Viewing and Downloading Images

The Standard Data Products (.imq extension) are compressed and require decompression software to be used before viewing. The Standard Decompressed Data Products (PDS format, .img extension) can simply be downloaded for viewing by clicking the appropriate link. You will need the NASAView software to view all decompressed images.

You can also either view or download the Actual MOC image (JPEG Version) and two related "context" images. To view these images, select (click) the appropriate browse image. On the resulting page, you can then download by selecting the image using the right-mouse button to activate the "Save image" option. You may also download the image directly from the browse page; select the appropriate image by using the right-mouse button to activate the "Save Link" option. The browse images themselves may also be downloaded by using this procedure.

(mouse-button options may vary depending on your platform and browser)

DISCLAIMER

THE BROWSE (JPEG) IMAGES RENDERED ON THIS WEB SITE ARE MEANT SOLELY TO FACILITATE THE SELECTION OF RAW MOC IMAGING PRODUCTS FOR DOWNLOAD AND USE BY THE SCIENTIFIC COMMUNITY. THE BROWSE AND JPEG IMAGES ARE NEITHER RADIOMETRICALLY NOR GEOMETRICALLY ACCURATE AND SHOULD NEVER BE USED FOR QUANTITATIVE OR INTERPRETATIVE PURPOSES.

PROCESSING APPLIED TO BROWSE (JPEG) IMAGES

Orientation - Because of the MOC's line-scan nature, depending on the orientation and sense of rotation of the spacecraft and the optical inversion properties of the camera system involved, MOC images can either be normal or flipped left-to-right (independent of whether the image is north up or south up.) This flag will be 'MIRRORED' if the image should be flipped left-to-right, and 'TRUE' if it is already in normal form. For those images having a value of 'MIRRORED', the JPEG images have been flipped left to right.

One of the inherent artifacts seen in the raw MOC images results from varying response in the sensitivity and bias of each detector in the push-broom array. The visual effect is seen as dark and light vertical (top to bottom) "streaks". An additional artifact is the aspect ratio distortion of the raw imaging caused by the observational characteristics of the imaging system. The aspect ratio effect causes surface features to appear geometrically distorted such as elliptical or oblong craters that would actually be round in an undistorted view.

The browse (jpeg) images created for the MOC Wide-Angle imaging contain all the artifacts of the raw imaging and only a contrast stretch was applied to the images.

Browse (jpeg) versions of the MOC Narrow-Angle underwent processing, as described below, to cosmetically reduce the radiometric "streaking" and provide an approximate aspect-ratio correction.

A first order correction has been applied to the images to minimize the "streaks". The cosmetic procedure first determined the average brightness value for each column in the image array. Next, each pixel of the column was divided by the column average and then multiplied by the average brightness of the entire scene. This method works well in most circumstances. However, scene dependent brightness variations can cause residual "streaking" in the processed images.

The downtrack and crosstrack pixel resolutions of a MOC image may be different, resulting in pixels that are rectangular on the surface rather than square. The crosstrack to downtrack ratio, or aspect ratio, typically ranges from 5.0 to 1.0 but is generally about 1.5 for the MOC Narrow-Angle images. The crosstrack resolution is dependent on spacecraft altitude and the camera focal length and pointing, whereas the downtrack resolution is dependent on the integration time of the push-broom array and the relative velocity of the spacecraft as it sweeps over the Martian surface. Other factors that affect the resolution are the camera operating modes that allow for pixel summing in either the crosstrack or downtrack direction. A simple aspect ratio correction has been applied by resampling the image to adjust the crosstrack resolution to match the downtrack resolution. When applying an aspect ratio correction, the procedure "splits the difference" in the downtrack and crosstrack resolution using a simple bilinear resampling method. The lower-resolution direction is increased in size, and the higher resolution direction has reduced to create approximately square pixels.

For most of the pre-mapping phase of the MGS mission, data quality did not allow error-free transmission of the instrument data to Earth. The MOC protocols (in particular, the formats for compressed image data) were designed for the bit error rates expected in mapping. As a result, considerable data losses were incurred in the image data. The majority of processing for pre- mapping data was done to minimize the effects of this data loss. Image data that could not be recovered exists as blocks of missing (black) lines that can intermittently be seen in the images.

Many of the MOC Narrow-Angle images contained brightness variations due to solar illumination or albedo differences over the scene. A high-pass boxcar filter was applied to adjust the scene brightness in order to reduce these variations. A 301 x 301 high-pass boxcar filter was applied to the data and an average of the original scene and the high-pass filtered scene was used to create the final images.