1. January 20, 2006 Geog 258: Maps and GIS
Remote Sensing
January 20, 2006
Geog 258: Maps and GIS

2. Outlines Principles of remote sensing
Three key aspects of resolution of remotely sensed image
Aerial photo vs. satellite image
Classifying aerial photograph
Geometric distortion of aerial photograph
Classifying satellite image

3. Principles of remote sensing
What you see is the amount of electromagnetic energy reflected by object; needs energy source, object, sensor
Electromagnetic energy can be divided into different spectral bands (visible light, NIR, microwave) given its wavelength
Electromagnetic spectrum
Electromagnetic energy interacts with object differently
Snow reflects most of energy (that’s why it looks bright)
Dry soil absorbs most of energy (that’s why it looks dark)
How object interacts with electromagnetic energy is a function of wavelengths  basis of image interpretation
Spectral reflectance signature
Electromagnetic radiation interacts with atmosphere as well. Some radiations (in visible, NIR and microwave bands) pass through atmosphere relatively well
Atmospheric window

4. Electromagnetic spectrum

5. Spectral signature
Explain why water looks darkish blue; Explain why vegetation looks greenish; Explain why sand looks reddish yellow

6. Atmospheric window
It shows how much electromagnetic radiation transmits atmosphere
In which bands, is % transmission high?
Advantage of using those bands?
Compare aerial photo and satellite image in terms of spectral bands

7. Aerial photograph
Optical sensor
Infrared sensor
Radar sensor

8. Different kinds of image
Pancromatic image
True-color image
False-color image

9. Pancromatic image
If airborne cameras use black/white film or satellite sensors use a single band, it produces pancromatic image (gray scale image)

10. Color composite Color primaries: RGB (Red, Green, Blue)
Many colors are formed by combining color primaries in various proportions
Look inside your inkjet color printer. Did you really buy 100 different color cartridges?
Same principles apply to producing color images taken from airborne cameras or satellite sensors

11. True-color image See Table 9.1 (p. 182)
How many bands are used for Landsat 4 and 5?
What’s the spectral range (and corresponding colors) for band name 3, 2, 1?
Each band collects different images (See Figure 9.27 at p. 182)
To produce color-image, bands 321 are assigned to RGB respectively
The color of resulting image resembles what would be observed by human eyes
That’s why it’s called true-color image

12. False-color image
If three bands are arbitrarily (i.e. doesn’t have to be RGB bands) assigned to RGB layers, it produces false-color image
SPOT XS1 (green band)
SPOT XS2 (red band)
SPOT XS3 (Near IR band)
R = XS3 (NIR band) G = XS2 (red band) B = XS1 (green band)
Why does vegetation appear dark in green band in pancromatic image?
Why does vegetation appear bright in NIR band in pancromatic image?
Why does vegetation appear red in this color composite?

13. Three key aspects of resolutions
Spatial: pixel (grain) size; minimum distance which can be recorded
Aerial photo has of course higher resolution
Temporal: how often is the image recorded
For satellite image, it can be regular (satellites are orbiting the earth in regular time interval)
Spectral: how many bands are used
Satellite sensors use different # bands; most of them less than 10 (e.g. TM uses 7 bands); sensors with # band > 10 are called hyperspectral (e.g. MODIS)
Important in determining the fitness of use

14. Types of sensor systems
Passive
Rely on external energy source
Optical sensor (reliant on solar energy)
Active
Sensor itself sends out radiation (like flash camera)
Radar sensor (reliant on microwave)
Which one can record the image of earth at night, and which one can’t?
Another reason for the popularity of Radar imaging is the ability of microwave to penetrate atmosphere (e.g. clouds); see Figure 9.4 (p. 164)
So what would be the advantage of Radar imaging compared to optical imaging?

15. Aerial photography vs. Satellite imaging (Airborne RS vs
Aerial photography vs. Satellite imaging (Airborne RS vs. Spaceborne RS)
Which began earlier? (with the development of what?)
Which would have a higher spatial resolution? (in which altitude?)
Are they analog or digital? (output format)
Which is more flexible? (can satellite be launched any time?)
Which would provide geographic coverage in a systematic manner?
Which is equipped with wider spectral bands? (does air photo has thermal-infrared sensors?)
How would post-processing be different?

16. Classifying aerial photo
Film emulsion
{black-and-white, true-color, color-infrared}
Camera’s height
{high, mid, low-altitude}
Camera’s vantage point
{vertical, oblique}
When color-infrared films were invented, and for which purpose?

17. Geometric distortion of air photo
Aerial photo gives us perspective view (it distorts geometry of geographic features)
Transformation from central to parallel perspective results in planimetrically correct photo or orthophoto

18. Sources of geometric distortion
Camera tilts
Scale is not uniform

19. Sources of geometric distortion
Variations in terrain
Remember photo is flat (2D) and objects should be seen from the point of infinite height in the planimetrically correct photo.
In the aerial photo,
a’ is displaced outward to a
b’ is displaced inward to b
due to terrain variations

20. Classifying satellite image
Spatial resolution
{high, low}
Types of energy source
{passive, active}
Types of spectral bands
{pancromatic, multispectral, hyperspectral}
Special purposes
{earth-observing, meterological, thermal-infrared}
From the list of earth observing satellite, place the sensors in classification schemes above

21. Review questions
What is the difference between aerial photo, orthophoto, and DOQs?
Discuss advantage and disadvantage of aerial photo vs. satellite imagery
What is the advantage of using active sensor systems?
What is the advantage of using spectral bands beyond visible wavelength ranges ( micron)?