Are NDVI Readings Sensors Dependent?

Tremblay, N., Z. Wang, B. Ma, C. Bélec and P. VigneaultSt-Jean-sur-Richelieu, Quebec

Yara FieldScan (N-Sensor)

Yara FieldScan (N-Sensor)

TouchScreen

Card DriveSpreader Controller

Irradiance Sensor

Left Sensors Right Sensors

Cab Top Mounted

In Cab Section

PC

DGPS or GPS (optional)

S2S1

GreenSeeker

Justifications

• Variable N Rates– Based on vegetation indices (e.g. NDVI)– Algorithms under development

• Importance of univariate population statistics in the design of fertilization algorithms (Pena-Yewtukhiw et al., 2006)– Divide the population into classes, and then treat each class

with a particular rate of fertilizer N• Are NDVI readings (therefore, the algorithms) sensors

dependent?

Outfit For Devices Comparison v. 2007

Sensors Sampling Density on Summer Wheat 2005

Yara FieldScan GreenSeeker

Yara FieldScan Scanning Area as Compared to GreenSeeker Positions

Yara FieldScan NDVI and GreenSeeker NDVI in Wheat

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0N

DV

I_G

S

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0

ND

VI_

GS

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0

ND

VI_

GS

ZG

S23

(30

DA

S)

ZG

S30

(33

DA

S)

ZG

S41

(40

DA

S)

2005

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0

ND

VI_

GS

ZG

S33

(42

DA

S) 2006

0.77

0.41

0.86

0.47

0.90

0.46

Variability in Wheat Field 2006

Wheat

• CV of NDVI values decreased with growth stages and were lower with the Yara FieldScan

• Yara FieldScan saturated earlier• However, in 2006, for NDVI lower than 0.5:

– NDVI_Yara’s CV: 26 % – NDVI_GS’s CV: 31 %

• GreenSeeker showed no progression with growth stage

Yara FieldScan

• 50-100 m²/s• Scan at an

oblique view zenith angle (64° on average)

Oblique vs nadir views

• Plant Chl concentration better predicted from oblique-looking reflectance measurements (Demetriades-Shah and Court, 1987)– Soil reflectance is reduced

• Spot area of oblique angle (45°) greater by 50 % (Poss et al., 2006)

• Saturates at lower values than nadir directions (Myneni and Williams, 1994)

• Likely more sensitive to canopy conditions at early stages

Corn 2005

Yara FieldScan NDVI and GreenSeeker NDVI in Corn

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0N

DV

I_G

S

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0

ND

VI_

GS

0.0 0.2 0.4 0.6 0.8 1.0NDVI_Yara

0.0

0.2

0.4

0.6

0.8

1.0

ND

VI_

GS

Ear

ly V

5 (4

2 D

AS

) V

5 (4

8 D

AS

)

V4

(43

DA

S)

20062005

0.32

0.30

0.64

0.47

x1.6 x2.0

Yara Devices

• N-Sensor• FieldScan

– 550, 660, 700, 740, and 780 nm• N-Sensor ALS

Reusch et al., 2002

Vegetation Indices and Saturation

• GreenSeeker

• NDVI from Yara FieldScan tend to saturate because of geometry• Optimized vegetation index

– VIopt = 100*(lnR780-lnR740), Reusch (2005) – Larger CV than NDVI on wheat

• ZGS 23 (8 vs 6 %)• ZGS 30 (8 vs 3 %)• ZGS 41 (9 vs 1 %)

– Other version: VIopt = 100*(lnR760-lnR730), Jasper et al. (2006)

http://www.ntechindustries.com/lit/gs/DataSheet_GS_Sensor.pdf

Conclusions

• More similarities between sensors with corn than wheat• NDVI from Yara FieldScan saturated earlier

– Oblique view– Potentially more sensitivity at early stages; less at late stages– Let’s try GreenSeeker looking forward on small corn for more

sensitivity at early stages– Opportunities for more adapted indices from other bands

• Notably different NDVI population statistics depending on crop features and growth stages– Implications for algorithms development

Acknowledgements

• M.Y. Bouroubi• GAPS program (Agriculture and Agri-Food Canada)• Summer students• synAgri for aSEC data collection• Éric Thibault (Pleine Terre s.e.n.c.)• Growers Landry, Bieri and Martel

For more information: –Contact Nicolas Tremblay: tremblayna@agr.gc.ca –web: http://res2.agr.ca/stjean/personnel/tremblay_e.htm

Thank You !