Synthetic Orthophotograph of the Mars Face Based on Stereographic Measurements
By: Lan Fleming




A previous article presented a set of elevation values derived from stereographic measurements for points on the Mars Face landform and the surrounding plains. Each elevation value was derived from the difference in position of some identifiable small feature between two images taken at different emission angles. It was noted that these displacements, which are due to parallax, could be used to create what could be termed a "synthetic orthophotograph," a view of the landform looking straight down at the surface from above. Such a view, also referred to as a "plan view," is critical for assessing the general symmetry of the landform. The skew-and-stretch procedure used by NASA to process MGS images cannot produce a true plan view unless the imaged surface is perfectly flat and planar. I refer to such enhancements as "partially rectified" images. For three-dimensional objects such as the Mars Face, the partial rectification retains distortions caused by parallax associated with the off-nadir (off-vertical) angle of the camera's line of sight.

Briefly, an orthophotograph can be synthesized from a partially rectified image by moving each pixel by an amount equal to the parallax displacement of the pixel from the off-nadir view to the plan view. The method of producing the otrhophoto is described in more detail subsequently for those who are interested.

Such an orthophotograph has been created from the April, 2001 Mars Global Surveyor image of the Mars Face using software written specifically for this task. It is shown in Figure 1.

Figure 1. Synthetic Orthophotograph based on the April, 2001 MGS image (MOC E0300824) of the Mars Face. Scale is 25% of full size.

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