The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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take advantage of post-mission improvements to the spacecraft 
ephemerides (Rappaport et al., 1999). Gridded Magellan 
altimetry data are also read in, for use as a source of vertical 
control. All these data sets are exported to SOCET SET by 
means of ISIS translation programs. 
SOCET processing optionally begins with the collection of 
tiepoints in the overlap areas between BIDRs and bundle- 
adjustment of the spacecraft trajectories. Some of the points 
may be constrained to lie at elevations given by altimetry. The 
trajectories are typically adjusted by introducing offset and 
linear drifts in the three orthogonal directions in-track, cross 
track, and radial. Adjustment parameters obtained from 
measurements on BIDRs can then be used in processing 
mosaicked data sets containing those BIDRs. 
Automatic DTM generation is achieved using SOCET SET’s 
Automatic Terrain Extraction (ATE) module (Zhang and Miller, 
1997). Although a lower limit on the useful spacing of DTM 
data that could be collected from the 75 m/pixel images would 
be 225 m (3 pixels) per post, we routinely generate DTMs at 
675 m/post, mainly to allow for greater averaging over speckle 
noise in the SAR images. Prior to running ATE, we "seed" the 
DTMs with manually collected points on ridge and valley lines, 
or with reliable altimetry data. This greatly improves the 
success rate of the automatic matching step, and generally 
limits the need for manual editing to bland areas, where the 
matcher fails entirely, and to ‘blunders’ found at the image 
edges where elevation values are extrapolated. The FMAP 
mosaics are normally used for ATE, avoiding the need to define 
a large number of image pairs made up of the smaller and more 
numerous BIDRs. The automatically generated DTM is viewed 
in stereo along with the images and interactively edited. Where 
possible, editing is based on the FMAPs, but some seam areas 
may need to be edited based on the individual BIDRs. After 
interactive editing, individual DTMs are combined into a single 
DTM for the entire map area. The merged DTM then requires 
additional interactive editing to replace gaps (due to missing 
data in orbits) with corresponding altimetry data. Finally, the 
BIDR images may be orthorectified and mosaicked, yielding an 
image base that registers more precisely to the DTM than the 
standard FMAP. The DTM and orthomosaic may be exported 
in various formats for analysis in ISIS and the production of 
publication-quality maps with other software such as ArcGIS 
and Adobe Illustrator. When mapping with FMAP mosaics, we 
produce 1:1,500,000-scale topographic maps, with a contour 
interval of 200 meters, with orthomosaic base, nomenclature, 
and collar information. We also produce color-coded shaded- 
relief/elevation maps (Figure 1) because they portray subtle 
topographic relations that assist with analyses of tectonic 
deformation, stratigraphic interpretation, flow direction, mass 
wasting, etc. The color-coding is chosen to show as much 
information as possible within a given map area, and hence is 
not necessarily consistent planetwide. 
Figure 1. Example map product: a color-coded shaded relief 
map of the 12°xl2° Joliot-Curie (06S066) FMAP quadrangle on 
Venus. Stereo data collected at 675 m/post have been edited 
interactively and merged with Magellan altimetry data. 
Enlargement at right gives an idea of the comparative detail 
level of the altimetric and stereo data sets (smooth and rugged 
strips). Full size Magellan stereo-derived maps are available at 
http://webgis.wr.usgs. gov/pigwad/down/venus_topo.htm 
2.3 Testing and Validation 
We have validated our Magellan stereomapping techniques as 
carefully as possible, given the limited availability of other data 
with which to compare the results (Howington-Kraus et al., 
2006). As a first step, mapping of a small area showed that 
ATE combined the high speed of automatic matching available 
in MST with much higher DTM resolution and gave results 
consistent with DSW-V, as expected given the commonality in 
the sensor model code used. Mapping of a larger area, the 
12°xl2° FMAP quadrangle 06S066 (Joliot-Curie) allowed us to 
develop procedures for making accurate controlled products. 
Magellan mosaics were previously known to contain 
discontinuities of as much as several km between data from 
orbits whose ephemerides had been calculated in separate 
solution blocks. Stereo viewing of the mosaics reveals these 
discontinuities as apparent “cliffs” in many cases. New 
ephemerides were computed after the mission on the basis of an 
improved gravity model, with a claimed reduction in position 
errors by 1.5 orders of magnitude (Rappaport et al., 1999). Our 
test mapping showed that the use of the improved ephemerides 
reduced discontinuities in the DTMs significantly, as well as 
presumably improving absolute positional accuracy, but that 
bundle adjustment based on image tiepoints was required in 
order to achieve pixel-level consistency. In both of these initial 
tests, some manual editing was required in relatively flat and 
featureless areas. Improved results were obtained in such areas 
by using the Cycle 2 images, in which the spacecraft was on the 
opposite side relative to Cycles 1 and 3. This provides greater 
stereo parallax, but also reverses any topographic shading, 
making stereo matching more difficult where such shading is 
present. In the lowlands of Venus, where relief and shading are 
low, the opposite-side imaging proved to be ideal. 
Mapping at high latitudes initially failed because of apparent 
multi-kilometer offsets between the images being used, even 
though the same BIDRs aligned properly at low latitudes. This 
problem was eventually traced to the use of an insufficiently 
precise value of the radar wavelength in the sensor model. 
Smaller discrepancies in cross-track coordinates were traced to 
the different atmospheric refraction models used in the DSW-V 
and the Magellan processor used to make the BIDRs. The 
Magellan model contains a bug that renders along-track 
adjustment of spacecraft positions impossible, so the DSW-V