METHODS of AERIAL PHOTOGRAPHY in vineyard mapping

 

Hi! And welcome to the third installment in our four-part series unpacking the science and methods behind application of remote sensing techniques enabling important analytics to vineyard management!

This week, we'll zoom out from the specifics of camera technology to a discussion of the methods of image capture used to generate analytical vine-vigor and disease maps.

Aerial imaging has been applied to agriculture since at least the early 1960s, when spy-planes were used to estimate crop production behind the Iron Curtain during the Cold War.

The areas of crop research, camera technology, imaging techniques, global positioning technology, and aeronautics have all undergone a quantum leap in the past five decades - bringing a whole new set of tools to the agricultural field. 

The most effective use of cameras to produce imagery of cultivated fields is to mount them to aircraft. Images captured from directly overhead at altitude (known as nadir photography) provide a real-time, map-like view of fields.
Images captured by this method also offer a much greater level of detail than satellite imagery - and their clarity suffers much less from distortion due to atmospheric effects and cloud cover.

With the advent of using airplanes to capture images of fields in crops, came the discipline of photogrammetry - the science of joining adjacent and overlapping photographs together to render mosaics, enabling the generation of expansive maps.

In the mosaicing process, images taken at different locations can be stitched together to form a larger image by matching features appearing in different frames.

In the mosaicing process, images taken at different locations can be stitched together to form a larger image by matching features appearing in different frames.

If you've ever taken a group picture, you are familiar with backing away from your subjects in order to increase the field of view, or FOV, allowing you to capture them all in the image. You've also noticed the further away you snap your photo, the less detail of your subjects your photograph captures.

The same concept applies to aerial photography; the higher up the camera, the more ground you capture, and the less detailed the photo. Thus, an airplane flying higher will capture a field or vineyard faster and in fewer pictures, but less details of the plants can be discerned.

Airplanes capture larger areas, faster, and with lower detail. Drones capture smaller areas, at greater detail, and operate in a slow and controlled fashion.

Airplanes capture larger areas, faster, and with lower detail. Drones capture smaller areas, at greater detail, and operate in a slow and controlled fashion.

As such, airplanes are a good choice for capturing very large areas and generating a coarse, but expansive dataset.

Inversely, remotely piloted drones, AKA unmanned aerial vehicles, which typically fly at 1/10 or lower of the altitude of manned aircraft, are a better choice for capturing smaller areas in crisp, high-resolution data. Drone-acquired imagery captured at an altitude of 350 feet can have over ten times the detail and resolution of that captured from manned aircraft flying at 3500 feet, even when the latter is using incredible, advanced 100-megapixel cameras. 

Although the quality of an aerial map can be judged on many parameters, the benchmark is ground-sampling distance, or GSD. This metric refers to the linear size of the area on the ground that is represented by one pixel - remember from last week's post that a pixel is the elemental component of a digital image - a square containing only one color value.
Thus, an image with a lower GSD features higher resolution, and vice-versa.

By way of example, if the GSD is three feet, that means all colors captured from a 9 square-foot area on the ground are represented by one pixel - a single square of one color, averaged from all the colors captured in that area.
If the GSD is six inches, then that same 9-square foot area is represented by 36 pixels - thus capturing much more detailed color information - and thus a much richer data input for analysis.

Two images of the same area in a vineyard. The image on top is was taken at low altitude using a 24-megapixel camera. It features low ground-sampling distance. The lower image was taken at ten times the altitude using a state of the art 100-megapixel camera, and features high ground-sampling distance.. For analysis of relatively small areas, the image at the top yields much more accurate information, for obvious reasons. 

Two images of the same area in a vineyard. The image on top is was taken at low altitude using a 24-megapixel camera. It features low ground-sampling distance. The lower image was taken at ten times the altitude using a state of the art 100-megapixel camera, and features high ground-sampling distance.. For analysis of relatively small areas, the image at the top yields much more accurate information, for obvious reasons. 

Imagery from manned flights are usually delivered on a set schedule - resulting from the aircraft performing regular imaging flyovers on a schedule that suits the imaging organization.
Mapping using drone-acquired imagery, on the other hand, can be arranged at any time suiting the end customer's needs - on very short notice, with a rapid turnaround time. 

Most drones performing precision agricultural imaging are of the rotorcraft variety - featuring four, six, or eight propellers providing lift - think of a miniature, multi-propeller helicopter.

A rotorcraft, or multi-copter type drone, in controlled flight.

A rotorcraft, or multi-copter type drone, in controlled flight.

This form factor provides extreme agility, slow, controlled flight (usually around 10 miles an hour) and the ability to hover. Thanks to extremely precise global positioning electronics, drones can create maps from images captured at exactly the same locations in your vineyard week after week, month after month, and year after year. This results in maps that are very consistent over time, recording spectral data from plant foliage under nearly identical conditions.
Manned aircraft, on the other hand, fly very high overhead at speeds usually in excess of a hundred miles an hour, making repeating image capture at precise locations challenging, and introducing considerable uncertainty related to speed, distance from the subject, and angle of spectral data reflected from plant foliage. 

A drone captures images within a very tight coordinate range in a repeatable fashion.

A drone captures images within a very tight coordinate range in a repeatable fashion.

Due to the high speed, high altitude, and the effect of crosswinds, an airplane has much less control over the exact location in which an image is captured.

Due to the high speed, high altitude, and the effect of crosswinds, an airplane has much less control over the exact location in which an image is captured.

When the goal is to create a very detailed map, the best approach is low, slow, and precise - best delivered by a remotely piloted drone fitted with a finely-tuned multispectral camera.

Stay tuned for next week's installment, where we bring together elements of all the posts to date for an unpacking of the unique and cutting-edge science and research that enables the delivery of our vigor and disease maps, which are the only maps on the market delivering reliable, actionable information.