NTech Greenseeker 505, Operating Manual

Page: 18 / 19

500-1-030 18 Rev I: 2006.05.04
APPENDIX C: Vegetation Indices Available in the GreenSeeker ® Sensor
There are several vegetation indices defined, evolving from more than thirty years of research in
remote sensing and aerial imaging. In precision agriculture applications, two of the most
commonly used indices are the Ratio and the Normalized Difference (NDVI), each comparing
the relative reflectance of plant material and soils at two wavelengths. Additional modifications of
the indices have been developed to compensate for various conditions of the plant canopy and
background soil.
Several indices are available from the NTech GreenSeeker
®
Sensors. The sensor always outputs
NDVI, plus an additional index (default is IRVI), which makes it possible to compare indices on
the same data at the same time. This can be changed by connecting the sensor directly to a
computer. Once selected, the output options are stored in the sensor, and remain in effect until
explicitly changed.
NDVI Normalized Difference Vegetation Index
SA-NDVI Soil Adjusted
WDR-NDVI Wide Dynamic Range
RVI Ratio
IRVI Inverse Ratio
Index Equations
The wavelength bands are in the visible and infra-red (NIR) regions of the spectrum. Sensors are
available in both Red and Green varieties, with bands centered at 530nm and 660nm, and the
NIR band centered at 770nm. Half-power bandwidths are approximately 25nm.
VIS NIR
VIS NIR ndvi
ρ ρ
ρ ρ
+
−
=
VIS
NIR
rvi
ρ
ρ
=
( )
( )
L ndvi sa
L
VIS NIR
VIS NIR
+ =
+ +
−
1
ρ ρ
ρ ρ
NIR
VIS irvi
ρ
ρ
=
VIS NIR
VIS NIR
a
a
ndvi wdr
ρ ρ
ρ ρ
+
−
=
The ‘a’ and ‘L’ values for the SA and WDR indices, respectively, are entered and stored in the Sensor.
SA-NDVI reverts to NDVI if L = 0, and WDR-NDVI reverts to NDVI if a = 1. A typical value for a
is 0.1; a typical value for L is 0.5.
References
University of Sheffield Remote Sensing: GEO6370 Vegetation Indices
http://www.shef.ac.uk/~bryant/6370/veg/vegsoil.htm
U.S. Water Conservation Laboratory: How a Vegetation Index Works
http://www.uswcl.ars.ag.gov/epd/remsen/Vi/VIworks.htm
Mark Servilla: The First Steps to Understanding Agriculture Remote Sensing
http://www.eomonline.com/modernagsite/archives/Servilla.html
USGS: Wide Dynamic Range VI application
http://www.gap.uidaho.edu/Bulletins/12/The Wide Dynamic Range Vegetation Index.htm
BGR: A Comparison of Slope-Based Vegetation Indices for Agricultural Applications
http://www.biogeorecon.com/vegindcs.htm