There are two ways of extending the downtrack dimension. First, we can use "lossless" compression to extend the distance by a factor of about 1.8. We do this with most of the MOC images. However, losses within the communication system occasionally create black bands (accounting for the loss of about 7-15% of the data) through the images. The MGS Project felt such losses would not be tolerable in the Cydonia images. The second way to increase downtrack dimension is to decrease resolution. If you sum the pixels two by two (that is, replace four pixels--two across by two down--with the average of the four), you can extend the image downtrack by a factor of four. This, then, changes the dimensions of the image from 2048 X 4800 to 1024 X 9600, and the images covers the same 4.8 km width but extends somewhat over 44 km downtrack. The resolution is lower (4.3 m/pixel vs. 2.1 m/pixel), but the image covers much more ground.
Since this resolution was still substantially better than the best Viking image (by about a factor of ten), the Project decided this was the best compromise in coverage and protection against data loss.