Image processing - using satellite images

This is a free resource courtesy of Earth Science Australia

Getting started

Satellite images can be very useful in providing valuable information but the human eye is not sensitive enough to interpret the subtle changes in brightness and colour.
To help we can use image processing to overcome this difficulty and to expose the hidden treasures of information found in satellite images. Image processing works because, unlike photographs, numbers represents the colours in satellite and digital images.

If you do not already have one installed on your computer consider installing this freeware image-processing program to complete the following activities:

Irfan view

http://www.irfanview.com/
IrfanView is a very fast, small, compact and innovative freeware (for non-commercial use) graphic viewer for Windows 9x/ME/NT/2000/XP/2003.
It tries to be simple for beginners and powerful for professionals.

Example 1: Studying the rocks on Venus

This example uses false colour image processing to bring out hidden features. It is an example of image enhancement.


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

No one has been able to set foot on Venus to sample the rocks and ages of the craters. To our untrained eye a black and white satellite image taken by a Russian Satellite many years ago shows little in the way of detail. The featureless surface of Venus gives little indication that the rocks and craters are of several ages.

Follow these steps to enhance the satellite image of Venus.

Step 1:
Open the image-processing program. If you installed IrfanView program open it.

Step 2:
Click on the image of Venus to open a larger version and save it to your computer.

Step 3:
Open the saved the image of Venus from within the image-processing program.
The image is made up of little squares called pixels, the more pixels, the more detailed the image. Three numbers corresponding to the three primary colours represent each pixel. Currently there are two common methods of describing the colours; namely Hex and RGB codes.

Step 4:
From within the image-processing program find the palette.
Within Irfan's menu select Image > Palette > Edit palette.


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

Step 5:
Double click on the 'grey square' or on the square that is seven in from the left. When you double click you will open another palette with colours.

Step 6:
Select and double click on a bright colour from the choices provided (I chose yellow)

Step 7:
To complete the process click O.K., then click O.K. again.

Repeat the process to further enhance the satellite image of Venus.

Your first colour changes to enhance the image would look something look like this.

Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

Repeat the steps and change more colours. Note with each successive colour change more detail appears. Until all the hidden details are revealed.


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

A geologist can then interpret this information. From the final image it would appear that here are three distinct rock types or ages each of which appears to be offset by a crack or fault not quite at right angles to the contacts (boundaries) of the rock groups. This is common on earth with rocks that are under some tension.

Example 2: Hawaii - exploring its waters


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

Mona Loa on the big island of Hawaii is the highest mountain on earth but we don't think of it that way because the bottom half of it is under water. It has some of the most diverse climate on the planet ranging from tropical rainforest around its biggest town Hilo (infamous for being frequently destroyed by tidal waves), to clear sub-arctic conditions at its summit to desert conditions in its rain shadow.

Step 1:
Open the image-processing program. If you installed IrfanView program open it.

Step 2:
Click on the image of Hawaii to open a larger version and save it to your computer

(Hawaii1.jpg).

This is a multi-spectral can image of Hawaii in false colour from Landsat 5 downloaded from a NASA site in Maryland, West Virginia. The image is supplied in 16 million colours and you will be instructed on how to quantize it to 256 colours. The circular feature on the left is the crater of this enormous volcano.

Step 3:
Open the saved the image of Hawaii from within the image-processing program.

Step 4:
Change the image to 256 colours. Within Irfan's menu select Image > Decrease colour depth > select 256 colours.

Step 5:
To complete the process click O.K.

Now we can enhance the image to find our more about the water.

More about the water

Notice the water in this image has been changed to black. Is there any standing water on the island itself? The answer is yes but the differences are too subtle and the pixel size too small for the human eye to detect. Most of the ocean and some of the volcanic peak appear blue indicating standing water.

From within the image-processing program find the palette.
Within Irfan's menu select Image > Palette > Edit palette (you will have to do this three times).

First time:
Click on the square shown in the top left hand corner and change the black to blue. Click O.K.
You can now see places that have cold water - notice the island is surrounded by cold (deep) water but also that there is a supply of cold (due to the altitude) water in the crater itself.

Second time:
Click on the dark square 11 down and 6 from the left and change it to red. Click O.K.
This shows warm water. Notice it is a very narrow band around the island. The ocean becomes very deep very quickly around the island. Click O.K.

Third time:
Click on the dark square 8 from the left in the top row. Change it to aqua.
This colour shows water in creeks and streams. Some of it is partly obscured by cloud but the enhancement allows you to trace the hidden watercourses.

Try some more images

Finding zones of maximum turbulence in a cyclone


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

This exercise uses the 'Sharpen' feature of image enhancement. It is a mathematical routine that identifies and slightly darkens edge features. It is used to bring out outline shapes from blurry of fuzzy images but must be used with care. Overuse results in the image becoming granular.

Step 1:
Open the image-processing program. If you installed IrfanView program open it.

Step 2:
Click on the image of the cyclone to open a larger version and save it to your computer (cyclone1.jpg).

Step 3:
Open the saved the image of the cyclone from within the image-processing program.

Step 4:
Sharpen then reduce the colour depth of the image to 16 colours. Within Irfan's menu select a) Image > Sharpen > OK
b) Image > Decrease colour depth > select 16 colours > OK.

Step 5:
Change the colours to see if you can pick up more subtle changes in the cyclone.

The Pyramids


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

Some people think the pyramids, at Giza in Egypt, were made by building, then removing a giant artificial hill. Disturbed sand comes up as a lighter colour.


Predicting floods on the Zambeze River in Africa


Courtesy of USGS Science for a changing world
http://astrogeology.usgs.gov

Reduce the colour depth of the satellite image of the Zambeze River to 16 colours. Work on changing the darker colours to black to make the areas prone to flooding stand out.

Need a hint?

Check your answer against an image of the Zambeze River actually in flood!

Notice that while our image enhancement easily outlines flooding areas on the delta, cloud cover makes us miss the major tributary on the top right hand side!

Remote sensing and satellite imagery must always be followed up with ground checks!

Additional images

Additional Images from the USGS Science for a changing world
(http://astrogeology.usgs.gov)