About half of the region was beyond the terminator during the three Mariner 10 encounters and hence not visible. The entire mapped area was covered by near-vertical photography from the second encounter, and the eastern part, from longitude 15° to about 110°, was covered by oblique photography from the first encounter. No third-encounter images were acquired. The entire visible area may be viewed stereoscopically by combining images from the first and second encounters taken at different viewing angles or by combining second-encounter images of the same area taken at different viewing angles. These combinations provided excellent qualitative control of topographic relief and a good quantitative photogrammetric base. However, sun-elevation angles of the images are limited to less than 25°, and image resolutions are no higher than about 0.5 km per picture element. Therefore, the south polar geologic map reflects mostly large-scale processes and topographic information, whereas other mercurian quadrangle maps benefit from greater albedo discrimination and, in some cases, higher resolution.
