1055
AUTOMATED NOISE REDUCTION IN HRSC MARS DTMS
Shih-Yuan Lin*, Jung-Rack Kim, Jan-Peter Muller
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, RH5 6NT, UK -
(syl, jk2, jpm)@mssl.ucl.ac.uk
KEY WORDS: Extra-terrestrial, Mapping, DTM, Surface, Matching, Geology, Noise, Detection
ABSTRACT:
Since the European Space Agency Mars Express spacecraft entered an orbit around Mars, the High Resolution Stereo Camera
(HRSC) placed onboard has been imaging the Martian surface. Due to the characteristics of high resolution and stereo imaging
capability of the HRSC, the construction of high resolution digital terrain models (DTMs) representing Mars surface topography has
been achieved. However, during the process of DTM creation, errors will occur which may lead to incorrect interpretation.
Therefore, the certification of DTM quality is required before any geological investigation is performed. This paper describes a new
method to detect and remove noise existing in Martian terrain models produced from HRSC stereo imagery. The proposed method is
here demonstrated using HRSC hi011 orbital images and positive results were achieved, in which the number and position of the
noisy heights were extracted. Furthermore the method proved of great potential in reducing vertical discontinuities between adjacent
DTMs when DTM mosaicing was performed. This solution for automated noise reduction has demonstrated the capability for
refining Mars DTMs produced from HRSC stereo imagery, which will be helpful in improving hydrological analysis and geological
investigations of the Martian surface.
1. INTRODUCTION
A fundamental scientific question in the exploration of the
planets within our Solar System is whether conditions
favourable to the development and presence of life exist or have
existed in the past. Central to this issue is the search for
evidence of past (and present) water, including the
determination of how much water was present at or near the
surface. Of all the planets in the Solar System, Mars is perhaps
the best (and nearest) candidate for addressing these questions,
and hence is the focus of significant planetary exploration at
present. Determining the inventory of water reservoirs and
morphological parameters on outflow channels on Mars
provides critical information for understanding the geological,
climatic and potentially exobiological evolution of the planet.
However, previous studies of the role of water on Mars have
been limited by the spatial resolution of topographic data
available to study Martian landforms. Whilst extensive but low
resolution Viking and MOC-WA (MOC-Wide Angle) imagery,
and high resolution, but spatially restricted, imagery taken from
MOC-NA (MOC-Narrow Angle) and Thermal Emission
Imaging System (THEMIS) have been available, the lack of
high resolution topographic data with which to investigate
meso-scale (10m-100 km scale) landforms in detail has
restricted the analysis and interpretation of Martian surface
features. A detailed investigation of landscape formation and
evolution requires high resolution (<<200m) and accurate
topographic data (Zrm<30m) from which fine-scale clues to the
origin of landforms can be resolved and quantitative
measurements of landscape properties determined.
Since the European Space Agency (ESA) Mars Express
spacecraft entered an elliptical orbit around Mars on the 25th of
December 2003, the High Resolution Stereo Camera (HRSC)
placed onboard has been imaging the Martian surface. Due to
the characteristics of high resolution and stereo imaging
capability of the HRSC, the construction of Digital Terrain
Model (DTM) of Mars surface topography with grid spacing of
30-75 metres is feasible (Albertz et al., 2005; Heipke et al.,
2007). By visualising the Martian terrain in a high resolution
three-dimensional model, the evolution of the Martian surface
and the geological processes involved can be effectively
analysed.
In order to select an area for future exploration, DTM data
quality is a very important aspect of the geological investigation
of the Martian surface. Errors/blunders and artefacts occurring
in any DTM will lead to incorrect interpretation and
understanding of the underlying physical processes. This paper
therefore aims to detect and remove such noise existing in any
Martian terrain model produced from HRSC stereo imagery.
Moreover, we deal with errors appearing in the final DTM
product, rather than the ones generated in the intermediate steps
in the process of DTM generation.
The strategy of noise reduction follows a two-stage process.
Surface matching is the technique first applied to determine
noisy DTM points and remove them in the DTMs.
Subsequently, the points removed are further examined using
the corresponding pixels in the ortho-image. Their colour digital
number (DN) values are inspected to determine if there is any
point being removed improperly. After extensive testing, the
overall solution for automated noise reduction, proposed in the
paper, is shown to be an efficient method for refining Mars
DTMs produced from HRSC stereo imagery. The method is of
great potential value for improving hydrological analysis and
geological investigations of the Martian surface. The details of
the method, as well as its implementation, results, quality
* Corresponding author.