SPECTRAL SIGNATURES. dr.ir. Jan Clevers. Centre for Geo-Information Dept. Environmental Sciences Wageningen UR

Transcription

1 SPECTRAL SIGNATURES dr.ir. Jan Clevers Centre for Geo-Information Dept. Environmental Sciences Wageningen UR

2 Subjects Spectral Signatures Spectral signature (reflection) Temporal signature Vegetation indices

3 Reflective Optical Window µm 1mm 1cm 1m 10m atmospheric transmittance blue green red UV VIS NIR MIR MIR TIR TIR blocking effect of atmosphere microwaves photography lidar human eye thermal scanners radar multispectral scanners microwave radiometry µm 1mm 1cm 1m 10m Wavelength optical reflective

4 Source of EM radiation 10 0 radiation (Wcm -2 µm -1 ) 10-1 curves for an average object on earth 10-2 reflected solar radiation emitted thermal radiation wavelength (µm) REFLECTION EMISSION

5 Reflection Reflection is the process of sending back EM radiation by an object Reflectance factor [0,1] -coefficient [0,1] -percentage [0%,100%] 1) ρ(λ) = Φ r (λ) / Φ i (λ) = reflected divided by incoming energy per wavelength λ or wavelength interval λ

6 Hand-held radiometer

7 2) ρ obj (λ) = L obj (λ)/l ref (λ) * ρ ref (λ) Reflection Bidirectional reflectance factor: the radiance of an object in a certain direction under certain conditions of irradiation, relative to the radiance of an ideal white diffuse surface in the same direction and the same irradiation conditions. Z nadir sun φ = 270º sensor φ = 180º θ X φ = 0º φ Y φ = 90º

8 Reference targets

9 Spectral band positions VISIBLE Meteosat TIRS Nimbus CZCS TIRS 1 2 NOAA AVHRR 3,4,5 (TIRS) Landsat MSS Landsat TM SPOT HRV-CCD XS-mode SPOT HRV-CCD P-mode DAEDALUS MSS 11, CAESAR-CCD MSS (land observation) reflectance (%) 60 dry soil wet soil blue green red water nearinfrared middleinfrared λ wavelength (µm) vegetation

10 Spectral Signatures: Soil Example influence of soil moisture content: 60 Newtona silt loam 0.8% Reflectance (%) % 8.3% 12.9% 16.9% 20.2% Wavelength (µm)

11 absorptance (chlorophyll) Spectral Signatures: Leaf % VIS NIR MIR transmittance absorptance (water) 20 reflectance Wavelength (µm)

12 Spectral information of a leaf Individual leaf VIS chlorophyll (absorptance) NIR cell walls - air transitions (reflectance) MIR water (absorptance)

13 Spectral Signatures: Leaf Plant type Reflectance (%) citrus tomato sorghum cotton Wavelength (µm)

14 Disease infection 100 Reflectance (%) (21) (24) Wavelength (µm)

15 Moisture content Reflectance (%) < 40% moisture content 40-54% moisture content 54-66% moisture content > 66% moisture content Wavelength (µm)

16 Spectral Signatures: Vegetation Reflectance (%) 50 barley 40 dry soil wet soil VIS veg. NIR veg. 1/2 VIS leaf > NIR leaf Wavelength (µm)

17 Temporal signatures: Barley Crop Reflectance (%) bare soil NIR red green days after sowing

18 Field trial with barley

19 Single-engine aircraft for 70-mm photography

20 Mounts and cameras for 70-mm photography

21 Microlight aircraft for 70-mm photography

22 Camera mounts for the microlight

23 50 Spectral bands for multispectral photography Reflectance (%) Wavelength (µm)

24 Green recording, 27 May 1982

25 Red recording, 27 May 1982

26 NIR recording, 27 May 1982

27 Reference targets

28 Vegetation Indices: simple band combinations in order to - strenghten the spectral contribution of vegetation - eliminate the disturbing influence of the atmosphere, soil, etc. Example: simple ratio: NIR/Red Normalized difference vegetation index : NDVI = (NIR-Red) / (NIR+Red)

29 NOAA-NDVI image of Africa March 1984 June 1984

30 Simple Reflectance Model (Clair) Starting point: calibrated reflectance factors Aim: the derivation of a suitable vegetation index or reflectance model for the estimation of biomass (or leaf area index, LAI) Main problem: influence of the soil reflectance (soil background) Main assumption: constant ratio of reflectance factors of bare soil NIR soil /Red soil = C Result: WDVI = NIR - C Red LAI = -1/α ln(1 - WDVI / WDVI ω )

31 Reflectance (%) Temporal signatures wet wet Barley crop: Red, NIR WDVI (%) dry 50 wet dry wet days after sowing Barley crop: WDVI 10 dry 0 dry days after sowing

32 Relationship WDVI - LAI LAI 8 6 LAI = -1/α ln(1 - WDVI / WDVI ω ) WDVI

33 LAI 8 Relationship NDVI - LAI 6 4 NDVI = (NIR - Red) / (NIR + Red) NDVI

34 LAI 8 Relationship NIR/Red - LAI NIR / Red

35 Example vegetation indices LAI R NIR NIR-R NIR/R NDVI wet dry colour NDVI = (NIR - R) / (NIR + R)