DN to Reflectance

HONDA Kiyoshi

Contents

Definition of NDVI Radiance Reflectance How to Calculate Radiance from DN Irradiance

Definition of NDVI

Red Visibleat eReflectanc:

InfraredNear at eReflectanc:

NDVI

VR

NIR

VRNIR

VRNIR

Reflectance

Ratio of Radiant Energy Reflected to Incident Energy Radiant Energy = Integral of vertical components of

Radiance Measured ( by satellite to be scaled back from DN )

anglezenith Solar :),cos(

/ IrradianceSolar Spectral:

/ Irradiance Spectral :

/ Surface theof Radiance Spectral :

eReflectanc Spectral :

2

2

2

sss

s

ss

mmWE

mmWG

msrmWL

E

L

G

L

E

θ

π L

DN ?

DN ( Digital Number ) you can see in the RS image files is not Reflectance.

So you have to derive Reflectance from the DN

Then what is DN ?

DN is a scaled Radiance DN is scaled from Radiance measured

by sensors

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter6/chapter6.html

Radiance

Energy in unit time, unit solid angle, unit wavelenth

msrmWmsrmsJ 22 // :unit

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter6/chapter6.html

Z

dω

dA

dλ

ddtdAd

EnergyL received

DN to Radiance (Lλ)

Have to find out LMAX, LMIN, QCALMAX, QCALMIN

biasoffsetQCALDN

LMINQCALMINQCALMAX

LMINLMAXoffset

QCALMINQCALMAX

LMINLMAXgain

LMINQCALMINQCALQCALMINQCALMAX

LMINLMAXL

or

offsetDNgainL

,

)(

QCALMAX

QCALMIN

LMIN, LMAX of LANDSAT7

LMIN, LMAX List

LMAX

LMIN

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

LANDSAT 7 Gain Setting for January

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter6/htmls/january_gains.html

How to Find the Constants for Scaling factor of Radiance

Metadata file .metSample ftp://cgceopush3.geo.msu.edu/push3_2/137/043/p137r43_7x000228/p137r043_7x20000228.met

Identify Hi-Gain or Low GainEROS Data Center Gateway

• http://edcimswww.cr.usgs.gov/pub/imswelcome/

Obtain Hi-Gain and Low-Gain parametersUSGS LANDSAT Page

• http://landsat7.usgs.gov/technical_details/calibration_files/

Header file/record Data Handling Manuals

Processed in Japan, China, Thailand ….

How to obtain TOA Irradiance Spectral Exo-atmospheric Irradiance

anglezenith Solar :),cos(

units alastronomicin distanceSun -Earth:

/ Irradiance SpectralSolor catmospheri-Exo Average:

/ Irradiance SpectralSolar catmospheri-Exo:2

2

2

sss

s

s

s

s

ss

d

mmWESUN

mmWE

d

ESUNE

E

L

G

L

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

Average Solar Spectral Irradiance ESUNλ

Average Solor Spectral Exo-atmospheric Irradiance

mmWESUN

mmWE

d

ESUNE

E

L

G

L

s

s

s

ss

2

2

2

/ Irradiance Spectral catmospheri-Exo Average:

/ Irradiance SpectralSolar icstomospher-Exo:

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

http://modarch.gsfc.nasa.gov/MODIS/ATM/solar.html

ESUNλ for ETM+

Sun Earth Distance ds

units alastronomicin distanceSun -Earth:

12

s

s

s

d

ESUNd

E

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

Relative Solar Irradiance 1/ (ds*ds)

0.90

0.95

1.00

1.05

1.10

0 30 60 90 120 150 180 210 240 270 300 330 360

Solar Zenith Angle

LANDSAT: Use Zenith Angle ( 90deg – Sun Elevation) at the centre of the image for the whole image180km -> +- 0.81 deg If Zenith Angle < 20deg, difference < 1%

Or

Calculate the angle from Lat, Lon, Julian day and time ( i.e POSSOL of 6S ).

http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

Atmospheric Correction

The reflectance we have obtained here is Apparent Reflectance or Reflectance at TOA

Atmospheric correction is needed to obtain Reflectance at Ground Level

You may run 6S with appropriate parameters Some products are Reflectance at Ground Surface, such as

MODIS, SPOT VEGETAION…

References Landsat 7 Science Data Users Handbook

Chapter6• http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter6/chapter6.html

Chapter 11• http://landsathandbook.gsfc.nasa.gov/handbook/handbook_htmls/chapter11/chapter11.html

Dr. Kawamura's page DN->Refelctance for Landsat ( Japanese ) http://www.geocities.jp/kensuke_kawamura/Others/LandsatCa

libration/LandsatCalibration.html

6S Manual