Dirty REMOTE SENSING · AES2012 L6 NDVI Remember the images are stored in the image file (eg. Jpeg)...

MERMS 12 - L4

Dirty REMOTESENSING

Stuart Green

Earthobservation.wordpress.com

Stuart.Green@Teagasc.ie

Lecture 3: First Steps in classifying

• You have your image, but is it any good?

• Is it full of cloud?

• Is it the right time of year?

• Is it the right resolution?

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Atmospheric effects

• The atmosphere, as we have seen, caninterfere with the picture the satellite istaking. Clouds and their shadows areobvious but atmospheric effects that arenot readily visible can distort the image,reduce the signal/noise ratio, introduceerrors in classification

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Atmospheric Correction

• Raw Image: The image recorded at thesatellite- this is uncalibrated data thatcontains all the errors from theatmosphere and the sensor. Level 0

• Level 1 usually corrects for satellite levelerrors and presents the data in DigitalNumbers. Sometime terrain correctionsare included.

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• Top of Atmosphere reflectance correctionconverts the L1 DN to a numberrepresenting the percentage of light thatfell on the earth as reflected back tosensor.

• Full atmospheric correction, to surfacereflectance. This is either a suppliedcommercial image or using free softwarelike LEDAPS you can perform thiscorrection yourself.

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• Atmospheric correction corrects forscattering and absorption in theatmosphere. At its most sophisticated itused models of the atmosphere like 6Salong with estimate of optical parametersof the atmosphere from satellites.

• At its simplest- we use the dark targetapproach

• http://ledapsweb.nascom.nasa.gov/MERMS 12 - L4

• If you need accurate, transferable,repeatable measurements you will need toatmospherically correct your imagery. Thiswill mean your image values areequivalent to surface measurements.

• HOWEVER Landsat data are available asa atmospherically corrected data set (level2a)

• http://www.cesbio.ups-tlse.fr/multitemp/?p=1774

First Automation find Vegation

Sources Online for Vegetationdetection

• http://earthobservatory.nasa.gov/Features/MeasuringVegetation/

• http://wiki.landscapetoolbox.org/doku.php/remote_sensing_methods:net_primary_productivity

• ftp://ftp.biosfera.dea.ufv.br/users/francisca/Franciz/papers/Running%20et%20al.%20Bioscience%202004.pdf

AES2012 L6 NDVI

AES2012 L6 NDVI

Video: Global Seasonal Vegetation Growth: a planetary phenology

AES2012 L6 NDVI

• Biomass: The mass per unit area of vegetation.• Cover: The vertical projection of the plant parts on the ground surface per unitareaof ground. Usually expressed as a percent. No species can have more than 100%cover.• Leaf Area Index: The ratio of the area of leaves and green vegetation in theplantcanopy per unit area of ground surface. LAI can exceed 1.The only way to get trueleaf area is to strip all the leaves off the plants and measure their area. All othermethods provide an “index” of this value• Normalized Difference Vegetation Index (NDVI): An index of vegetationgreenness derived from remote sensing methods. Often used as an indexof biomass.

How do we measure what veg is there?

FPAR measures the proportion of available radiation in the photosynthetically activewavelengths (400 to 700 nm) that a canopy absorbs.

AES2012 L6 NDVI

AES2012 L6 NDVI

AES2012 L6 NDVI

We can measure with thesateelite sensors the ratio ofred to NIR light- this ratio is

called aVegetaion Index

AES2012 L6 NDVI

AES2012 L6 NDVI

Vegetation Indices?

• The gigantic chlorophyll absorption welldistinguishes vegetation from non-vegetation.

• Its size tells us chlorophyll concentration inthe leaf and the canopy.

• Many vegetation indices are a simplisticattempt to estimate the size of thisabsorption well.

AES2012 L6 NDVI

Vegetation Indices

Vegetation indices (VI’s) can be broken upinto two basic categories:

Ratio based indices – VI’s based on the ratio oftwo or more radiance, reflectance, or DN values(or linear combinations thereof).

Difference indices – VI’s based on the differencebetween the spectral response of vegetation andthe soil background.

AES2012 L6 NDVI

Common Ratio Indices

Simple Ratio Index (SR) = NIR/R

Normalized Difference Vegetation Index (NDVI) =

NIR R

NIR R

AES2012 L6 NDVI

Estimating the size of the absorptionwell

wavelength

400 600 800 1000 1200

reflecta

nce(%

)

0.0

0.1

0.2

0.3

0.4

0.5

density 1

density 2

density 3

density 4

density 5

density 6

sunlit soil

What are Vegetation Indices?

AES2012 L6 NDVI

• (B4-B3)/(B4+B3) for Landsat

• A pixel by pixel mathematical process

AES2012 L6 NDVI

AES2012 L6 NDVI

Remember the images are stored in the image file (eg.Jpeg) as xy matrices with a pixel in one band

corresponding with the pixel in another band with samexy coordinates

45 12 19

44 10 16

27 90 56

10 67 12

99 70 53

2 98 1

55 79 31

143 80 69

29 188 57

B1 B2 B1+B2

AES2012 L6 NDVI

Other Indices

SAVI =

NDII= (VNIR-NIR)/(VNIR+NIR)

Create and NDVI image

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• See https://www.youtube.com/watch?v=F3rsSq0-mmE

• And http://virginiaview.cnre.vt.edu/education.html#RSinArcGIS10

Practical

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• Load Curragh17.Tiff into ARC MAP andopen the ToolBox window

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• WE need to convert the image to a floatingpoint format

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• Do this for Band 4 (red) and Band 5 (NIR).

• Call the outputs RED and NIR

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Use Map Calculator to calculatethe NDVI image

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This is the calculation

• Press OK

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Use the Info button to clickaround and see the values

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Load up the Sites.shp file

• What are the NDVI values for the 6 sites.

• What’s the relationship between landcoverand NDVI? (list the landcover types andNDVI score)

• Click around the image- find an NDVIvalue that represents “no vegeation”

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Click onto “reClass->reclassify” and load yourNDVI image into pop up menu- we are going to

create a vegeation/no-vegetaion map.

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On the reclassify menu click “classify…” andchange the number of classes to 2. Then change

the 1st break values to be your bare soil ndvi value(o.3 in this example) and the second to 1. Click OK

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Clcik the output button and enter a value aname in YOUR directory – do not add an

extension. Click OK

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Your output will look a little like this.You have created a vegetation/no-vegetation mask

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