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Measure the Magnitude of the Effect Cirrus Clouds
Step 1
Step 2
Step 3
Step 4
7.6
Wind
Note:
7.7
Vegetation
ASD.Document 600540 Rev. C
Simply standardize the field spectrometer on the reference panel.
Continue to view the reference panel with the instrument.
If the atmospheric conditions are stable, the computed reflectance of the panel
will be a flat spectrum with near 100% reflectance.
If atmospheric conditions are unstable, the computed reflectance of the panel
will vary over time and will show absorption minima or maxima (depending
on whether atmospheric water vapor is increasing or decreasing) at the
wavelengths corresponding to the water vapor absorption features.
In this way, it can be determined whether spectral data with sufficient
accuracy can be acquired.
Wind can be a source of error if the material being measured moves during the
time the spectrum is acquired.
If a spectrum is slowly scanned, changes in the amount of shadow in the
instrument field-of-view will result in erroneous "features" in the spectrum.
Vegetation canopies, with their large portion of shadow, are especially
susceptible to wind induced errors.
Instruments using an array detector or that scan the spectrum rapidly are not
significantly affected by wind.
Because of the complex three dimensional geometry of a plant canopy, light
returned from the canopy is a complex mixture of multiply reflected and/or
transmitted components (Curtiss, 1990; Curtiss and Maecher, 1991; Curtiss
and Ustin, 1989).
The canopy level optical signal is dependant upon:
•
Illumination and viewing geometry.
•
Canopy structure.
•
Leaf optical properties.
•
The optical properties of other vegetative and non-vegetative components
within and below the canopy.
59
Chapter 7 Field Measurements
FieldSpec® 3 User Manual
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