Proceedings of the Symposium on Global and Environmental Monitoring: Proceedings of the Symposium on Global and Environmental Monitoring

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Persistent identifier:: 856665355

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts ; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856665355

Language:: English

Document type:: Multivolume work

Volume

Persistent identifier:: 856669164

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Scope:: XIV, 912 Seiten

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856669164

Illustration:: Illustrationen, Diagramme, Karten

Signature of the source:: ZS 312(28,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: International Society for Photogrammetry and Remote Sensing, Commission of Photographic and Remote Sensing Data

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: [FA-1 TOPOGRAPHIC ANALYSIS]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: A Combined Shape-From-Shading and Feature Matching Technique for the Acquisition of Ground Control Points in SAR Images of Rugged Terrain. M. Adair, B. Guindon

Document type:: Multivolume work

Structure type:: Chapter

4 2ì 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. 811 ?, "■ ' ‘ ■' ' I'f- ■ : 8*$ téià 8 èìÈ é Wmm

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