Digital Imaging and Remote Sensing Laboratory Correction of Geometric Distortions in Line Scanner Imagery Peter Kopacz Dr. John Schott Bryce Nordgren Scott Brown May 8, 1998

Digital Imaging and Remote Sensing Laboratory Coverage Line Scanner Background Theory Geometric Distortions Results Future Work / Recommendations

Digital Imaging and Remote Sensing Laboratory Line Scanner Example across track along track FOV IFOV sampled ground pixel Airborne / Spaceborne EO Imaging Instrument Scanning mirror allows for collection of ground data one line at a time geometric distortions...

Digital Imaging and Remote Sensing Laboratory Geometric Distortions Degradation in visual appearance of the image Due to the Platform Attitude (orientation): Roll, Pitch, and Yaw Due to the Sensor’s Design Characteristics: Tangential, V/H roll distortion….

Digital Imaging and Remote Sensing Laboratory Roll Distortion Roll Distortion introduces a shift in the acquired scan lines. tangent distortion….

Digital Imaging and Remote Sensing Laboratory Tangent Distortion - All pixels collected at equal angular increments. - Unequal ground representation between the collected ground pixels (D edge > D nadir ) V/H distortion….

Digital Imaging and Remote Sensing Laboratory Oversampling…. V/H - ratio of aircraft velocity and altitude oversampling - scan rate is too fast when compared to the ratio undersampling - scan rate is too slow when compared to the ratio V/H Distortion

Digital Imaging and Remote Sensing Laboratory V/H Distortion - oversampling Undersampling…. Scan lines are acquired too fast, causing an overlap

Digital Imaging and Remote Sensing Laboratory V/H Distortion - undersampling proposed hypothesis…. Scan lines are acquired too slow, leaving gaps

Digital Imaging and Remote Sensing Laboratory Hypothesis Develop a set of C algorithms to correct the discussed geometric distortions in line scanner imagery. Research Progress….

Digital Imaging and Remote Sensing Laboratory Uncorrected Input 512 Scan Parameters 512 scan lines 512 pixels per line altitude = 1000 ft Field of View = 45 o

Digital Imaging and Remote Sensing Laboratory Corrected Output Roll Correction - (shifting of scan lines rectifies the image) V/H Correction - (eliminated the stretching in scene objects) Tangent Correction - (pixels represent equal ground areas)

Digital Imaging and Remote Sensing Laboratory Nearest Neighbor Resampling Mean Radiometry (DC Distribution) Preserved Histogram Comparison Input Image (512 x 512)Output Image (551 x 334)

Digital Imaging and Remote Sensing Laboratory Uncorrected Input 512 Scan Parameters 512 scan lines 512 pixels per line altitude = 250 ft Field of View = 45 o Distortions: V/H (Undersampling) Tangent

Digital Imaging and Remote Sensing Laboratory Corrected Output V/H Correction - (eliminated compression effects in scene objects) Tangent Correction - (pixels represent equal ground areas) Equal ground representation along and across track (square ground pixels)

Digital Imaging and Remote Sensing Laboratory Input Image (512 x 512)Output Image (541 x 1336) Histogram Comparison Nearest Neighbor Resampling Mean Radiometry (DC) Preserved Research Progress….

Digital Imaging and Remote Sensing Laboratory Uncorrected Input 512 Scan Parameters 512 scan lines 512 pixels per line altitude = 250 ft Field of View = 45 o slant path (45 degrees) Distortions: V/H (Undersampling) Tangent (ex:curvature along the diagonal)

Digital Imaging and Remote Sensing Laboratory Corrected Output V/H Correction - (eliminated compression effects in scene objects) Tangent Correction - (pixels represent equal ground areas) Improvement in the appearance of the diagonal

Digital Imaging and Remote Sensing Laboratory Algorithm Improvements Single resampling after Roll, Tangent, and V/H corrections limits further image degradation (ex: blurring due to bilinear resampling) Choice of Resampling Algorithms: Nearest Neighbor or Bilinear Corrects multiple bands simultaneously Error Analysis….

Digital Imaging and Remote Sensing Laboratory Scientific Analysis accurate determination of ground pixel’s position dictates the appearance of the resulting image. determines which flight parameters are the largest sources of error : aircraft velocity, aircraft altitude, roll angle, start angle, IFOV? How ?...

Digital Imaging and Remote Sensing Laboratory Error Sensitivity Analysis Based on the governing equations, estimate the errors across track (X) and along track (Y) Error Across Track….

Digital Imaging and Remote Sensing Laboratory Error Across Track H  start +  roll IFOV ground pixel ctr[x] = H * tan (  start +  roll + (IFOV*x) + (IFOV/2) ) Plots….

Digital Imaging and Remote Sensing Laboratory Roll Angle Effects Largest source of error across track Sensor’s ability to accurately determine a ground pixel’s position decreases for pixels near the edges

Digital Imaging and Remote Sensing Laboratory Altitude Effects Second Largest source of error across track Nadir pixel unaffected by the sources of error Error Along Track….

Digital Imaging and Remote Sensing Laboratory Error Along Track H  n  y= n * ( (H *IFOV) - (Velocity * time_per_scan) ) Plots….

Digital Imaging and Remote Sensing Laboratory Velocity Effects Largest source of error along track Position Error increases with aircraft velocity

Digital Imaging and Remote Sensing Laboratory Altitude Effects Second Largest source of error along track Future Work….

Digital Imaging and Remote Sensing Laboratory Conclusion - Future Work Correction algorithms successfully improve the visual appearance of the image Incorporate the algorithms with (MISI) line scanner Incorporate other geometric distortions, such as pitch and yaw Acknowledgments….

Digital Imaging and Remote Sensing Laboratory Acknowledgments I’d like to thank the following people for contributing to this research: Dr. John Schott Bryce Nordgren Scott Brown Rolando Raqueño