by Andreas Neuman

Published on April 20, 2016

Remote sensing is one of the considerable advances in farming of the past 20 years. Lately, the arrival of drones allows technology that was prohibitively expensive to be affordable for most farmers, and with a precision previously unachievable even by the most costly options. This new accuracy allows growers to rapidly analyze specific plants instead of only large areas, thus making it useful not only for big landowners but also to small producers. But once you have your imagery in hands, what are the next steps?

While remote sensing is a fantastic technology, doing field scouting, walking in the vineyards, is still needed and plays a critical role in modern precision agriculture. The key difference is now the field work is predominantly “ground truthing” – analyzing the cause(s) of issues instead of searching for them. Remote sensing allows people to see problems that are easily missed from the ground level, making field scouting faster and comprehensive. Though, it is important to “ground truth” information from the imagery to what’s happening on the field, either by visual inspection, targeted sampling or by correlating the imagery with data from other sensors.

When using a vigor map, it is important to understand that high vigor is not always optimal and stress is not always bad. Viticulturists and winemakers need for instance to do BRIX measurements in the zones that have previously been identified by the use of the Normalized Difference Vegetation Index (NDVI) or other vegetation indices such as the Normalized Difference Red Edge Index (NDRE) or the Soil-adjusted Vegetation Index (SAVI) imagery. A vine with more stress often has fruit with more desirable characteristics for the winemaker than healthier grapes do. For most growers, what is more, important than identifying individual stressed plants is that variance in the field is first identified and then the causes of the variance are understood. Growers must also determine at what scale variance becomes significant. For example, growers of grapes destined for premium wine may very well determine that using an NDVI scale with 10 or 20 classifications so that they can treat individual plants is the correct scale to operate with. However, growers of grapes destined for table wines may wish to use a simpler NDVI scale with four classifications based upon average NDVI values in one, two or three-meter zones.

As discussed above, when designing field surveys based upon NDVI imagery the goal is to ground truth the findings on the same scale that the NDVI was produced. Growers (and in many cases their consultants as well) who use NDVI imagery will design a ground truthing plan that efficiently ensures that a representative sample is taken for each classification so that they can accurately extrapolate that information and apply it throughout zones with similar NDVI readings. They also help to determine what is causing the variance in the field and a cultivation plan can be put in place to create a more homogenous crop.
The team of Viña Undurraga performing field scouting in a vineyard south of Santiago
So, in terms of the requirements for manual labor in the field, a premium wine grape grower will likely have more ground truthing to perform than a grower of table wine grapes simply because of the higher level of granularity with which they choose to evaluate variance. However, both growers should realize high ROI from their use of labor as the ground truthing has been optimized for the expected return on labor. Jeanne Lacombe, director of the Pape-Clément vineyard in the Bordeaux region in France, explains in an interview from 2014, how they used an NDVI imagery just before harvest, to delimitate different maturity zones. They detect the zone with less vigor and then send somebody on the field to check the maturity of the grapes. She explains that if the vines enter a period of dormancy, the maturity of the grapes stops and they need to be harvested as soon as possible to avoid a decrease in acidity. Without imagery or a proper guided ground check, this would have been close to impossible to detect and cultivate efficiently. The imagery shows where samples need to be taken.

While it is its backbone, precision agriculture goes far beyond remote sensing imagery. Viticulturists and other fruit growers use ground sensors (FDR sensors), weather stations, spectrometers, BRIX readings in addition to soil analysis, visual observations, among other current and historical datasets.

To avoid misconception, here is a short list of what is possible with remote sensing and NDVI imagery and what is not (yet):

  • Remote sensing imagery tells you at a fraction of the cost (both in terms of time and money) of doing field inspections where the problems and zones of interest are.
  • A vigor map and soil moisture data from ground sensors allows growers to detect a lack of water (usually identified as stress of an area of relatively lower vigor plants) or excessive irrigation, provoking excessive vegetation growth or on the opposite, rotten roots.
  • An NDVI map allows growers to map where individual plants or sections are stressed, but targeted manual inspections performed by workers in the field are still needed to determine the exact causes.
  • Mathematical models are being developed to diagnose specific risks such as exact disease, pest, virus, or bacteria and even to monitor the chemical composition of crops. Terroir matters and it may take a season or two of ground truthing NDVI imagery to perfect a model, so the sooner a grower starts incorporating modern precision agriculture tools the sooner they can benefit from algorithms tailored for their fields.
  • NDVI imagery ground-truthed by standard assessments (BRIX, acidity, polyphenols analysis, etc.) allows growers to clearly delineate sub-zones based upon variance in a vineyard for selective harvest.
  • The stress caused by nutrient deficiency will be detected in NDVI imagery and can be ground proofed by soil and/or plant sampling to perform variable-rate nutrition plan.
“now the field work is predominantly “ground truthing” – analyzing the cause(s) of issues instead of searching for them.”
What is the conclusion? Remote sensing can reduce the requirement for manual field inspections but is not yet ready to replace them. The good news is, the field inspections are much more efficient with the use of remote sensing – instead of searching for problems field workers now can start at the next step of analyzing known issues and take action in time to positively affect their yield.

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