The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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thousands for Magellan) and random rather than north-south 
orientation made it more convenient to store the beam-and-burst 
information as a raster map in the same projection as the image, 
rather than as a tabular database. The needed ancillary data for 
each burst, including both its footprint boundary and the space 
craft position and velocity, are obtained from a binary table 
known as the SBDR (Stiles, 2008a). A final difference between 
the Magellan and Cassini processing comes about because there 
is no pre-Cassini topographic information for Titan. Cassini 
BIDRs are therefore projected onto a spherical reference 
surface rather than onto a low-resolution DTM. Because the 
equations of projection onto a sphere can be expressed 
analytically, no resampling coefficients are required. Care is 
needed, however, to use the adjusted spacecraft position and 
velocity to calculate range-Doppler coordinates from the 
ground point location, but to use the original position and 
velocity estimates to calculate map coordinates from range and 
Doppler, in order to be consistent with the way the BIDRs were 
generated. 
Our approach to processing the Cassini RADAR stereopairs has 
also been informed by our experience with Magellan. In 
particular, the practice of “seeding” the automatic matching 
process with a loose set of surface points selected interactively 
has once again been shown to reduce the need for final editing. 
The initial mapping results reported below were obtained 
without any bundle adjustment, but we expect that as we 
analyze a larger number of overlap areas, some adjustment will 
be necessary to achieve consistent results at the sub-kilometer 
level of precision. 
3.3 Results 
Evidence about the topography of Titan prior to our beginning 
radargrammetric mapping with SOCET SET came from a 
variety of sources, all of which suggested that both local and 
global relief is low, with elevation variations greater than about 
1000 m rare. Radarclinometry (shape-from-shading) initially 
revealed only a few hundred meters relief in areas where it 
could be applied (Kirk et al., 2005) though more recent results 
reach 1500-2000 m for some mountains (Radebaugh et al., 
2007). Topographic profiles have been obtained over a limited 
number of short arcs by operating the RADAR as an altimeter 
(Johnson et al., 2007) and over longer arcs by an ingenious 
method that compares the signal strength from adjacent 
overlapping beams to determine heights along each SAR image 
(Stiles et al., 2007). The profiling methods agree well where 
they have been compared (Gim et al., 2007), and both show 
relief of a few hundred meters or less. Finally, preliminary 
estimates of topography from stereo, again indicating relief of 
hundreds of meters (Kirk et al., 2007) were based on automated 
image matching by Scott Hensley at JPL and on manual 
parallax measurements by us, but in either case a simple 
parallax-height scaling based on the two radar incidence angles 
was used in lieu of a rigorous sensor model to estimate relative 
elevation differences. Hensley has since implemented a 
rigorous Cassini RADAR sensor model for his Magellan- 
derived matching software (written communication, 2007). 
Our plans to map the complete set of RADAR overlap areas 
now that a rigorous sensor model is available for SOCET SET 
have been delayed somewhat by the discovery of substantial 
(up to 30 km) positional mismatches between many of the 
image pairs that would introduce spurious parallax and/or 
prevent stereo matching altogether. These offsets have been 
traced to the need for an improved model of Titan’s rotation, 
and have been reduced to sub-km levels by adjusting the 
orientation of the spin axis, the rotation rate, and the first 
derivatives of these parameters (Stiles et al., 2008). The 
nonsynchronous spin rate, in particular, implies that the ice 
crust of Titan is decoupled from the deep interior by a 
subsurface liquid water “ocean” (Lorenz et al., 2008b)—an 
interesting example of a significant geophysical discovery 
arising from a routine use of radargrammetry to improve 
cartographic products. Reprocessing of the complete set of 
Cassini BIDRs based on the new rotational model will be 
completed in the late spring or early summer of 2008. 
Meanwhile, several overlapping images obtained in 2007 
February to April were made at the same rotational phase, so 
that the misregistration caused by using the older rotation 
model is negligible. Fortunately, the overlap between these 
images covers one of the most interesting regions of Titan, an 
area of extensive dark areas interpreted to be lakes and seas 
(Stofan et al., 2007) near the north pole. 
Figure 3. Color-coded topographic map of part of Titan’s north polar “lake country” based on stereoanalysis of RADAR images 
from flybys T25 and T28. Polar stereographic projection, north approximately at top. Island at right center is Mayda Insula, 
discussed in text. 
Figure 3 shows our topographic model of the overlap between 
the images from the T25 (2007 February 22) and T28 (2007 
April 10) flybys. Both flybys illuminated the area from the 
south, yielding same-side stereo with vertical precision 
typically on the order of 100 m for single pixel (175 m) 
matching error. The procedures for making this DTM were 
tested by initial mapping of Mayda Insula, a 90x150 km island 
centered near 78° N 312° W (Kirk et al., 2008c). Bundle 
adjustment was not needed for this data set, because cross 
stereobase misregistration was less than one pixel, and the 
unadjusted stereo elevations along the SAR topography profile 
agreed at the 50-100 m level with the absolute elevations of 
the latter data set. We note that that this agreement is obtained 
even where the two incidence angles are similar, at the east end