From Sky to Earth . . . Researchers
Capture "Ground Truth"
To obtain views of the soil surface from various angles, soil scientist Susan
Moran uses a specially mounted radiometer that turns in many directions. The
data she obtains will be used to correlate soil texture with crop vigor.
Farmers will soon be able to get a bird's-eye view of the "back
forty" with a single click of their computer mouse.
That's the goal of Agricultural Research
Service scientists and private industry cooperators working together under
one of the largest cooperative research and development agreements (CRADAs) in
the history of the USDA research agency.
The project aims to provide farmers with satellite-based information on the
health of their crops so they can apply spot-specific remedies and improve
longer term management practices.
RESOURCE21, LLC, of Englewood, ColoradoARS' CRADA partnerplans
to launch up to four satellites devoted to remote sensing for farmers. [See
also "Orbiting Eye Will See Where Crops Need Help," Agricultural
Research, April 1996, p. 12.]
"Our job is to help develop and refine the software that interprets the
satellite data," says James S. Schepers, the ARS CRADA coordinator for the
project. Schepers leads the research team at the ARS Soil and Water
Conservation Research Unit in Lincoln, Nebraska.
Four private companies and six ARS laboratories are participating. Companies
include The Boeing Company, an aircraft maker in Seattle, Washington; Farmland
Industries, Inc., a national agriculture cooperative based in Kansas City,
Missouri; Marconi Integrated Systems, Inc., a remote sensing firm from San
Diego, California; and the Institute for Technology Development, Inc., a
nonprofit company in Ridgeland, Mississippi.
ARS received more than $900,000 for research at laboratories in Lincoln;
Shafter, California; Phoenix, Arizona; Ames, Iowa; Beltsville, Maryland; and
Once the technology is in place, Farmland Industries wants to deliver it to
600,000 farmer-members. The farmer-owned cooperative has 1,500 local co-op
associations in 25 states, and each association has at least one farm supply
store. Trained experts at these outlets would use the satellite-based system to
further help farmers.
ARS physical scientist Mike Schlemmer (left) checks a soil sample while ARS
soil scientist Dennis Francis withdraws another. In the center, University of
Nebraska graduate student Shannon Osborne operates a GPS device that allows
them to relate aerial images to the sampling sites.
"We want technology that makes money for farmers," says Gary W.
Colliver, director of agronomy services at Farmland. "We're looking at the
whole package. Remote sensing complements other information, such as soil data
collected by farm consultants."
Colliver adds that there are other uses for remote sensing besides crop
monitoringfor example, evaluating on a large scale cataclysmic events
such as hailstorms or damage from plant diseases or insect pests.
To Check Data Accuracy
To get the project under way, a small RESOURCE21 plane toting company-owned
sensorssimilar to those that will be mounted on satellitessped over
ARS research plots in Arizona, California, Iowa, Nebraska, and Texas during the
past two growing seasons.
The sensors are digital cameras that view crops or soil in several bands of
reflected lightboth visible and near-infrared. The cameras record energy
as digital numbers representing the amount of light hitting the sensor. ARS
provided data to help RESOURCE21 convert the digital numbers to numbers that
represent surface properties like reflectance. The company can use these
reflectance numbers to create maps for farmers that represent crop and soil
conditions. ARS also helped ensure the accuracy of the computer programs that
produce the maps.
To represent field conditions, ARS researchers took detailed, systematic
measurements of crop growth and development. Called "ground truth,"
these measurementscaptured by more than a dozen different kinds of
scientific instrumentsdetermine how well the imagery in digital format
correlates with scientists' on-foot field measurements.
Thanks to day after day of clear, sunny skies, research fields of California
cotton were the most intensively scrutinized of any in the 1997 field tests.
One intent of the in-air and on-ground observation in California was to reveal
how quickly the imagery could detect and track cotton plants when they emerged
from the soil.
To Sound an Early Warning
That information is critical, says ARS plant physiologist Stephan J. Maas,
because the imagery could alert farmers to problems in time for them to take
action. Maas and colleague William R. DeTar conducted the tests at ARS' Western
Integrated Cropping Systems Research Unit in Shafter.
"Our results," says Maas, "indicate that the imagery is
sufficiently accurate to perceive whether the crop is coming up well enough for
the grower to let it continue for the rest of the seasonor whether it is
coming up so poorly the grower needs to replant while there's still time."
"Later in the season," Maas adds, "the imagery can tell you
if gaps are appearing in the plant canopy. Because the imagery is keyed to
global positioning satellites, you can get the exact coordinates of the trouble
spot in the field. You can find out if there is something wrong with your
irrigation system, or if insects are attacking the crop, or if there's some
other type of problem."
ARS physical scientist Charles Walthall (right) and William Bernard, director
of remote sensing for 3DI, LLC, a geographic technologies company in Easton,
Maryland, adjust an airborne hyperspectral sensor used in imaging crop and soil
One of the tougher tests for the computers analyzing the image data was to
correctly differentiate dark soilwetted by a weekly furrow
irrigationfrom dark-green, healthy leaves of the canopy. For this task,
the mathematical models that tell the computer how to interpret the imagery may
need to be fine-tuned. Other conditions also affect the ability of the imagery
to capture an accurate picture.
"Every time the plane flies, the position of the sun and amount of
atmospheric haze and visibility are different," says Susan Moran, an ARS
soil scientist at the U.S. Water Conservation Laboratory in Phoenix. "This
renders each image slightly different from the previous one, whether or not the
soils or crops have changed. Users need a way to compare only those variables
that relate to crop health."
So Moran and ARS colleagues Paul J. Pinter, Jr., Edward M. Barnes, and
Thomas R. Clarke developed tools that compensate for these extraneous factors.
One was a calibration procedure that changes the digital numbers to reflectance
values that represent surface conditions more accurately and consistently from
one image to the next.
First they calibrated the camera output against an object of known
reflectancea surprisingly tricky task. Moran placed commercially produced
canvas tarps at the field sites. The tarps are chemically coated to produce a
Then Moran's team developed equations that convert the digital numbers to
numbers that represent the reflectance of a given image. Normal manufacturing
processes and exposure to harsh field conditions cause tarp variability that
required Moran to produce a unique equation for each tarp. She also had to
teach the users how to place the tarps on the ground and how to clean and store
them to ensure accurate readings.
"If a tarp is dirty, its reflectance can change by up to 70
percent," Moran says.
The calibrations compensated for atmospheric conditions, but images still
couldn't be comparedbecause the viewing angle could differ for each
flight, skewing the results.
"If you look straight down on a crop, getting what's called a nadir
view, you might see that it has 50 percent plant cover," says Moran.
"But if you look at the same crop from an oblique angle, it could
incorrectly appear that there is almost 100 percent plant cover."
To solve this problem, former ARS physical scientist Jiaguo Qi developed a
simple-to-use computer program that converts the reflectance from any viewing
angle to the standard nadir view. RESOURCE21 has already started using both the
tarps and the model.
To Account for Variation
Researchers at the ARS labs in Ames and Beltsville are also working to
compensate for environmental variables.
For example, scientists led by ARS plant physiologist Jerry L. Hatfield at
the National Soil Tilth Laboratory in Ames are designing statistical techniques
to interpret what's known as temporal variationthe patterns of change
seen in aircraft and satellite images over time.
Sara Loechel, a remote sensing researcher at the University of Maryland, labels
fields, woodlands, and streams on a computer, while ARS agronomist Craig
Daughtry locates the same features on an aerial photograph of the Beltsville
[Maryland] Agricultural Research Center. The computer image will form the base
layer of a geographic information system map. Other data about soils, crops,
and management practices will be added, along with remotely sensed images of
crops to update their growth and development.
By viewing the same fields as those scanned by the remote sensors,
scientists are able to determine the patterns of the soil color and topography
and crop growth. The researchers then analyze the patterns for clues about soil
conditions and crop growth over the growing season.
This will allow farmers to pinpoint specific problem sites in a field and
apply nutrients and pesticides only where needed.
At the ARS Remote Sensing and Modeling Research Laboratory in Beltsville,
research agronomist Craig S. Daughtry and physical scientist Charles L.
Walthall are examining digitized images for spatial variabilitythe
differences in height, plant growth, and appearance from one part of the field
to another. They're also analyzing spectral properties, or the differences in
color within a field and why one area may look greener than another.
But instead of direct experimentation, the scientists are simulating crop
and soil reflectances using computer models. To test their models, they obtain
data from 3DI, LLC, a geographic technologies company in Easton, Maryland, that
uses airborne hyperspectral sensors to scan target areas.
"We can look at more variables and situations in a simulation than we
could experimentally," says Daughtry. "Our modeling efforts should
tell us what kind of differences the sensors can detect, such as how small a
change in leaf area or color could be discerned. This information will help us
determine the best light wavelength bands to use to interpret soil and crop
conditions over the growing season," he says.
To Detect Yield-Limiting Factors
ARS scientists in Lubbock and Lincoln are testing remote sensing's ability
to detect conditions such as water stress and nitrogen deficiency that can
reduce crop yields.
In 1998, the second year of trials in Lubbock, a sensor-equipped plane flew
over cotton- and cornfields every day to view the effects of the worst
April-through-July drought in Texas High Plains history. The project's
scientists were able to test remote sensing of nitrogen deficiencies in crops
under extremely dry, as well as fully irrigated, conditions.
"The drought and high air temperatures were so bad," says Dan R.
Upchurch, "that when we cut back on irrigation by only a third, we grew 80
percent less corn."
"But it was a good year for remote sensing trials because we nearly
always had clear skies for aerial viewing of plants under extreme drought
conditions." Upchurch leads research at the ARS Cropping Systems Research
Laboratory in Lubbock.
The scientists compared two levels of watering in both corn and cotton to
see how reduced watering affected crops during a severe drought. For each water
level, they tested five levels of nitrogen fertilizer application.
To verify the aerial readings, Upchurch and agricultural engineer Donald F.
Wanjura took ground measurements such as leaf water potential, a measure of how
tightly water is held in leaf tissue. They also collected data from a field
weather station and a set of infrared thermometers that measure leaf
temperature. They took light reflectance measurements of the fields with a
boom-mounted camera perched above the canopy.
Soil scientist Dennis Francis compares soils collected at an ARS Management
Systems Evaluation Area (MSEA) in Shelton, Nebraska. The project looks at water
and nitrogen stress in corn and at water stress in soybeans.
The purpose of the experiment was to see whether cameras can detect plant
nitrogen deficiency before visible signs appear, under both dry and wet
Lincoln researchers are working to develop signaturessort of like
fingerprints made of the different light wavelengthsto indicate nitrogen
deficiency or water stress.
"For each area, a different stress may predominate," says
Lincoln's Schepers. "In Texas, that's water. In Nebraska, nitrogen is of
more concern," he says.
A Beneficial Relationship
Richard Baumeister, director of product development for RESOURCE21, values
ARS' nationwide network of laboratories, which allows widespread geographic
testingplus ARS expertise in research and validating remote-sensing data
on the ground.
"ARS scientists know how to set up nitrogen and drought-stress
experiments, while we provide the aerial imagery that is helpful to those
experiments," Baumeister says.
"This is a mutually beneficial relationship," he continues.
"We want to enhance the management practices of farmers so they make the
best possible yields. Sure, we want to make money, but we can't do that if the
farmers don't, too."
David G. Mohr, RESOURCE21's director of new business development, adds,
"We rely on ARS to make our research reliable and credible. ARS helps us
follow the proper research protocol to test our products, making sure our
results are valid and applicable to the entire country."By
Kathryn Barry Stelljes,
Don Comis, and
Marcia Wood, Agricultural
Research Service Information Staff. Dawn Lyons-Johnson, formerly with ARS,
contributed to this article.
This research is part of Integrated Farming Systems, an ARS National Program
described on the World Wide Web at
For more information on this project, contact
James S. Schepers, USDA-ARS
Soil and Water
Conservation Research Unit, 119 Keim Hall, University of Nebraska, Lincoln,
NE 68583-0915; phone (402) 472-1513, fax (402) 472-0516.
From Sky to Earth. . . Researchers Capture "Ground Truth"
was published in the March 1999
issue of Agricultural Research magazine.