File:Mars Viking MDIM21 ClrMosaic 1km.jpg

English: This global image map of Mars has a resolution of approximately 1.0 km/pixel at the equator. It is a reduced version of a much larger map with 231 m/pixel resolution at the equator. The colorized mosaic was completed by NASA Ames which warped the original Viking colorized mosaic and blended over the latest black/white mosaic. The full resolution mosaic is known as Colorized Mars Digital Image Model (MDIM) 2.1. MDIM 2.1 replaces two earlier mosaics produced by the USGS from the same set of approximately 4600 Viking Orbiter images. The positional accuracy of features in MDIM 2.1 is estimated to be roughly one pixel (200 m), compared to 3 km for MDIM 2.0 released in 2001 and >6 km for MDIM 1.0 released in 1991. In addition to relatively imprecise geodetic control, the previous mosaics were affected by changing definitions of cartographic parameters (such as the definition of zero longitude), resulting in an overall longitude shift of as much as 0.2° between the early MDIMs and other datasets. The new mosaic uses the most recent coordinate system definitions for Mars. These definitions have been widely adopted by NASA missions and other users of planetary data and are likely to remain in use for a decade or more. As a result, MDIM 2.1 not only registers precisely with data from current missions, such as MGS and 2001 Mars Odyssey, but will serve as an accurate basemap on which data from future missions can be plotted.

The images used to make MDIM 2.1 were obtained primarily through the red, clear, and minus-blue filters of the Viking Orbiter imaging system, and thus provide a monochromatic view of Mars weighted toward the red end of the visible spectrum. Images were obtained with a wide range of solar incidence angles. It is unfortunately not possible to correct the appearance of both albedo (reflectivity) variations and topographic features for these incidence angle variations simultaneously. The images have therefore been highpass-filtered at a scale of ~50 km to remove regional albedo variations and then normalized so that equal topographic slopes appear with equal contrast everywhere. Photometric processing for MDIM 2.1 incorporates a model of the transmission and scattering of light in the atmosphere that is substantially improved over that used in MDIM 2.0. Residual tonal mismatches between different images after photometric correction were corrected based on a least-squares adjustment of image brightness and contrast. Because of these photometric and cosmetic improvements, it was possible to use a less severe highpass filter than for MDIM 2.0, improving the overall appearance of the mosaic.

Further information on the Viking image map may be found in this Planetary Society blog post: A new map of Mars from some pretty old data