Polarimetric mosaicking of ortho-rectified ALOS/Palsar data
Yrjo Rauste(1), Anne Lönnqvist(1), Heikki Ahola(1), Matthieu Molinier(1) and Tuomas Häme(1)
(1) VTT, P.O. Box 1000, 02044 VTT, Finland
The objective was to produce a mosaic of several polarimetric ALOS/Palsar scenes in such a form that polarimetric processing methods - like polarization synthesis and various decomposition techniques - can be applied to the mosaic product.
A study site was selected in Finnish Lapland (centre 68.5 degrees north, 27.5 degrees east) in an area where polarimetric acquisitions of neighboring orbits overlap. Four scenes from April 2007 (2.4.2007 and 19.4.2007) were used.
Polarimetric Palsar scenes were pre-averaged (in Stokes matrix form) over 6 lines in azimuth direction. HH amplitude images of these 6-by-1 averaged scenes were used to produce correlation-based tie points between scenes. Ground control points (GCPs) were measured between Palsar scenes and Finnish topographic map data (1:20 000).
Tie point observations (77) and GCPs (13) were submitted to a least squares adjustment, where three geometry revision parameters were determined for each scene: translation in northing and easting and image rotation. The tie point residual mean square error (RMSE) was 8.9 m in northing and 9.5 m in easting. RMSE for GCP observations was 13.6 m in northing and 19.8 m in easting.
Each polarimetric scene was ortho-rectified using a DEM and taking into account the geometry revision parameters. The pixel spacing of the DEM and the resulting mosaic was 25 m.
The mosaic shows good geometry. There was a notable radiometric difference between image strips that were acquired 17 days apart during the snow melting season. Because the radiometric difference was due to changes in physical snow conditions no effort was made to hide this difference by image normalization techniques.