ASD Document 600540 Rev. J
61
FieldSpec® 3 User Manual
www.asdi.com
Chapter 7 Field Measurements
In addition to the errors produced by time varying atmospheric water vapor,
partial cloud cover also greatly increases the intensity of diffuse skylight
illumination. This tends to “fill in” shadows and reduce the contrast between
surfaces with dissimilar surface textures.
Note:
If the goal is to collect field spectra for image calibration or interpretation,
spectra should be collected under illumination conditions similar to those at
the time the image was collected.
Measure the Magnitude of the Effect Cirrus Clouds
Step 1
Simply standardize the field spectrometer on the reference panel.
Step 2
Continue to view the reference panel with the instrument.
Step 3
If the atmospheric conditions are stable, the computed reflectance of the panel
will be a flat spectrum with near 100% reflectance.
Step 4
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.
7.6
Wind
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.
Note:
Instruments using an array detector or that scan the spectrum rapidly are not
significantly affected by wind.
7.7
Vegetation
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.
The canopy level optical signal is dependant upon:
•
Illumination and viewing geometry.
•
Canopy structure.
Summary of Contents for FieldSpec 3
Page 1: ...FieldSpec 3 User Manual ASD Document 600540 Rev J 2010 by ASD Inc www asdi com...
Page 7: ...ASD Document 600540 Rev J vii FieldSpec 3 User Manual www asdi com...
Page 10: ...ASD Document 600540 Rev J x FieldSpec 3 User Manual www asdi com Notes...
Page 50: ...ASD Document 600540 Rev J 40 FieldSpec 3 User Manual www asdi com Chapter 5 Batteries Notes...
Page 90: ...ASD Document 600540 Rev J 80 FieldSpec 3 User Manual www asdi com Appendix A Troubleshooting...
