Image Interpretation

Color Composites

Terra, July 6, 2002 Engel-Cox, J. et al. 2004. Atmospheric Environment

Information Content

• While scientists are primarily interested in geophysical quantities such as pollution concentrations and temperatures – space-borne sensors only measure radiances

• These radiances must be converted to geophysical quantities.

Image Interpretation – Level 1

Radiances are converted to reflectances

Image Interpretation – Level 1

Radiances are converted to temperature

Color Composite

Yellow is a mix of red and green; orange is a mix of more red and some green; white is an equal mix of all three primaries, and black is simply the absence of any colored light of any wavelength

Spectral Signatures

• Spectral signatures allow identification of classes (or features)

• Example : Water does not reflect beyond ~0.8 micron

Imagery

Visible imageryClouds are whiteWater is black

Color CompositeClouds are whiteWater is black, Vegetation is green.

Why is this important?

• You can only understand spectral signatures from Level 1 imagery

• This is the foundation for image classification, feature identification and understanding how to separate various classes.

• Example : How does one separate aerosols from clouds or the background?

Image interpretation

• 550 nm imagery shows smoke, clouds, ocean and land

• Note that eye captures textural information as well

Spectral Signatures of Aerosols and Clouds

MODIS Wavelength (um)

WaterClouds

MODIS Wavelength (um)

In thermal IR smoke andOcean temperatures aresame sub micron smoke does notEmit at these wavelengths.Clouds have colder temperatures

Smoke has sub micron size, thereforeNot reflective at 2.1 umWater clouds at 2.1 um reflect based on particle sizeOcean reflects less than smoke at 0.55um

How to pick bands for Red, Green, Blue• Depends upon your image• Example R : 645 nm, G : 865 nm, B : 645 nmNote that all of this is reflected solar radiationNote that 865 nm reflectance is in both the Red and Green and

Blue BandsSince the green is at 865 nm if there is healthy green vegetation

then then the color composite will show up as green for vegetation

Clouds are spectrally flat and reflect about the same in R, G, and B and therefore will be white

Color Composites

Fusing Level 1 and Level 2• AOD is a measure of the

extinction and scattering of light by particles in a total column from the satellite to the ground

• AOD is proportional to particulate concentration

• AOD is dimensionless; values typically range from 0 (clear, no haze) to 1 (very hazy, smoky, or dusty) in the US

• Clouds block the measurement of AOD!

Next Steps

• In the afternoon session you will be ‘playing’ with GLIDER to interpret level 1B imagery

You will be creating this

Other examples from Volcanic Ash

R, G, B = 0.47,0.55,0.65 um R,G,B = 0.65, 2.13, 11.0 um

ASH