Abstract
We have used high-resolution digital terrain models (DTMs) of two rover landing sites based on mosaicked images from the High-Resolution Imaging Science Experiment (HiRISE) camera as a reference to evaluate DTMs based on High-Resolution Stereo Camera (HRSC) and Context Camera (CTX) images. The Next-Generation Automatic Terrain Extraction (NGATE) matcher in the SOCET SET and GXP® commercial photogrammetric systems produces DTMs with good (small) horizontal resolution but large vertical error. Somewhat surprisingly, results for NGATE are terrain dependent, with poorer resolution and smaller errors on smoother surfaces. Multiple approaches to smoothing the NGATE DTMs give similar tradeoffs between resolution and error; a 5 × 5 lowpass filter is near optimal in terms of both combined resolution-error performance and local slope estimation. Smoothing with an area-based matcher, the standard processing for U.S. Geological Survey planetary DTMs, yields similar errors to the 5 × 5 filter at slightly worse resolution. DTMs from the HRSC team processing pipeline fall within this same trade space but are less sensitive to terrain roughness. DTMs produced with the Ames Stereo Pipeline also fall in this space at resolutions intermediate between NGATE and the team pipeline. Considered individually, resolution and error each varied by approximately a factor of 2. Matching errors were 0.2–0.5 pixels but most results fell in the 0.2–0.3 pixel range that has been stated as a rule of thumb in multiple prior studies. Horizontal resolutions of 10–20 image pixels were found, consistently greater than the 3–5 pixel spacing generally used for stereo DTM production. Resolution and precision were inversely correlated; their product varied by ≤20% (4–5 pixels squared). Refinement of the stereo DTM by photoclinometry can yield quantitative improvement in resolution (more than a factor of 2), provided that albedo variations over distances smaller than the stereo DTM resolution are not too severe. We offer specific guidance for both producers and users of planetary stereo DTMs, based on our results.
Highlights
Detailed topographic data are foundational to geoscience and engineering operations on other planetary bodies just as they are on Earth, making the assessment of digital terrain model (DTM) quality of great interest
Slopes computed from the best block matching digital terrain models (DTMs) underestimated the true values on all terrains by amounts ranging from 0.4◦ to 1.2◦ (Figure 7)
Our strongest conclusion is qualitative: that comparing reference and target DTMs made from stereo images of very differing resolution provides a useful way to assess the resolution and precision of the lower-resolution target DTMs when—as is common in planetary exploration—ground data are not available for this purpose
Summary
Detailed topographic data are foundational to geoscience and engineering operations on other planetary bodies just as they are on Earth, making the assessment of digital terrain (or topographic) model (DTM) quality of great interest. Multiple measures of DTM quality are needed to address all aspects of these seemingly simple questions, starting with the size of features that are reliably detected (the horizontal resolution and local vertical precision of the DTM). These questions all involve local properties of the DTM, which are mainly determined by the algorithm used to generate 3D points and the process by which those points are interpolated to fill the DTM grid, as well as the quality and geometric properties of the images. They form the focus of this paper. Other questions involving broader-scale reliability such as leveling and absolute accuracy depend mainly on the control process and are not addressed here
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