ndvi anomaly, kenya, january 2009 vegetation indices enhancing green vegetation using mathematical...
TRANSCRIPT
![Page 1: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/1.jpg)
![Page 2: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/2.jpg)
NDVI Anomaly, Kenya, January 2009
![Page 3: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/3.jpg)
Vegetation Indices
Enhancing green vegetation using mathematical equations
and transformations
![Page 4: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/4.jpg)
Learning Objectives• What are vegetation indices?• What do we hope to accomplish with
them?• Understand the relationship between
spectral indices and spectral reflectance curves.
• What features of vegetation spectra are most indices based on?
• What are advantages and disadvantages of various algabraic indices?
![Page 5: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/5.jpg)
What is a “vegetation index”?
• A mathematical combination or transformation of spectral bands that accentuates the spectral properties of green plants so that they appear distinct from other image features.
![Page 6: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/6.jpg)
What Should Vegetation Indices Do??
• Indicate the AMOUNT of vegetation (e.g., %cover, LAI, biomass, etc.)
• Distinguish between soil and vegetation• Be insensitive to atmospheric and
topographic effects if possible
![Page 7: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/7.jpg)
How is Vegetation Spectrally Distinct?
• Reflectance in individual wavelength regions (bands)?
• Shape of spectral curve created by looking at more than one wavelength region?
• Changes in spectral curves with amount of vegetation?
• Others?
![Page 8: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/8.jpg)
Soil Reflectance
• Can be bright in NIR (like vegetation)– dry soil especially bright – wet soil much darker than dry soil
• Soil can have low visible light reflectance (like vegetation)
![Page 9: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/9.jpg)
Group Exercise
• Given typical green vegetation spectral reflectance, and reflectance of soils ranging from dark to bright, propose an algebraic combination of two Landsat 8 bands that will distinguish the plants from the soils!
![Page 10: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/10.jpg)
Vegetation vs. Soil and Water
![Page 11: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/11.jpg)
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
![Page 12: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/12.jpg)
Difference Vegetation Index (DVI)
• Probably the simplest vegetation index– Sensitive to the amount of vegetation– Distinguishes between soil and vegetation– Does NOT deal well with the difference
between reflectance and radiance caused by the atmosphere or shadows• So for example…can’t distinguish vegetation from
soil in shady areas very well.• A problem when there is topography.
![Page 13: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/13.jpg)
Ratio-based Vegetation Indices
• 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
![Page 14: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/14.jpg)
Why Simple Ratios Reduce Atmospheric and Topographic Effects
LNIR = ENIRtNIRrNIR/π
LRed = ERedtRedrRed/π
So =
Partly cancels irradiance from equation and therefore topographic differences
Partly cancels transmittance and therefore atmospheric effects
![Page 15: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/15.jpg)
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
![Page 16: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/16.jpg)
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!
![Page 17: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/17.jpg)
NDVI Applications
![Page 18: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/18.jpg)
![Page 19: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/19.jpg)
But…Problem with NDVI (and some other ratios)
• Sensitive to soil background reflectance• Non-linear changes in index as amount of
vegetation changes• Not insensitive to atmosphere• Affected by geometry• Saturation problems• So…use with caution. Great for many
applications but not all!
![Page 20: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/20.jpg)
Soil Background Effects
![Page 21: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/21.jpg)
IR
IR
IR
IR
IR
IR
IRIR
GR
B
(Amount changes depending on soil)
![Page 22: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/22.jpg)
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 1.
![Page 23: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/23.jpg)
Vegetation Amount (LAI)
![Page 24: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/24.jpg)
![Page 25: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/25.jpg)
Choosing an Algabraic Index• Most difference indices fall short in terms
of dealing with atmospheric and topographic effects
• Most ratio-based indices are functionally equivalent (work about the same)
• Some ratio-based indices are computationally “cleaner”
• NDVI is often the index of choice and generally performs pretty well, but you must be aware of potential issues
![Page 26: NDVI Anomaly, Kenya, January 2009 Vegetation Indices Enhancing green vegetation using mathematical equations and transformations](https://reader036.vdocuments.site/reader036/viewer/2022081504/56649c885503460f94940577/html5/thumbnails/26.jpg)
Next Lecture…
• Indices based on data transformations and “feature space”