Know the Details Behind the Data
A coming-of-age technology is reaching the point where it’s a common tool for farmers. NDVI (Normalized Difference Vegetation Index) mapping provides an in-season snapshot of your crop—and knowing the level of detail provided by the map will determine how you use the tool.
NDVI maps are built using pixels, which are data points that represent a given width and length. The smaller the pixels are, the higher the resolution, and the more details you can see.
"When looking at a map with 30-meter resolution, you can see variation across the field and start to recognize management zones," says Isaac Ferrie, who works on the Farm Journal Test Plots team and has five years of experience helping farmers with their NDVI images. "If you compare a
30-meter resolution map to a 10-meter resolution map, the management zones are more defined as resolution increases. We’ve had 3-meter and 1-meter maps that show nitrogen deficiency from a plugged anhydrous knife and you can even see tile lines."
Layers of information.
NDVI maps can be gathered using equipment-mounted sensors, airplane imagery or satellite imagery.Meter and submeter resolution are available through some aerial imaging services and satellite imaging companies.
"Many commercial maps are krieged, which is a process that is used to simulate higher resolution by using a lower-resolution source image," Ferrie explains.
When a map is krieged, the values of the pixels of the source image are used to extrapolate data values and break up the image into smaller pixels.
"We’ve overlaid 30-meter NDVI maps that were krieged to reflect a higher resolution, and it was similar to a well-calibrated yield map," Ferrie says. "Krieging is an effective way to simulate a higher-resolution map, but it can exaggerate small areas with large variations. "
In addition to understanding resolution, knowing how you want to use the maps is also important, Ferrie adds.
"If you want the map to act like a yield monitor, a 15-meter or 30-meter map would suit your needs," he says. "But if you are seeking a map with higher detail than a yield map, to see smaller management zones or pinch rows behind the planter, a higher resolution would be needed."
Some companies are giving customers NDVI maps as an added service. Ferrie provides the following tips for getting the most from these maps:
- Know when the image was taken. In corn, the NDVI maps that most closely correlate with yield maps are taken right before tasselling. For in-season management changes, consider collecting the map data earlier.
- Ask for a true-color photo. In some cases, areas that look like a poor crop on the NDVI map may actually be clouds, shadows or in-field obstacles.
- Know the resolution. Ask your source for the maps what the image resolution is and if it’s been krieged.
- Ground truth. NDVI images show where the problem is in the field. The image can’t tell you what’s wrong.
- Late Spring 2011