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A Combined Shape-From-Shading and Feature Matching Technique
for the Acquisition of Ground Control Points in SAR Images of Rugged Terrain
M. Adair
Intera Technologies Ltd.
Ottawa, Ontario, Canada
and
B. Guindon
Canada Centre for Remote Sensing
Ottawa, Ontario, Canada
ABSTRACT
This paper describes an image feature matching
technique which can be used as a tool in ground
control point acquisition for SAR images of rugged
terrain. Topographic features, such as valley
networks, can be readily extracted from both
digital elevation data and from SAR images using
shape-from-shading. First, in each network over
lay, individual valley pixels are linked into
contiguous segments. Valley pixels which are
logically linked to two or more segments are
denoted as nodes. An iterative relaxation method
is then employed to identify node correspondence
in the overlay pairs. Matching experiments have
been carried out with SEASAT coverage of the Adams
Lake region of central British Columbia and an
available 1:250000 scale DEM. Image coordinate
transformation errors of approximately 50 meters
RMS have been achieved.
INTRODUCTION
The utility of spaceborne SAR systems will be
greatly enhanced if the data can be rectified to
user specified map projections, generally through
the use of ground control points. This is rendered
difficult in the case of SAR imagery of rugged
terrain because the radiometry is dominated by
terrain effects, which effectively masks point
like features, and due to geometric distortions of
foreshortening and layover. Terrain can be viewed
as 'rugged' from a SAR point of view if it
exhibits local slopes whose steepness imply near
layover conditions. For SEASAT this implies
regions with extended slopes of 20 degrees.
Control points can be acquired through the
correlation of real and simulated image chips,
however this requires that the DEM exhibit the
same level of spatial detail as the image itself
(Guindon and Maruyama, 1986). In the case of
spaceborne SAR sensors with spatial resolution of
approximately 25 meters, accurate 1:50000 scale
terrain information is required. However, for most
of the earth's surface, only lower quality DEM's
are available.
This paper describes an alternative approach to
the image rectification problem using a feature-
based image matching technique. Ground control
results from the correspondence of spatial point
patterns of nodes in the valley floor network in
areas of rugged terrain. In one case the network
is derived from a low resolution digital elevation
model (DEM) and in the other case it is derived
from a SAR image using a shape-from-shading
technique which extracts the topographic structure
of terrain from a single SAR image (Guindon,
1989). The processing is carried out in three
steps: (1) detect the valleys in a DEM and trans
form this valley 'mask' to a SAR projection (the
'DEM mask'); this mask can then be compared with
the valley mask extracted using the shape-from-
shading technique (called the 'SAR mask'), (2)
linking and labelling each of the masks to detect
points where two or more valley segments meet
(called 'nodes') and, (3) matching of the spatial
point pattern of the nodes using an iterative
relaxation technique. This set of matched nodes
can, in principle, be used as a source of control
points for image rectification. Each of the steps
are described further below.
DEM VALLEY EXTRACTION
The detection of valleys in the DEM is carried out
in two passes over the DEM and is based on the
method of Qian et al. (1990). The first pass
labels pixels as candidate valleys by examining
the elevation values of three pixels in each of
four directions (horizontal, vertical and the two
diagonals), looking for a local minimum at the
centre pixel. The second pass over the DEM locates
the valley floor from the candidate pixels by
tracing the path of steepest descent from pixel to
pixel. This gives a vector representation of the
valleys which is then transformed to the nominal
SAR projection to match the SAR mask. In the
present case the SAR projection is a ground range-
azimuth projection with terrain height related
parallax.
The DEM of the study area, near Adams Lake, B.C.,
was rasterized on a 50m grid using 1:250000 scale
digital elevation contour data provided by the
Canada Centre for Mapping. The region is charac
terized by elevation variations of 1500 to 2000
meters and terrain slopes of up to 20 degrees in
steepness.
Portions of the DEM and SAR valley masks are shown
in Figures 1 and 2. The SAR mask is derived from
a SEASAT SAR image with 50m pixel resolution.
While there are differences in the local details
of each valley segment, the main segments are
visually similar. The SAR mask shows valley
segments which run mainly in the azimuth direction
(top to bottom) due to the radiometric sensitivity
to the range component of terrain slope (Guindon,
1989). Other differences are due to layover in the
mask derived from the radar image.
MASK LINKING AND LABELLING
The next step of the process is to link and label
both of the valley masks. Every pixel is labelled
as part of a valley segment or as a node. The same
linking algorithm is applied to both valley masks.
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