Worksheet to Chapter 5 - Teacher Instructions

Image Processing
Georectifying - Enhancement and Mosaicing

Objectives:

To produce meaningful and valuable satellite images as we know them from everyday life, or an image mosaic like in worksheet 4 (about Visual Image Interpretation of Europe), the raw satellite data has to be processed, enhanced and mosaiced together. Moreover images have to be georectified to compare the data with other sources. In this process a relationship between the image pixels and the position of the corresponding points on the earth's surface is established.

Didactical comment:

• The worksheet 'Image processing' illustrates that satellite image data has to be processed, enhanced, and run through different work stages before it is useful.
• The students should understand that interferences such as clouds or sun reflections must be eliminated by blending different images to get a 'clear' view of the surface.
• The students should describe and reconstruct the different image processing steps.

Solution to the worksheet 'Image Processing'

1. Explain why we enhance data.
   • Non-corrected satellite images depend on atmosphere and weather conditions.
   • Enhancements like geometric and radiometric corrections make it easier to visually extract information and apply it to various problems.
   • An atmospheric correction includes additional information from other sources about interferences in the atmosphere between a satellite and the earth surface, such as for example aerosols. With this additional information the interferences can be erased.
2. Speculate how the satellite scenes in the image above were manipulated. Which type of image processing did they go through?
   • The images were first radiometrically and spectrally corrected, which means atmospheric influences (haze) were reduced and colours optimised.
   • Next the images were geometrically corrected. This establishes a correlation between the image pixels and the position of the corresponding points on the earth's surface. Therefore, images obtained at different dates can be combined to a mosaic.
3. Describe and interpret variations in colour between the different scenes.
   • The ocean is represented in different colours in the satellite images. Colours range from medium blue to black.
   • Some images in the area of former Yugoslavia show a darker green value compared to the other images of Europe.
   • Images from North Africa show completely different colours due too deserts, geological formations etc.

   a. What could the white spots above Iceland represent?

   • They represent clouds over the island.
   • MERIS is an optical sensor which can not penetrate clouds or rain in contrast to active Radar systems and therefore clouds are visible in white.

   b. What might be the white areas in the Southern Mediterranean Sea?

   • These white areas are reflections of the sun in the Mediterranean Sea.
4. Speculate how the satellite scenes in the image above were manipulated. Which type of image processing did they go through?
   • The satellite scenes have to be spectrally adapted to the neighbouring images to obtain evenly distributed colours.
   • Missing images have to be built into the mosaic.