and JPEG2000 formats needs to be added to this page.]

The images sent back from space missions are data -- the brightness or darkness of each pixel in an image represents a precise measurement of the intensity of light that is reflected or emitted by some object of interest to planetary scientists.  Therefore, it is critically important to researchers that the value of every pixel in a planetary image be unaffected by the way it is stored.  It is also important that every image be associated with metadata -- literally, "data about data."  Image data from planetary missions would be nearly useless unless they were accompanied by such information as when it was taken, by what spacecraft, with what instrument, pointed at what target, and so forth.

VICAR Format

Image data from the earliest planetary missions that returned imaging data (e.g., Mariner 9 to Mars, Mariner 10 to Venus and Mercury, Viking Orbiter and Lander missions to Mars, and the Voyager mission to the outer planets) was processed at the Jet Propulsion Laboratory (JPL) in the 1970s and 1980s.  At that time, almost all of the data formats now in routine use did not exist.  All of the images from these early missions were processed using a suite of image processing programs known as VICAR (for “Video image access and retrieval”).  The final image data products were sent to the data archives in VICAR format.  VICAR format includes a file header (usually ASCII text format) containing the metadata and image data stored line by line in individual records as binary data.

In the 1970s and early 1980s, the primary storage medium used to archive planetary image data was magnetic tape.  After several years, physical deterioration of the tapes was observed.  In addition, the computers and operating systems originally used to store the data onto the tapes were becoming obsolete.  In the mid-1980s, NASA funded JPL to recover as much as possible of the image data from the original tapes and archive the data onto contemporary media.  The data recovery task produced CDs containing as much data as could be recovered at that time.  The data format was not changed during the preservation project, which means that data from missions archived during the data preservation task are still in VICAR format.  Internet access to the CD-based data sets is provided by the PDS for those missions.  Public domain software exists to read the data and convert it into data formats more commonly in use by today’s commercially available image processing software (click here for a list of software useful for opening vicar formatted data).

PDS Format

During the 1980s, the Planetary Data System established standards governing the format of image data delivered to the PDS.  Later missions conformed to that format standard, and the PDS provides online software enabling users to download data from the PDS archives and convert it into other useful data formats. PDS formatted images usually have a .IMG file extension and have detached ASCII-formatted text labels containing the image's metadata.

"Raw" Formats

The word "raw" is currently used in two contexts to describe spacecraft images.  Recently, as the Internet has become widely accessible to the general public and standard image formats have evolved, several NASA planetary missions, starting with Mars Pathfinder, have made mission data available via the Internet within days or even hours of the receipt of data from the spacecraft.  These early versions of the images are often called "raw" images.  They are stored in lossy formats (such as JPEG), have typically not been calibrated by their instrument teams, and are usually not accompanied by much metadata, so they are not useable for scientific research.  They are also usually "stretched" to enhance their contrast, which makes it impossible to generate "true color" images from them.  They are released on the Web only to permit the public to watch the current status of a mission through a spacecraft's eyes.

"Raw" is also used to describe image data that is represented as it was returned from the spacecraft, before any calibrating or processing has been performed to correct for idiosyncrasies of the instrument, geometry, lighting conditions, etc.  Vidicon cameras especially produced images with noticeable geometric distortions and brightness variations, but even images taken with modern CCD cameras must be calibrated before they are scientifically useful.  Each instrument's science team processes, corrects, and verifies its own image products.  The PDS receives both the initial releases of raw image products from current missions and the final scientifically corrected versions.  ESA's Planetary Science Archive operates similarly.

Archival data records of planetary image data are available at a variety of "levels."  There are standards that define the various levels of data processing that can be performed on planetary image data.  The standards were developed by a National Research Council Committee on Data Management and Computation (CODMAC) in the 1970s.  The standards are shown in the following table.

Many thanks to Bill Green for his help in developing this page.