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An important aspect of remote sensing research is the problem of comparing
data taken with various instruments having different characteristics. Some
questions that should be addressed within this context are: How do you compare
data obtained from radiometers that measure radiation in different wavelength
regions? How do you relate data taken with a wide field of view (15° for most
hand-held instruments) to data obtained with different fields of view, or to an
aircraft- or satellite- based scanner where the instantaneous field of view is
very small? What does an instrument see in terms of plants and soil back ground?
What are “vegetation indices,” and how are they used? The overriding question is:
How can we best take spectral data that are understandable and transferable to
other situations?
In 1979, the SEA/AR Wheat Yield Modeling Group contracted with NASA to
construct approximately 12 hand-held radiometers for delivery in 1980. These
radiometers, designed by Tucker et al. (1980) at the Goddard Space Flight Center
(GSFC), contain three bands that are similar to three bands of the Thematic
Mapper, the radiometer that is to be carried on LANDSAT-D (scheduled for launch
as LANDSAT-4 in 1981) (Tucker 1978a). This instrument, designated as the Mark II
3-band, was developed after Tucker had gained a considerable amount of
experimental experience with a two-band instrument described by Pearson et al.
(1976) (herein called the PMT 2-band). Another radiometer adaptable to hand-held
use has been available commercially for several years. This is the Exotech model
100A “LANDSAT Ground Truth” radiometer, whose bands, as the name implies,
correspond to bands 4 through 7 on the MSS carried by the currently orbiting
LANDSATs. All of these instruments measure portions of the electromagnetic
spectrum that are in the visible and the near infrared (IR) regions.
Our introduction to hand-held spectral radiometers occurred after a meeting
in January 1977 with Barrett Robinson and Marvin Bauer of the Laboratory for
Applications of Remote Sensing (LARS), Purdue University. They loaned us
equipment, and Barrett Robinson spent a day instructing us in the use of the
Exotech model 100A. We are starting our fourth year of measurements with the
Exotech, but due to weather conditions and a few other reasons, we have little
field experience with the Mark II 3-band.
Many of the same researchers who will be using these instruments to measure
reflected radiation have also ordered a newly developed hand-held IR
“thermometer” that measures emitted thermal radiation in the 8 to 14
µ
m (or 10.5
to 12.5
µ
m) wavelength regions, which can be related to surface temperatures.
This instrument, produced by the Telatemp Corporation, weighs about 1.1 kg and
has a pistol grip, which allows it to be held like a handgun. We have used the
Telatemp for 2 years; and for 4 years preceding that we used a Barnes PRT-5 IR
thermometer.
During these years, we have learned a bit about the use of hand-held
radiometers--much of it by trial and error--and as in most endeavors, hindsight
has been an excellent teacher. Thus, with the impending deliveries of hand-held
radiometers to our colleagues, we thought that a workshop, in which we discussed
much of what we know about the use and misuse of hand-held radiometers, would be
beneficial.
In preparing for the workshop, a set of notes developed. We share these
notes with a word of caution: We do not have all the answers, some things may
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