In 2010, the Farm Journal Test Plots crew mapped 1,500 acres of corn with three ground-based Normalized Difference Vegetation Index (NDVI) sensors. Of that, 600 acres were also mapped with aerial and satellite images.
One of the goals of the Farm Journal Test Plots is to stay up-to-date with emerging technologies that can help you farm more efficiently and take yields higher. In the past five years, one of the biggest technologies to come on the scene has been NDVI (Normalized Difference Vegetation Index) mapping.
NDVI maps are a tool for in-season scouting and year-to-year field management. The data collected can help you refine management zones and production practices.
Acres mapped. In 2010, the Farm Journal Test Plots crew went to the field with all of the commercially available technologies to create NDVI maps. There are multiple ways to collect data for the maps: equipment-mounted sensors, images taken from airplanes and satellite imagery.
For the ground-based sensors, the crew used OptRx available from Ag Leader Technology, CropSpec from Topcon and GreenSeeker from Trimble. The aerial images were gathered by GeoVantage, and the satellite imagery was provided by Satshot.
The crew gathered NDVI images with all of the sources across 600 acres of corn. Using the ground-based sensors, an additional 900 acres of corn was NDVI mapped.
|With one set of sensors externally mounted on the tractor cab and two more across the sidedress toolbar, the test plots crew used three displays to map the real-time data.
Installation of the sensors required a simple setup: a head unit, module and monitor to operate in the cab. The OptRx and GreenSeeker sensors were installed on a sidedress toolbar, a CropSpec sensor was mounted on both sides of the tractor cab and the fields were mapped when sidedressing nitrogen. Although the sensors can apply algorithms to determine nitrogen rates, the crew used the sensors solely for mapping.
"The advantage of the ground-based sensors is that the farmer controls when the map is created," says Isaac Ferrie, who works on the test plots. "These sensors provide instant data and require little or no calibration."
Ferrie notes that there is a learning curve when operating the sensors and that farmers are responsible for processing their own maps. The resolution of the map varies by system; all of the sensors we used were set to create a map based on the 40' swath of the sidedress toolbar.
"With the GreenSeeker and OptRx sensors, we were able to gather maps that were very comparable in results," says Brad Beutke, who also works on the test plots. "With the CropSpec mounted on the cab, it takes a much wider angle reading of the crop, and we had some interference with dust. After refining those maps, taking into account the direction of the wind, we were able to achieve maps that mimicked those of the other sensors."
The ground-based sensors generate a NDVI map with each sprayer or sidedress pass. The aerial imagery maps provided by GeoVantage are captured by using an airplane outfitted with cameras.
"To schedule the flights, the company needs a significant amount of acres in the area
to send a plane," Ferrie says. "But this method of mapping doesn’t require the farmer to buy, install and calibrate equipment. The maps are uploaded to a password-protected website for download."
The satellite images from Satshot are collected every time a satellite orbit crosses your fields. This allows maps to be gathered multiple times throughout the season, but the data can only be collected during daylight hours and some shots can be disturbed by cloud cover.
"Satshot can access historical data from your fields as well," Beutke says. "Not only can you get multiple maps for the current season but multiple years, when available."
Dig into details. The test plots crew has learned that the real value of NDVI maps is their detail compared with yield monitor maps. NDVI maps give a snapshot of the crop when management changes can be made. When both maps are collected, the NDVI map can be an in-season progress report and the yield map the report card.
"For example, if the NDVI map shows nitrogen loss early enough in the season, there could still be time to rescue that loss," Ferrie says.
The crew identifies the following three areas for measuring the usefulness of NDVI maps: resolution, timeliness and cost.
The resolution of the map determines the details in the data.
"It’s important to understand resolution, which is the length and width of each measured point or pixel," Ferrie says. "How you plan to use the map will determine the resolution you need to seek out."
He explains that with 30-meter resolution, you see variation across the field and start to see management zones. At 10-meter resolution, the management zones are more defined. At 3-meter to 1-meter resolution, you can see streaks in nitrogen application and identify compaction in the pinch rows and water pockets. Submeter resolution is available at a greater cost.
"Most commonly, the satellite images for agricultural use provide 30-meter to 10-meter resolution but can be ‘krieged,’ which simulates a higher resolution," Ferrie says. "There are higher resolutions available from the satellite services at varying costs."
The GreenSeeker has multiple sensors and the NDVI value it computes is the average of the entire swath width. It has the ability to record individual sensors with dedicated software. The OptRx and CropSpec sensors in this project also used average NDVI values across the working width, but they can report data based on individual sensors.
Timing mapping efforts is key. For example, the test plots crew ordered the aerial maps on June 7 and drove the fields that day with ground-based sensors. The aerial map was flown on June 17, and Satshot captured a usable map on June 23.
Another factor in timeliness is measuring plant health when it most reflects yield potential.
"Some of the best maps that correlate to yield maps are shot right before tassel," Ferrie says. "With this type of technology, you can use high-clearance equipment with the ground-based sensors or use the aerial and satellite images, but it may be too late to rescue lost yield potential."
A notable difference between the ground-based sensors and aerial mapping services is the light source they depend on. All of the ground-based sensors we used are active light sensors and provide their own light source for gathering data, whereas the aerial images use passive light from the sun to get the NDVI readings.
"Two problems you have with aerial imagery is that clouds can block a clear image of the field and can interfere with the sunlight and alter the NDVI reading." Ferrie says. "One tip we give to farmers who are having aerial or satellite images created is to have the company provide a color photo with the map."
He says that some areas that looked like a poor crop have proven to be shadows, possibly caused by clouds, buildings or wind turbines.
Technology tool. From this demonstration, the test plots crew learned that NDVI mapping can serve as a farm management tool to maximize yields.
"The technology is getting better and more affordable," Beutke says. "To get the greatest value, understand the information you’re collecting, know its uses and limitations and put that information into practice. The maps provide an understanding of the variation and when it appeared, but you have to ground-truth its source."
Thank You to Our Test Plot Partners
Each Farm Journal Test Plot is a cooperative effort. Thanks go to: Ag Leader Technology, Chad Fick and Dave King; Topcon and Carol Snyder; Altorfer Ag, Derek Strunk and Travis Cornwell; Trimble, Russ Leinhart and Wayne Wenzel; GeoVantage and Nick Morrow; Satshot and Nathan Faleide; Rod Wilson; Crop-Tech Consulting, Brad Beutke and Isaac Ferrie.
- Early Spring 2011