  We selected approximately 10,500 Narrow Angle Camera (NAC) and Wide-Angle
Camera (WAC) stereo images obtained by the MESSENGER spacecraft during its
orbital mission with average resolutions of 150 m/pixel to compute a Digital
Terrain Model (DTM) for the so-called H6 quadrangle (Kuiper), extending between
22.5{\deg}S to 22.5{\deg}N and 288.0{\deg}E to 360.0{\deg}E. Among the images,
we identified about 21,100 stereo image combinations with each combination
consisting of at least 3 images. We applied sparse multi-image matching to
derive approximately 250,000 tie-points representing 50,000 ground points. We
used the tie-points to carry out a photogrammetric block adjustment, which
improves the image pointing and the accuracy of (3D) ground points from about
850 m to approximately 55 m. Subsequently, we applied high density
(pixel-by-pixel) multi-image matching to derive about 45 billion tie-points.
Benefitting from improved image pointing data, we compute about 6.3 billion
surface points. By interpolation, we generated a DEM with a lateral spacing of
221.7 m/pixel (192 pixels per degree) and a vertical accuracy of about 30 m.
The comparison of the DEM with the Mercury Laser Altimeter (MLA) tracks
obtained over 4 years of MESSENGER operation reveals that the DEM is
geometrically very rigid. It may be used as a reference to identify MLA
outliers (when MLA operating at ranging limit) or to map offsets of laser
altimeter tracks, presumably caused by residual spacecraft orbit and attitude
errors. After the relevant outlier removal and corrections, MLA profiles show
excellent agreement with topographic profiles from H6 with root mean square
height difference of only 88 m.
