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agricultural drone

Aerial imagery from a drone obviously provides you a birds eye view of your farm which is useful up to a point, but  when used in combination with photogrammetry and GIS software, even a simple drone can become a powerful farm management tool.


Modern drones are pretty much able to fly themselves, and can be set to follow predefined flight paths over a target area to capture hundreds of high resolution photos at a relatively low altitude. This provides high detailed imagery of a crop or target area, with each pixel representing between 2 -15 cm on the ground depending on how high the drone was flown, and the type of camera used. 



Photogrammetry software can take thousands of drone photos and stitch them together to make accurate 3D maps. Because the drone records the GPS position and altitude of each photo, the subsequent maps are also geo-rectified, meaning that they can be used as an accurate positioning platform for a variety of applications. Photogrammetry software is also able to calculate the height of the ground, and other features, in the imagery, which has a number of very useful applications including:


  • Contour mapping

  • Irrigation and drainage design

  • Tree and crop height calculation

  • Accurate field and area measurements 

  • 3D modeling and volumetric calculation


Contour Map generated from drone imagery using photogrammetry 


Crop Assessments with Drone and NDVI. 

Standard drone maps give you a basic visual overview of a crop, that in many cases can reveal problem areas that you might not have known about, especially when access is difficult, but these on their own are rather limited in terms of quantifying how a crop is actually performing. It is well known that a crop can be under stress long before it is visually noticeable. There are several different camera options available that are commonly used in agriculture to provide deeper insight into crop health, and are used to detect plant stress, even before it is visible. The most common and cost effective of these are the Near Infra-Red (NIR) cameras which are used to generate NDVI maps (Normalised Difference Vegetation Index), and other reflectance maps. 


In very basic terms, how it works is that plants that are not under any stress, absorb as much light as they can get, especially in the Near Infra-Red light spectrum, whereas a plant that is under stress, for whatever reason, will start to reflect light in the NIR band. NIR cameras capture this reflectance, and by using various algorithms in GIS software, an NDVI map is generated. Essentially we are mapping chlorophyll activity. 


Each pixel on the map has a numeric value, and different colours are used to represent a range of values, to make the maps visually understandable. A high NDVI value (on a healthy plant) will therefore often be represented as a dark green colour whereas a low NDVI value (dry, bare soil) is normally represented with a red colour. Different colours represent varying degrees of stress/chlorophyll activity, and this can be tweaked to highlight certain issues within a crop. 

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Standard Drone Imagery 

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NDVI Imagery of Avo Trees

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Tree Classifications based on average NDVI rating of each tree overlaid on drone imagery (Blue=Excellent, Greens=Good, Orange/Yellow=Stressed)

In my experience NDVI is very useful in detecting irrigation problems,drainage issues, and weeds. Which, with an experienced eye, can be diagnosed with a fair degree of accuracy from drone imagery alone. I am however reluctant to make any conclusions as to the cause of stress without proper ground truthing. NDVI maps do however provide a powerful tool in drawing attention to issues that may not have been ordinarily detected, and to quickly and accurately ascertain the extent of the problem and to track the progress and effectiveness of management responses.


NDVI maps should be used in conjunction with soil mapping and leaf analysis to gain a full perspective of what is happening above and below ground and how the two relate.  By comparing NDVI to soil maps, one is able to quickly work out whether a problem is related to soil, or some other cause such as a disease or pest infestation, and guide you to where to start scouting. NDVI can also be used to better inform where soil and leaf samples should be taken, and help map out the extent of different soil conditions within a field. This can also help guide variable rate application, and zone demarcation. 

How often to fly a field depends largely on the crop type and how fast one can react to any issue identified in the imagery. More information is better, but if it cannot be acted on, then it quickly becomes useless and managers can start to suffer from data overload, and simply stop looking at it. On tree crops such as Avos and Macs, we recommend 2-3 flights per year. 

  1. Early growing season, after significant rains, to identify problems early and give enough time to react. 

  2. Mid to late season, to assess; management responses to 1st survey,  pest and diseases and yield potential. 

  3. Mid winter, useful to identify irrigation problems. 

Tree Counts and classification

Data Management

Drone mapping generates big data, which is often difficult to open and slow to use. It is important to make the maps and data easy to use and accessible in the field if it is going to be effective. Over time we have tried to reduce the amount of data that a farmer sees and simplified our maps to be usable on smartphones and tablets as a GPS background. 


For tree crops we calculate a NDVI rating for each tree and classify the trees into 5 classes from Excellent to Bad. Each orchard or compartment is reported on with a table that shows the total tree count and the number of trees in each class together with a colour coded tree map. Number of trees/hectare, average size, diameter, and a summary of previous reports are also reported, so farmers can track if problems are getting better or worse.  

Maps are presented as digital A0 printable PDFs (which are usable on smartphones as well) and also as Google Earth overlays. Reports are presented in PowerPoint and PDF format that allow for easy sharing and presentation purposes with the entire management team. 

Crop Spraying Drones

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The XAG Crop Spraying Drone in action


Another exciting development has been the introduction of crop spraying drones. Although they are still fairly controversial, the technology does pose some remarkable opportunities to farmers in our difficult conditions. The potential benefits are fairly obvious, in that it frees the farmer from having to rely on crop spraying contractors, and could potentially lower the cost of aerial applications over small areas. It can also replace the knapsack sprayer in some circumstances, and is able to operate over very difficult terrain, safely and very accurately. 


Apart from the legal issues that surround their use, drones also still face a few practical problems in that their payload is still relatively small at 10-15 liters depending on the model, and the battery life is relatively short at around 10-15 minutes. Most of the chemicals we use on cane have a label rate of 30 L/ha which means that at least 2 flights per hectare are required. That said there are a number of farmers and contractors using drones for ripening, at 10 - 15 L/ha and they are achieving very good results. 


The good news is that some of the local chemical companies are starting to take notice, and field trials are starting to take place with drones here in South Africa. There has also recently been a shift in the regulations and there is now a legal route for those wanting to use drones for crop spraying. 


At the lower application rates (10L/ha), drones are able to spray 4 - 5 ha per hour, which make them very useful for small areas but not very practical for large flat fields where conventional aircraft can operate far more cost effectively. 


The 2 leading crop spraying drone manufacturers are DJI and XAG who are both represented here in KZN, with the XAG now being available with finance through Coastals. Pricing starts at around R210 000 for the base model with a 10 L capacity and goes up to R260 000 for the 16L model.


XAG are an exciting company that manufacture drones specifically for agriculture. To date they have more than 42 000 agricultural drones in operation in 38 countries, and are represented here in South Africa by Specialised Agricultural Services and HVL Drones. 


All XAG drones are fully autonomous making them very simple to fly, and come equipped with an RTK GPS base station and rover to ensure pin-point accuracy and safety. Quick release tanks with an automatic chemical filler, fast battery charging, and rotary atomisers are all standard, making the XAG one of the most efficient and accurate crop spraying drones available. 


XAG also produce a mapping drone, designed to work in conjunction with their crop spraying drones, which will lay way for precision spraying applications based on the maps it produces. This means you can fly and map your farm, identify problem areas and send the crop spraying drone to spray specific locations all from one device and one map. Farming is certainly getting smarter 

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