So imagine my surprise to see a “Special Report” from FarmLogs called "Why Satellite Imagery is Better than Drone Imagery." In my 49 years of remote sensing and GIS work with NASA, Jet Propulsion Labs as well as designing vegetative health indexes for DigitalGlobe, EarthMap Solutions, Monsanto, and Trimble – I felt the call to respond to this assertion.
My first reaction of course was to understand the marketing aspect of “At FarmLogs, our technology is powered by satellite imagery”. So by design, their assessment may be biased to justify their business strategy. My second understanding was that this article is slanted toward large-area, continuous canopy type crops. I would like to note that orchards, vineyards, and some other heterogeneous crops need to be mapped using images that have sub-meter resolution pixels. So what was my overall assessment of this “Special Report”?
TO CHOOSE … OR NOT TO CHOOSE?
To begin, FarmLogs seems to be saying that a farmer must choose either to use satellite-imagery based maps or to use drone-based maps. But, a farmer can choose to use a combination of both of these sources of imagery plus manned-aircraft based maps and information. The mix depends on what purpose is being served - and on the weather of course. If a farmer has a field crop, then the lower resolution satellite imagery choice might be all that is needed. But, some applications need high-resolution imagery to map features that a farmer needs. Tree by tree and vine by vine assessments in orchards and vineyards need imagery that has a spatial resolution smaller than 0.7 meters … smaller than the best resolution (1.2 m) available from commercial satellite sources. Leaf-level assessments need imagery that that has resolutions better than 10 centimeters.
The article states that the satellite resolution is 5 meters. This seems to be a reference to RapidEye or to Planet Labs. As stated, the average revisit for RapidEye imagery is about five days. However, a drone can be used to collect imagery on any date of interest. For same situations, such as thermal assessments for irrigation, afternoon images are needed. All satellite-based image collections occur during morning hours. And, contrary to the article, drones do not need to have sunny, cloud-free conditions. Partly cloudy skies are a problem for any aerial image based mapping system; however, completely overcast skies are just found for drone-based imaging. But, completely overcast skies prevent satellite-based or manned aircraft systems from seeing the ground.
Response 1: Acquiring satellite imagery requires less of your time
The farmer is not the person who should be taking time to acquire and process drone-based images. This can (and should) be done by a service provider who has the training and skills and permission from the FAA to do so. The resulting maps that are provided to the farmer based on satellite imagery are just as easy to use as the maps that are provided to the farmer based on drone images.
Response 2: Satellite imagery doesn’t need to be stitched together
Again, this is done by the service provider. Many kinds of software exist that “stitch together” drone images. And, these software packages deliver more than just orthorectified mosaics. They deliver point clouds data files (a bit like LiDAR data) and an up-to-date Digital Surface Model (DSM) maps that can be useful to some farmers for mapping variations in the height of the crop. This cannot be done based on satellite images. If the field of interest has spatial objects in it … trees, vines, and row crop features ... that can be resolved by each high-resolution image, then the farmer does not have to lay out a grid in the field of interest.
Response 3: Satellites provide imagery that’s georeferenced
This implies that drone-based imagery in not accurately georeferenced. In fact, the orthorectified products from photogrammetry software are accurately georeferenced … certainly good enough for agricultural purposes (and without the need for Ground Control Point targets to be deployed or used). With ground control, accurates can be as good as the pixel sizes.
Response 4: You don’t need a special license or special training to use satellite imagery
But, again, if drone-based maps are being provided to a farmer by a service entity, the farmer is not the one who needs a special license or special training to use that imagery.
Response 5: You can use satellite imagery to quickly spot seasonal field patterns
Using satellite imagery is not that simple. Using maps made … by someone else … from satellite images is simple. But, the same is true for maps made … by someone else … from manned-aircraft or drone images.
Response 6: Satellite imagery is more cost effective
One should not compare the cost per acre for satellite-based images to the cost per acre for drone-based images. The information per acre is not the same for these two sources. Satellite imagery that has a spatial resolution better (smaller GSD) than 10 meters is not free imagery. And, most commercial satellite image providers require the buyer to buy a much larger area than the area of a given field of interest. Satellite data are not useful for very application.
SUMMARY – CHOOSE PURPOSE FIT
So, again, the farmer does not need to choose between maps made from satellite imagery and maps made from manned aerial or drone-based images. They each serve different purposes. Only some purposes are served by one or the other. If so, then satellite imagery is a better choice. But, a satellite image is not on demand as is a drone-based image. And, in many cases, a manned aircraft image is the best choice.
Overall, the most obvious payoff of imagery management would be the extra bushels of corn, soybean and wheat at harvest. For the farmer struggling to make it through depressed commodity prices, using a full view of the field to optimize inputs and take timely actions are hard to ignore. My recommendation to growers: use imagery as a tool during this growing season. Work with your agronomist and advisors to select the imagery you need – and when - this season to make application decisions that are above or below the line for return on investment.
Author’s Note: Drones must operate at altitudes at or below 400 feet (not 500 feet as stated in the article). And, the report does not state that some fields of interest are too close to airports or other restricted areas to allow drones to operate. The final FAA rules have not been issued yet … but small drones with small, yet powerful cameras may be less restricted than larger drones. So, small drones with the right cameras may be the best and economic choice for a given farmer.
by Jack F. Paris, PhD
President of Paris Geospacial, LLC
Imagery as a Farm Asset:
Satellite AND Drone AND Manned Aerial
Farmers will benefit from the right imagery at the right time with the right tools – from satellites, manned aerial, and drones.
" Imagery continues to evolve as an asset that helps growers to make informed
business decisions on their farms. And, now more than ever, imagery is a rapidly growing market space. This market growth is highlighted by the 2015 Precision Agriculture subsector investments where drone and satellite imagery investments accounted for 76% of the $661M invested."