International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
Once the images are adequately registered to each other, as well
as the LOLA data, the process of extracting DEMs can begin
with NGATE (SOCET SET- Next Generation Automatic
Terrain Extraction). NGATE performs image correlation and
edge matching for every pixel in the image to create a dense
model. The DEM is then resampled to at least three times the
ground sampling distance of the image in order to reduce noise.
In the nominal phase of the mission (50 km circular orbit)
DEMs were typically sampled at a two meter pixel scale.
Results from NGATE require very little editing if the stereo pair
was acquired under nominal conditions (35° to 65° incidence
angle and 10° to 45° convergence angle). Low incidence angles
can cause LROC images to be difficult to co-register between
images while images with a high incidence angle are
significantly affected by shadows and thus require extensive
editing. NGATE is not optimized to work with the linear
pushbroom images. In order to increase effectiveness, a pair-
wise rectification is performed on the images used for DEM
extraction. Pair-wise rectification rotates the images so that the
epipolar lines are horizontal and scales are set to a common
pixel scale. The rectified images make stereovision easier on the
analyst, and are required for the accurate generation of the DEM
(Tran et al. 2010).
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Figure 4. Workflow diagram for DEM production.
Orthorectified images are created upon completion of the DEM
using SOCET SET's Orthophoto Generation. Orthophotos are
images that have had all distortion due to camera obliquity and
terrain relief removed. An orthophoto represents what you
would see if you were looking at the ground orthogonally from
a distance above (every pixel is viewed as if nadir). In addition,
a hill shade image, color shaded relief image, slope map, and
confidence map are provided in GeoTIFF format. These
products are made using the Geospatial Data Abstraction
Library (GDAL) (Warmerdam 2008).
4. ERROR ANLAYSIS
The overall quality of the DEM is a measurement of both the
absolute and relative accuracies. The absolute accuracies are
determined by how far the coordinates of the DEM align with
the true latitude and longitude. LOLA altimetry provides the
absolute geodetic reference frame for the NAC DEMs. A
crossover correction analysis with the LOLA data shows
improvement in the RMS differences between the LOLA track
data to 10.18 m along track, 8.37 m cross track, and 1.8 m
radially (Mazarico et al 2011). On average then, it can be
expected that the positional accuracy of LOLA data approaches
these levels.
4.1 Relative Error
The theoretical expected vertical precision of NAC DEMs can
be calculated based on the spacecraft orbit and camera
geometry. While acquiring stereo images, the spacecraft is
either pointed nadir or rolled about the flight line. In some cases
near the poles, LRO is pitched forward to acquire stereo images
(Tran et al. 2010). The convergence angle is the magnitude of
the parallax angles between the two stereo pairs. SOCET SET
calculates the linear error between stereo mates upon
completion of the DEM. The linear error gives an estimate of
the overall precision of the DEM at the 90% confidence level
(Subramanian et al. 2003). The horizontal linear error in the
DEM is the same as the spatial resolution of the DEM. The
relative precision of DEMs produced from nominal phase
images is theoretically 0.5 meters, but can be as large as 2.0
meters. DEMs produced in the commissioning phase and frozen
orbit will have an expected vertical precision of as much as 3.0
m.
4.2 LOLA Registration
Alimetric observations obtained by LOLA provide
measurements of +0.1 m between the spacecraft and the lunar
surface (Smith et al. 2010). However, uncertainties in the
spacecraft positioning can result in offsets (£15m) between
altimeter tracks over many orbits. The LROC team is currently
developing a tool to automatically register alimetric
observations to NAC DEMs (Speyerer et al. 2012). Using a
generalized pattern search (GPS) algorithm, the new automatic
registration adjusts the spacecraft position and pointing
information for the times when NAC images are acquired as
well as when LOLA collects measurements of the same region.
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improvement in the RMS error and a partition of the final
registration.
The absolute horizontal and vertical accuracy of the DEM
largely depends on the accuracy of the LOLA profiles.
Currently, our strategy is to register the DEM to only one
profile with the help of the GPS. This procedure ensures that the
model is fixed in the down track direction. Other profiles that
are coincident with the DEM may not be consistent from
profile-to-profile and have errors due to small unknowns In
spacecraft position. Areas that are relatively flat are used as
elevation controls throughout the remaining portion of the
DEM, which more accurately accounts for artificial tilts In the
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