This data set contains the Ceres digital terrain model (DTM) based on
the Dawn High Altitude Mapping orbit (HAMO) Framing Camera 2 (FC2)
imagesand derived by using the stereo photogrammetry (SPG) method. The
HAMO DTM covers approximately 98% of the Ceres surface (few permanently
shadowed areas near the poles required interpolation). The a DTM has
a lateral spacing of ~136.7 m/pixel (60.0 pixel/degree) and a vertical
accuracy of about 10 m.
Dawn mission is equipped with two identical framing cameras (FC1 & FC2)
[SIERKSETAL2011] which have one clear filter and seven band pass
filters. At Ceres, only the FC2 was used to acquire science images
while the FC1 was held in reserve. Clear filter images which were
taken during HAMO were used to produce a global DTM of the
illuminated part of Ceres [PREUSKERETAL2016]. Dawn orbited Ceres
during in 6 cycles between August 16 and October 23, 2015 at the
HAMO altitude of ~1475 km. A cycle is a single complete mapping of
surface at a fixed attitude (nadir or off-nadir). The framing camera
acquired about 2350 clear filter images [PREUSKERETAL2016] during the
HAMO phase. The images were taken with different viewing angles and
similar illumination conditions by slewing the spacecraft to various
off-nadir attitudes. These images are analyzed by using the SPG method
[PREUSKERETAL2011, RAYMONDETAL2011] to produce the Ceres HAMO DTM.
For more information on the contents and organization of the volume
set refer to the AAREADME.TXT file located in the root directory of
the data volumes. A description of the map projections used in this
data sets is provided in the dsmap.cat file in the catalog directory
of this archive volume.
The stereo-photogrammetric processing of Ceres images is based on a
software suite that has been developed at the German Aerospace Center
(DLR BERLIN-ADLERSHOF) within the last decade. It has been applied to
several planetary image data sets and covers the entire workflow
from photogrammetric block adjustment to digital terrain model (DTM)
and map generation [PREUSKERETAL2016]. Details of the SPG data
processing pipeline can be found in [RAYMONDETAL2011] or
[PREUSKERETAL2011]. However, for those who do not have easy access to
those texts, a STEREO_PHOTOGRAMMETRY document derived from those papers
is included in the DOCUMENTS directory of this archive volume.
All HAMO clear filter images were constrained by stereo requirements
(Table 1) and achieved at least triple stereo image coverage for the
Table 1 Requirements for stereo processing
Differences in illumination <10 degrees
Stereo angle 15-55 degrees
Incidence angle 10-90 degrees
Emission angle 0-55 degrees
Phase angle 10-180 degrees
entire illuminated surface. In total, about 10,000 independent
multi-stereo image combinations were used to determine selected
image tie points by multi-image matching for the set-up of a
three-dimensional (3D) control network of ~30,000 surface points.
The control point network defines the input for the photogrammetric
least squares adjustment where corrections for the nominal orientation
data (pointing and position) are derived. The 3D point accuracy of the
resulting ground points have been improved from +/-200 m to
+/-16 m (0.12 pixel). The Ceres spin axis orientation, formerly
determined from Dawn Survey observations [CERES_COORD_SYS_151014,
this volume], to: right ascension = 291.431 +/- 0.01 (degrees),
declination = 66.761 +/- 0.01 (degrees). The Ceres spin rate has also
been updated from the Earth-based observations [CHAMBERLAINET2009] to
W1 = 952.15323 +/- 0.00005 degrees per day. Finally, the value of W0
was changed to 170.488 +/- 0.01 (degrees) so that the IAU approved
tiny crater 'Kait' (PREUSKERETAL2016, Figure 1) remains located at
zero longitude [ROATSCHETAL2016]. All updates to the IAU 2015
parameters are within the stated error limits of those parameters and
have equal or lower error estimates.
Table 2 Differences from IAU 2015 values for Ceres
IAU 2014 Pruesker et al. 2016
Right Ascension 291.418 +/- 0.03 291.431 +/- 0.01 degrees
Declination 66.764 +/- 0.03 66.761 +/- 0.01 degrees
W1 952.1532 +/- 0.00003 952.15323 +/- 0.00002* deg/day
W0 170.650 170.488 +/- 0.01 degrees
* One sigma error estimate, F. Preusker, personal communication
Finally, 10,000 individual multi-image matching processes at full
image resolution were carried out to yield ~2.8 billion object points.
The achieved mean forward ray intersection accuracy of the ground
points is +/- 16 m. Based on the HAMO DTM, the Ceres shape is best-fit
with a triaxial ellipsoid (482.8, 480.6, 445.0 km) with its long
axis at 45.9 degrees east.
All of the archive data files are found in the DATA directory of
the PDS archive volume DWNCHCSPC_2. Data are stored in PDS image
format (.IMG) with attached PDS3 labels. In addition to the archive
DTM, browse versions are provided in JPEG format (.JPG) in the
BROWSE directory and in TIFF format (.TIF)in the EXTRAS directory.
CE_HAMO_G_00N_180E_EQU_DTM Global DTM, equidist cylindrical projection
CE_HAMO_G_90N_000E_STE_DTM North polar stereographic projection
CE_HAMO_G_90S_000E_STE_DTM South polar stereographic projection.
All of the Ceres SPG DTM products have a SCALING_FACTOR of 1.0 and
and OFFSET of 470000 m (as noted in the PDS product labels). Conversion
from Digital Number to HEIGHT, i.e. elevation in meters, is:
HEIGHT = (DN * SCALING_FACTOR).
The conversion from Digital Number to PLANETARY_RADIUS in meters is:
PLANETARY_RADIUS = (DN * SCALING_FACTOR) + OFFSET
where OFFSET is the radius of the reference sphere.
The DTM archived here is inconsistent with the IAU (2015) approved
Ceres coordinate system that is based on Dawn Ceres Approach and
Survey images (described above). SPICE users of these HAMO SPG
DTM products will require a specific SPICE PCK file containing
the planetary constants described above (dawn_ceres_SPG20160107.tpc).
This file can be found in the GEOMETRY directory on this archive volume.
Modeling of the shape of Ceres is an ongoing process. Dawn has acquired
much higher resolution images of Ceres from its Low Altitude Mapping
Orbit (LAMO) that can be used for stereo photogrammetric analysis in
the near future. Those images were still being acquired at the time
that this document was written. The Dawn mission does not anticipate
petitioning the IAU to consider updating the current Ceres coordinate
system definition until the high resolution images can be analyzed.
This DTM and the associated coordinate system update are preliminary
products created at the end of the Dawn prime mission as contractually
required. These products will be updated after the end of the extended
mission at low altitude.
Roatsch,T., E. Kersten,K.-D. Matz,F. Preusker, F. Scholten, S. Elgner,
S.E. Schroeder, R. Jaumann, C.A. Raymond, C.T. Russell,
DAWN FC2 DERIVED CERES HAMO DTM SPG V1.0,
DAWN-A-FC2-5-CERESHAMODTMSPG-V1.0, NASA Planetary Data System, 2016.