Introduction to Remote Sensing

Purpose:  To learn about how remote sensing data is collected, transmitted and used.  To become familiar with the types of remote sensing data that are now readily available on the web and  to learn to interpret, analyze and enhance maps produced with remote sensing data.  To introduce appropriate 2-D area rendering techniques for remote sensing data.

Overview:  Remote sensing data has become  very important in a number of scientific fields.  In this lesson students view, discuss, and compare different types of maps produced from remote sensing data.  Then using image processing software they analyze and enhance sea surface temperature maps. Only a brief introduction to remote sensing data is provided in this lesson.  There are many excellent internet sites on this topic.  If students have internet access this makes a great research and presentation topic.  Once students are familiar with the images available and the image processing software they can also develop good portfolio  projects applying these techniques.

Topics:

    Scientific Visualization: 2 D Area rendering techniques

    Science:  Earth Science: location and mapping, meteorology, oceanography, space exploration, current research.

NC Scientific and Technical Visualization Objectives:

This lesson serves as an introduction to some of the software and techniques needed for the following level II objectives:

Level II

2.01B   The student will identify and apply the guidelines for 2-D color usage.

3.01      Identify and explain 2-D area rendering techniques.

The student will identify sources of data and applications for 2-D area rendering. The student will identify and apply color lookup table techniques. The student will identify and apply image processing techniques. The student will identify and apply measurement techniques.
         The student will identify and apply output techniques.
 

NC Earth and Environmental Science Goals and Objectives (objectives from 1994 revision):
    Work on this lesson can contribute to understanding in the following areas:


Tools      Scion Image  or NIH Image,  Internet Access
 

The directions included in this lesson are for Scion Image Release Beta 3b for Windows NT.  Directions may need some adjustment for NIH Image or other versions of Scion Image.  Teachers should be sure to do this project several days in advance on the computers the students will be using to check for adjustments in the directions. This lesson can be done without internet access but will be much enhanced with internet access.

Teacher Background

Remote sensing provides many types of scientists with data about our earth and beyond.  In this lesson we will be concentrating on the data about Earth provided by satellites.  These satellite images used to be available only to government agencies and advanced research centers but now much of the data is available to the public.  Some data such as sea surface temperatures and weather maps can be accessed in near real time on the web.   Other satellite images can be ordered in digital form by anyone with web access.  The data can be analyzed for a wide variety of projects.   A quick summary of how remote sensing works and what it is used for is provided below for teacher reference and for those classes without internet access.  For those classes with internet access, links are provided in the Reference Section so that students can research these topics further.

An important source of remote sensing data is satellites.  Satellites have sensors which collect data and record electromagnetic radiation from the Earth.  The electromagnetic spectrum is divided into regions based on the wavelength of the energy.  For example, visible light has short waves compared to radio and TV waves.  Satellite sensors can detect one or more  of the types of electromagnetic radiation that are reflected from or emitted by the Earth.  These include visible, ultraviolet,  and infrared radiation.  Also, some satellites send out radiation, either radio or light waves, and record the signal that is reflected back.  These systems are called radar if they use radio waves or lidar if they use light waves.  The satellites collect the data and send it to earth in digitized form.  (To help students understand how this works try the pixel transmission activity).

These different types of  data can be used to assess temperature, weather conditions, type, amount, and health of  vegetation, ozone levels, volcanic eruptions, polar ice melting, etc. For example, thermal infrared radiation can tell us the temperature of the surface emitting the radiation which can be used to construct sea surface temperature maps, cloud top temperature maps, and volcano hot spot maps.  By comparing images taken of the same place over time we can detect patterns of change.     The data is called multispectral if data from more than one part of the spectrum is collected simultaneously.   When this data is displayed different colors are used to represent data from different spectral regions.  If the spectral regions being displayed are all in the visible light region and the display uses red, green and blue  to represent the red green and blue light recorded by the satellite sensor then the image is true color and will look like a photograph.  However most satellite images are not true color.  A false color composite uses different colors to represent the different bands - for example red might be used for data from the near IR,  blue for the far IR, and green for ultraviolet.  You can experiment with using different colors to display data at the Virtually Hawaii's Spectral Imager site:  http://hawaii.ivv.nasa.gov/space/hawaii/vfts/oahu/rem_sens_ex/rsex.spectral.4.html   which is part of their excellent online guide to remote sensing.  In a psuedo-color image the color of each pixel depends on the value of  one measured quantity.  The sea surface temperature picture displayed below is an example of a psuedo-color image from NOAH.  In this image the color of each pixel depends on the water  temperature which was calculated from the satellite data.  We will be using this image in the exercises below.

Sea Surface Temperature

Remote sensing is a great topic to research on the internet as lots of tutorials and interesting images are available.  It would be far preferable to have students research these topics themselves.  If Internet access is available divide students into groups and have them research and report back to the class on the following:

The Electromagnetic Spectrum
Types of Satellite Data
Displaying Satellite Images
Satellite Orbits
Spectral Signatures
Weather Map Symbols
Weather Forecasting
Remote Sensing and Volcanos
Remote Sensing and Climate Change
Images of the Gulf Stream

Once students have an idea of what remote sensing data is, what it is used for, what data is available, how it is collected, and if possible where and how to to find it on the internet they are ready to work with some actual images.  These directions are designed for the sea surface temp image that you can download here but would work with minor modifications for other images you or your students might be interested in.  Your browser may give you a choice between opening this file or saving it to disk.  Save it to disk and then open it using Scion Image or NIH Image.

 download here
 

There are several difficult concepts for students to understand in this exercise.  Depending on their  mathematical proficiency, students may need help understanding what is being done when the scale calibration is done.  A possible way to teach this is to use thermometers which can be read in both ºF  and ºC.  Help students to understand the conversion between these two scales which are being used to measure the exact same thing.   Have them note that there are 180ºF between freezing (32ºF) and boiling (212ºC) on the Farenheit scale while there are 100ºC between freezing and boiling on the celcius (centigrade) scale.  This gives us a ratio of  180 to 100 which reduces to 9/5.  So each rise of one degree celcius is equivalent to almost a 2 degree (9/5) rise in degrees farenheit.  Since 0ºC = 32ºF the formula for changing celcius to farenheit is F=(9/5)C +32.  Essentially we go through the same process in calibrating our temperature scale for this image only we are converting pixel values to degrees celcius.  Also we only have to convert 2 measurements.  Once these two measurements are entered into the computer  the software fits a straight line conversion equation and can give you the temperature for any other pixel value.

The density slice from 24 to 27 degrees Celcius  nicely outlines the gulfstream as shown:


 
 
 

At the end of the directions for the various operations is a list products you can ask students to do as assignments to turn in or present and  questions  to assign or discuss.

The reference section below contains  a list of links to get started finding data, down loading  images and  researching  remote sensing.
 

References

 Remote Sensing

  Weather Data   Sea Surface Temperature  Volcanic hot spots   Remote Sensing Online Lessons

Student Assignment Sheet  - link to separate page for separate print out.
Name______________________
Remote Sensing - Image Processing

Purpose:  To interpret, analyze and enhance maps produced with remote sensing data.  To introduce appropriate 2-D area rendering techniques.

Tools:  You will be using Scion Image, an image processing and analysis tool.  When you first enter Scion Image four tool and information boxes will pop up in addition to the Menu Bar.   These are the LUT or look up table which assigns colors to pixel values, the Toolbar, The map box and an information box.  You may need to click on the full page button to get Scion image to cover your desktop.  Some of the tools you will be using are labeled below.  Information about the others can be found in the contents under tools in Scion's help menu.


 
 
 

Getting Started  Download the sea surface temperature image provided.   If you are online,  click here to go back to the download image directions.  If you are not online your teacher will have downloaded the image.

  1. Open this image in Scion Image.  The original name of the image  was s9900709.ns7  The s is for the Southeast region, 99 is the year, 007 is the day of the year (the days are numbered from 1 to 365 so the month is not included in the filename.  The 9 means that the picture was taken between 9:00 and 10:00.  The image has been converted to a special tiff file for you to work with.  (Warning:  if you try to download and use the files direct from coastwatch you will lose some of the information and some of the exercises below will NOT work properly although they may appear to.)     The n stands for North Carolina and the s7 stands for night sea surface temperature.  Each pixel in this image represents a 1.41 km square.  The average temperature of the sea surface for this area was determined from the satellite data and stored as a value for that pixel.  Each temperature value then has a color assigned to it.

  2.  
  3. Examine the image closely.   The Latitude and Longitude grid have been overlaid on the map.  The land, interior water surfaces, and some clouds over the ocean are blue - only the sea surface areas have accurate temperature data in this image.  Note that the sea surface temperature gets warmer as you go offshore.   This is because of the gulf stream which brings warm water from the tropics north along our coast.

  4.  
  5. Use the magnifying tool to examine the outer banks more carefully.  By clicking on the magnifying tool and then on the image you will zoom in.  When the image is larger you will no longer be able to see it all at once but you can use the hand tool to move it around. Eventually if you keep zooming in you will see that the image is made up of pixels.  In this case each pixel represents the average temperature calculated for a square 1.41 km on each side. Zoom out again by holding down the control key while you click the magnifying tool.


Measuring Distance:  You can use the tools to measure distances in the image if you calibrate the distance scale first.  We will measure the length of North Carolina's coast.  To do this:

  1. Pull down the Analyze menu and go to set scale.  A dialog box will appear.

  2.  
  3. Change the units to kilometers.

  4.  
  5. Change the measured distance to 1 pixel and the known distance to 1.41because each pixel is known to represent an area of 1.41 square kilometers.  The pixel aspect ratio is 1 because our pixels represent the same distance for the length and the width. .

  6.  
  7. Now you can measure distance with a line selection tool.  If you hold down the mouse button you can choose a freehand  or segmented line as well as the straight line.   Because the coast is irregular the freehand tool will allow you to trace the coastline most accurately.

  8.  
  9. Choose the freehand tool, start at the state line with Virginia and trace along North Carolina's coast and the Outer Banks down to the State line for South Carolina.  The line will disappear as soon as you release the mouse.

  10.  
  11.  Pull down the analyze menu and choose measure.

  12.  
  13.  Then (also in the analyze menu) select show results.

  14.  
  15. A results dialog box should pop up.  Since your line has no width, area and mean will be 0.  The length should be approximately 600 km.

  16.  
  17. You can go back to set scale and change the scale to miles by just changing the units to miles.  Re measure and find that the coastline is approximately 380 miles long.

  18.  
  19. You can use the select rectangular area tool to find the area of a selected region as well.
Experimenting with the LUT:  LUT stands for look up table.  Images are stored as tables of  values.  In this case each value is the average temperature for 1.41 square kilometers each of which is represented by one pixel in the image.The computer uses the LUT  to see what color each temperature value should have.   You can change the LUT.  This will change the appearance of your picture  by changing what color is mapped to a particular temperature.  It will not change the actual temperature data.
  1. Under the Options menu you can change to gray scale or any of several other LUTs listed under color tables.  You can also invert the LUT  under the LUT options menu choice.  Experiment with these different options noting the advantages and disadvantages of various choices.  You can always go back to your original image by choosing revert to saved in the file menu.  If you decide to save any other version of your map be sure to use save as and give it a different file name.

  2.  
  3. There are several other ways to manipulate the LUT.  You can use the map box (choose it under the window menu if  it is hidden)  to adjust the brightness and contrast.  You can use the little sliders or the line graph.  Practice this with a gray scale image.  Notice what happens to the grayscale LUT when you adjust the contrast and when you adjust the brightness.  (question 4)

  4.  
  5. You can move whichever LUT you have selected directly.   With the LUT tool selected click and hold on the LUT.  As you slide the LUT up and down different color ranges are used.
Featuring North Carolina
  1. The index number of the pixel value used to overlay the state border lines and rivers is 4.  To make these borders and rivers stand out clearly you can map that value to a different color.  Choose the eyedropper tool.  Move the cursor over the LUT - you should see the index value change from 0 at the top of the LUT to 255 at the bottom.  Get the value to exactly 4 and double click.  Now choose a color for state borders and rivers.  In the same way you can choose a different color for the latitude and longitude lines.  Their index value is 2.

  2.  
  3.  If you have a lot of patience you can  use the paint bucket tool to fill in North Carolina's land area  with another color of your choice. This will help orient the viewer by allowing him or her to distinguish North Carolina from the surrounding states.   You will have to use the zoom tool and the pencil tool to be sure that the land area is completely enclosed by lines.   Otherwise your color will leak out along the coast.  When that happens you can use undo or revert to saved until you get it right.  You will also need the zoom tool if you decide to work on the Outer Banks.

  4.  
  5. Use the text tool to label the rivers.
Calibrating your temperature scale:   This operation allows you to give Scion Image the information to calculate the temperature scale.  Each pixel in the image has a value between 1 and 255.   The temperature range that the image was transformed with was -5 to 35 ºC.  (Temperatures outside the chosen range would map white or black.)  So a temperature of -5ºC gets a pixel value (P)  of 1 and a temperature of 35ºC gets a pixel value (P) of 255.

This gives us a conversion formula, where P stands for Pixel Value:

 ºC = (40/255)P - 5

So what you have to do is:
  1. Use the  select region tool to select a small area just a few pixels square, someplace in the ocean.

  2.  
  3. Select measure in the analyse menu.

  4.  
  5. Then select calibrate in the analyse menu.  A pop up menu will come up with the average measured pixel value for your square.  This is P.

  6.  
  7. Substitute the measured value for P in the conversion equation above to calculate the degrees Celcius.  (You can do the math with a calculator -  your computer probably has one you can use in the Accessories folder. (look in the start menu in the programs menu for accessories then look for calculator)
  8. Enter your result in the known column.
  9. Repeat in a different area of the ocean so that you get a different measured value, again use your formula to calculate and then enter the known temperature.
  10. Click on the straight line button, enter degrees for the unit of measure and click ok.
  11. Now as you move your cursor over the image the value in the Info box should be the temperature in degrees celcius.
  12. You can use the plot profile tool to plot the temperature of  points on a line as you move offshore.   A line running straight offshore from the NC Virginia state line will have a plot profile similar to the one below.   The sharp spikes are where the latitude and longitude lines are (pixel value =0).  Note the long distance of cooler water followed by a rapid shift to warmer water where your line intersects the gulf stream current  followed by cooler water on the other side.


   use arrow keys to adjust plot box size
 

Density Slicing  Density Slicing allows you to highlight a  range of values in red.  It allows you to pick a range that you are interested in - say water between 24º and 27 ºC.

  1. Revert to your saved image and gray scale it.

  2.  
  3.  Choose the LUT tool.  Then choose density slicing from the options menu.

  4.  
  5. You can click on the top or bottom of the density slice to increase or decrease the size of your slice or click in the middle of the slice to move it up and down the LUT.  If all of your calibration is still included the info window will tell you the upper and lower value of your slice.

  6.  
  7. Move and size your slice until 24 to 27 degree water is shown in red.
To Do:
  1. Produce a  plot profile of the water temperature going directly south parallel to the longitude line from  from Cape Fear (which is the point on the coast that sticks out into the ocean just  south of Wilmington.

  2.  
  3. Measure in kilometers  how far off shore the warmest part of the gulf stream is at Cape Fear.

  4.  
  5. Measure the average temperature of Albemarle Sound. (hint:take separate measurements on either side of the longitude line and average them)

  6.  
  7. Produce a density slice map highlighting the water that is between 22 and 24 degrees Celcius.
Questions:
  1. What do we mean when we say is this a psuedo colored image?

  2.  
  3. When and why might it be useful to change the color of North Carolina's land area?  Would  you choose colors to hide, minimize or highlight the latitude and longitude lines?  Explain the reason for your choice.

  4.  
  5. Which LUT did you think did the best job of bringing the viewers attention to the gulf stream pattern?  Why?

  6.  
  7. How do the brightness and contrast adjusters differ in their effects?

  8.  
  9. Why can't all of North Carolina's outer bank islands be distinguished on this image

  10.  
  11. What practical value might these images have for fishermen?




 

Evaluation Criteria:

 Success will be measured by the following criteria:
 
Produced required profile plot.
        20
Accurately measures distance.
20
 Accurately measures temperature
20
Produces required density slice map
20
Answers questions
20
Total
100

 
 

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last update 10/07/99