Radiometric and geometric characteristics of Pleiades images
Pleiades images are distributed with 50 cm ground sampling distance (GSD) even if the physical resolution for nadir images is just 70 cm. By theory this should influence the effective GSD determined by means of point spread function at image edges. Nevertheless by edge enhancement the effective GSD can be improved, but this should cause enlarged image noise. Again image noise can be reduced by image restoration. Finally even optimized image restoration cannot improve the image information from 70 cm to 50 cm without loss of details, requiring a comparison of Pleiades image details with other very high resolution space images. The image noise has been determined by analysis of the whole images for any sub-area with 5 pixels times 5 pixels. Based on the standard deviation of grey values in the small sub-areas the image noise has been determined by frequency analysis. This leads to realistic results, checked by test targets. On the other hand the visual determination of image noise based on apparently homogenous sub-areas results in too high values because the human eye is not able to identify small grey value differences – it is limited to just approximately 40 grey value steps over the available gray value range, so small difference in grey values cannot be seen, enlarging results of a manual noise determination. A tri-stereo combination of Pleiades 1A in a mountainous, but partially urban, area has been analyzed and compared with images of the same area from WorldView-1, QuickBird and IKONOS. The image restoration of the Pleiades images is very good, so the effective image resolution resulted in a factor 1.0, meaning that the effective resolution corresponds to the nominal resolution of 50 cm. This does not correspond to the physical resolution of 70 cm, but by edge enhancement the steepness of the grey value profile across the edge can be enlarged, reducing the width of the point spread function. Without additional filtering edge enhancement enlarges the image noise, but the average image noise of approximately 1.0 grey values related to 8 bit images is very small, not indicating the edge enhancement and the down sampling of the GSD from 70 cm to 50 cm. So the direct comparison with the other images has to give the answer if the image quality of Pleiades images is on similar level as corresponding to the nominal resolution. As expected with the image geometry there is no problem. This is the case for all used space images in the test area, where the point identification limits the accuracy of the scene orientation.
Comprehensive evaluation of Pléiades-1A bundle images for geospatial applications
This paper presents the results of comprehensive evaluation of Pléiades 1A which is the first civilian satellite of Europe with sub-meter resolution. The analyses consist of radiometric evaluation, georeferencing accuracy assessment, pan-sharpening performance, digital surface/terrain model quality and vector map production. The effective resolution is estimated with a factor slightly below 1.0 for triplet panchromatic images, and signal to noise ratio is in the range of other comparable space borne images. The georeferencing accuracy was estimated with a standard deviation in X and Y directions in the range of 0.45 m by bias-corrected and sensor-dependent rational functional model. 3D standard deviation of ± 0.44 m in X direction, ± 0.51 m in Y direction and ± 1.82 m in the Z direction were reached in spite of the very narrow angle of convergence by the same mathematical model. The generated digital surface/terrain models were achieved with ± 1.6 m standard deviation in Z direction in relation to a reference digital terrain model. The pan-sharpened images were generated by various methods, and were validated by quantitative and qualitative analyses. Moreover, a vector map was generated in the level of detail 0 to analyse information content.
Assessing student learning in higher education
Chapter 1. Introduction to the Text, Chapter 2. What is Assessment?, Chapter 3. Student Learning, Chapter 4. Methods and Strategies: An Overview, Chapter 5. Assessing Essays, Chapter 6. Multiple Choice Questions, Chapter 7. Assessing Practical Work, Chapter 8. Assessing Projects, Chapter 9. Assessing Problems, Chapter 10. Assessing Oral Communication, Chapter 11. Self and Peer Assessment, Chapter 12. Self Assessment: Related Approaches, Chapter 13. Using Computers in Assessment, Chapter 14. Changing Assessment, Chapter 15. Reliability, Validity and Examining, Chapter 16. Quality, Standards and Underlying Issues, Bibliography Notes and comments on activities Some further reading Appendix One: Examples of examination questions in Arts, Law and Social Sciences