Maa-57.2040 Kaukokartoituksen yleiskurssi

General Remote Sensing

Image enhancement II

Autumn 2007

Markus Törmä

Markus.Torma@tkk.fi

Image indexes

• Idea is to combine different channels from multispectral image so that desired feature is enhanced– ratio, difference or combination of these– larger value, feature is more present

• It is useful to know spectral characteristics of different material when developing index

• Vegetation indexes most important group

Spectra taken from ASTER Spectral Library

Vegetation index

• Vegetation index is a number that is– generated by some combination of remote

sensing bands and – may have some relationship to the amount

of vegetation in a given image pixel

• Vegetation indices are generally based on empirical evidence and not basic biology, chemistry or physics

• A FAQ on Vegetation in Remote Sensing http://hyperdaac.webthing.com/html/rsvegfaq.txt

Basic assumptions made by the vegetation indices

• Some algebraic combination of remotely-sensed spectral bands can tell you something useful about vegetation

– There is fairly good empirical evidence that they can

• All bare soil in an image will form a line in spectral space– This line is considered to be the line of zero vegetation

• Isovegetation lines: lines of equal vegetation1. All isovegetation lines converge at a single point

– Measure the slope of the line between the point of convergence and the red-NIR point of the pixel

– E.g. NDVI, SAVI, and RVI

2. All isovegetation lines remain parallel to soil line

– Measure the perpendicular distance from the soil line to the red-NIR point of the pixel

– E.g. PVI, WDVI, and DVI

RVI (ratio vegetation index)• RVI = NIR / PUN• values: 0 - inf

NDVI: Normalized Difference Vegetation Index

• NDVI = (NIR-PUN)/(NIR+PUN)

• values: -1 - +1• most used and

well-known• water: low

(negative) values• forest 0.5-0.8• open land 0.5-0.6

NDVI

• April 19• Clouds: grey• Areas with

chlorophyll: white• Snow in Lapland: dark

grey• Water: black

NDVI

IPVI: Infrared Percentage Vegetation Index:

• IPVI = NIR/(NIR+PUN)

• values: 0 - +1

Some more

• Difference Vegetation Index (DVI):

DVI = NIR - PUN

values: -max(PUN) - max(NIR)

• Transformed Vegetation Index (TVI):

TVI = ((NIR-PUN)/(NIR+PUN)+0.5)0.5 x

100

Soil line

• Line in spectral space– describes the variation

of bare soil in the image

• Line can be found by locating two or more patches of bare soil in the image having different reflectivities and finding the best fit line in spectral space

Vegetation index• Some vegetation indices use information about soil line• Perpendicular Vegetation Index

PVI = sin(a)NIR-cos(a)red

– a is the angle between the soil line and the NIR axis

• Weighted Difference Vegetation Index

WDVI = NIR-g*red

– g is the slope of the soil line

Vegetation index

• Some vegetation indices try to minimize soil noise– All of the vegetation indices assume that there is a single soil

line

– However, it is often the case that there are soils with different red-NIR slopes in a single image

– Changes in soil moisture change index value

– Problem of soil noise is most acute when vegetation cover is low

• Soil Adjusted Vegetation Index

SAVI = (( NIR-red )/(NIR+red+L))(1+L)– L is a correction factor which ranges from 0 (high vegetation

cover) to 1 (low cover)

Normalized Difference Moisture Index• NDMI = ( NIR - MIR ) / ( NIR + MIR )

– E.g. ( ETM4 - ETM5 ) / ( ETM4 + ETM5 )

Normalized Difference Snow Index• NDSI = ( GREEN – MIR ) / GREEN + MIR )

– E.g. ( ETM2 – ETM5 ) / ( ETM2 + ETM5 )

Spectral Indices Disadvantages

• Not physically-based– Empirical Relations– Correlation not Causality– NDVI vs. Tourism in Italy

• Only small amount of spectral information used– Rarely simple relationship between variable

and index

Difference in vegetation indexes:difference in vegetation

• Compute vegetation indexes for images taken at different times– Simple way to characterize changes in

vegetation

Tasseled cap transform

• Linear transform for multispectral images

• Multispectral image is tarnsformed to images describing some scene property– brightness– greenness– moisture – haze

• Originally developed for Landsat MSS, then TM, ETM and other instruments

Tasseled cap transform

• Kauth and Thomas noticed that growing cycle of crop– started from bare soil– then to green vegetation

and – then to crop maturation

with crops turning yellow

http://www.cnr.berkeley.edu/~gong/textbook/chapter6/html/sect65.htm

Tasseled cap transform

• They developed linear transformation to characterize that

• Landsat MSS: – Redness (soil)– Greenness

(vegetation)– Yellowness– Noise

http://www.cnr.berkeley.edu/~gong/textbook/chapter6/html/sect65.htm

Tasseled Cap (Landsat-7 ETM)

• ETM-image should be converted to radiances• Brightness = 0.3561 * Ch1 + 0.3972 * Ch2 + 0.3904 *

Ch3 + 0.6966 * Ch4 + 0.2286 * Ch5 + 0.1595 * Ch7

- Corresponds to soil reflectance

• Greenness = -0.3344 * Ch1 - 0.3544 * Ch2 - 0.4556 * Ch3 + 0.6966 * Ch4 - 0.0242 * Ch5 - 0.2630 * Ch7

- Amount of vegetation

• Moisture= 0.2626 * Ch1 + 0.2141 * Ch2 + 0.0926 * Ch3 + 0.0656 * Ch4 - 0.7629 * Ch5 - 0.5388 * Ch7

- Soil and vegetation moisture

Brightness

Greenness

Moisture

R: brightnessG: greennessB: moisture

Karhunen -Löwe transform

• Aim is to decrease number of channels and preserve information

• Idea: remove correlations between channels– same information in different channels

• E.g.: TM-image, 6 channels transformed image, 3 channels

Karhunen -Löwe transform

• y = A * x

• x original pixels

• y transformed pixels

• A transformation matrix

• Transformation matrix compresses information to less number of channels than originally

Karhunen-Löwe muunnos

• Different transformation matrices:– Principal component analysis / transformation:

variance of data is maximized– Canonical correlation: maximize class

separability

• Based on turning of coordinate system according to largest variance

Principal Component Analysis

• PCA: Principal Component Analysis

• Mean vector of data

• Covariance matrix of data– describes the variance of data according to

different coordinate axis

• Hypothesis:– large variance much information

Principal Component Analysis

Channel 2

Channel 1

1. PC

Principal Component Analysis

• Landsat ETM:6 channel, 6-dimensional space

• Usually 3 first principal component as computed

PCA example 1• Porvoo: Landsat ETM 743 and PCA 123• Principal component images have been computed from all

ETM-channels

PCA example 1• Landsat ETM 743 and PCA 1

PCA example 1• Landsat ETM 743 and PCA 2

PCA example 1• Landsat ETM 743 and PCA 3

PCA example 1• Landsat ETM 743 and PCA 4

PCA example 1• Landsat ETM 743 and PCA 5

PCA example 1• Landsat ETM 743 and PCA 6

PCA example

• Proportion of variances of different principal component images 1. 73 %

2. 19 %

3. 3 %

4. 0.7 %

5. 0.3 %

6. 0.2 %

• Three first: about 99% information

Decorrelation strecth

• Image enhancement method

1. Make PCA-images

2. PCA-images are scaled (streched) so that their variance is equal to variance of first PCA-image

3. Make inverse PCA, i.e. return to original image-space

Data fusion: Spatial resolution enhancement

• Generally:– Good spatial resolution bad spectral or radiometric

resolution

– Bad spatial resolution good spectral or radiometric resolution

• For example: – Spot-5 PAN: 5m, 0.48 - 0.71 µm

– Spot-5 XS: 10m, Green: 0.50 – 0.59 µm, red: 0.61 – 0.68 µm, NIR: 0.78 – 0.89 µm, 20m, SWIR: 1.58 – 1.75 µm

Spatial resolution enhancement• Sköldvik Landsat ETM 342 and PAN

Spatial resolution enhancement• Sköldvik Landsat ETM 342 and PAN- ja XS-average image

Spatial resolution enhancement• Sköldvik Landsat ETM 342 and data fusion by principal

component method

Examples of processing chains

Finnish IMAGE2000 for Corine 2000 Land Cover Classification

NAPS/AKO at Finnish Environment Institute

Processing of Finnish IMAGE2000

• 36 Landsat ETM-images

•Orthocorrection by Metria Sweden

•25 m pixel size

•Average RMSE error of test points 12.9 m

• Cloud and shadow masking by visual interpretation

• Atmospheric correction using VTT-SMAC

• Topographic correction in Northern Finland

• Mosaicking according to vegetation zones

Production line for EO data (MODIS, NOAA AVHRR)

Algorithm & Cloud masking End-product

Data delivery-WWW

-Map user interface

-numerical data

Product calculation & data delivery (SYKE)

water protectionwatershed researchclimate change research

forest industry

tourism

citizens

hydropower industry

runoff forecasts

End users

Dat

a

Image processing:

1. Unpacking

2. Radiometric calibration and atmospheric correction

3. Geometric correction

Data in usable form for the algorithms

Automated processing system (SYKE)

EO-data distributer (FMI, K-Sat, …) D

ata

FTP-box Archieving