Download Ratios - onlinegeographer

BAND RATIOS
Today, we begin to speak of the relationships between two+
bands.
BUT WHY?
• To highlight certain features
• To “correct” for topography
EFFECT OF TOPOGRAPHY
ON SCATTERPLOTS
Grassy
fields
Water/
shade
Flat terrain
Terrain without
topography
Bare ground
Terrain with topography
Thanks to Robin Weeks
HOW?
• Simplest ratios are simply one band divided by another. Though they get a bit
trickier in reality.
• The trick is truly knowing how different materials reflect – and using that to your
advantage.
• Let’s start with vegetation.
VEGETATION RATIOS
• Well, what bands tell us things about vegetation?
HOW CAN WE USE THIS WITH DIGITAL
IMAGERY?
• Many vegetation indices are based on accentuating the
DIFFERENCE between red and NIR reflectance in image pixels
Big
Difference
Small
Difference
RATIO-BASED VEGETATION INDICES
• RVI = NIR/Red
• Simplest ratio-based index is called the Simple Ratio (SR) or Ratio
Vegetation Index (RVI)
• High for vegetation
• Low for soil, ice, water, etc.
• Indicates the amount of vegetation
• Reduces the effects of atmosphere and topography – because the
ratios should be the same regardless of sun intensity!
PROBLEM WITH SR
• Division by zero
• Wide range of possible values depending on amount of red reflectance
• These problems addressed by development of the NDVI
NORMALIZED DIFFERENCE
VEGETATION INDEX
• NDVI = (NIR – Red)/(NIR + Red)
• Ranges from -1 to 1
• Never (rarely?) divide by zero
• Indicates amount of vegetation, distinguishes veg from soil, minimizes topographic effects,
etc.
• A good index!
• Does not eliminate atmospheric effects!
BUT… WHAT IF YOUR FIELD AREA HAS A
LOT OF EXPOSED DIRT/ROCK?
INDICES GET “TUNED” TO TRY TO REDUCE
THESE PROBLEMS.
• E.g., Soil Adjusted Vegetation Index (SAVI)
• Uses a soil background “fudge factor”
SAVI = [(NIR – Red)/(NIR + Red + L)] * (1 + L)
L is a soil fudge factor that varies from 0 to 1 depending on the soil.
Often set to 0.5 as a default.
OK
• First, let’s goto ERDAS and look at Ellensburg in NDVI (raster, unsupervised, indices)
• Then, scatter plots!
SCATTERPLOTS
1.
2.
3.
4.
5.
Classifier Icon
Select Signature Editor
In signature editor Select Feature
Then Create Feature Space Layers
Input Raster Layer (navigate to source file) ( the output image automatically is entered in the Output
window)
6. In the Create Feature Space Image window click on the layers you wish to plot in the Feature Space
Layers window (to do this click on the tabs on the right under FS Image holding the shift key down to
select the spectral plots you wish to generate)
7. Tick output to viewer
8. The plots will appear on the screen - See more at:
http://learningzone.rspsoc.org.uk/index.php/Learning-Materials/Image-Processing-for-ERDAS/1.3.Two-Wavebands-Scatterplot#sthash.4dLo6Gkw.dpuf
CLASS EXERCISE 1
• A flood index along the Nile River in Egypt.
• Which bands do you choose? Assume Landsat 8
CLASS EXERCISE 2
• Mineral mapping in Australia
• Assume you can see, in roughly equal portions, rocks, soils, and scrubby vegetation.
BACK TO ERDAS TO….
• Look at some of the canned ratios.