A new type of aerial photography that helps to monitor the condition of plants, prevent problems in the early stages and, as a result, increase yields, has been heard by many workers and leaders of the agricultural sector who follow the development of modern technologies. In this article, we will talk about the physical meaning, application and features of multispectral shooting using the professional drone DJI P4 Multispectral as an example and touch on points that have not been sufficiently covered in articles in Russian before.
Why are there 6 cameras on one gimbal?
Instead of the usual small camera of the Phantom 4 drone, we see a massive device with six lenses on the drone’s three-axis gimbal. Each of the cameras, united by a common shutter, has a matrix that is sensitive in a certain region of the electromagnetic spectrum, also called a channel.
The P4 multispectral camera takes pictures in the following channels (the wavelength in nanometers is indicated in parentheses):
- red edge (RE, 730 nm ± 16 nm): in this channel, located at the border of infrared radiation, the pigment chlorophyll is most noticeable, with the participation of which photosynthesis occurs;
- near infrared (NIR, 840 nm ± 26 nm); the channel is used for similar purposes and is located on the border of visible light and mid-infrared radiation;
- Green (G 560 nm ± 16 nm): used to visualize the vegetative activity of plants and their aging process, important in maturation and preparation for harvesting crops;
- Red (R 650 nm ± 16 nm): in the agricultural sector it is of paramount importance for the analysis of the quality and condition of the soil, it also allows visualizing man-made objects on a multispectral image;
- Blue (B 450 nm ± 16 nm): used to display water on images, as well as to study the bottom of reservoirs;
- Visible radiation: Take standard RGB photos.
Normalized Relative Vegetation Index (NDVI) and other indices
The normalized relative vegetation index (also in Russian it can be called the normalized relative vegetation index) is calculated from the results of shooting in the red and near infrared channels, it ranges from 0 (no vegetation) to 1 (maximum vegetation) and indicates the relative volume of green plant biomass, in real conditions cannot exceed 0.95. The value of the index indicates the state of health of plants and their development. The software used for the P4 Multispectral drone allows you to visualize NDVI on the map in various colors, including in real time. The specialist can easily find the problem areas of the field on the orthophoto map and provide information to his colleagues.
The DJI Terra software (1‑year basic license included with the purchase of the drone) that works with the P4 Multispectral also calculates and visualizes the following indexes:
- GNDVI (green normalized relative vegetation index) — similar to NDVI, but using a green channel instead of a red one. It characterizes the photosynthetic activity of plants and is used to search for plantations experiencing a deficiency or excess of moisture; plants of other species (weeds) located on the territory of agricultural land are also determined;
- NDRE (Normalized Red Edge Difference Index) — serves to determine the nitrogen concentration in the leaves by assessing the activity of photosynthesis. It is used mainly in the study of old and unsatisfactory crops;
- LCI (soil color index) — designed to assess soil fertility;
- OSAVI (vegetation index with optimized soil) — used to study the condition of sparse young green vegetation, taking into account the influence of the soil.
Georeferencing and Positioning Accuracy
Since the main goal of multispectral imaging is to detect a problem at an early stage and prevent its development, it is critical to know the exact location of the area in the field that requires increased attention in order to decide on further actions. For this, the Real Time Kinematic (RTK) on-board module and the D‑RTK 2 ground station are used, using the signals of all common satellite systems and storing data to work in conditions of interference and low signal. The TimeSync system synchronizes the work of positioning systems and cameras, as a result, providing an accuracy of snapping images on the ground up to 1 cm. In practice, this means that in the process of post-processing you will find not only problem areas, but also individual plants.
Research at every stage of crop growth
- Preparation for the sowing campaign: at this stage, maps of the land plot are compiled, soil fertility is assessed using the LCI index, and a decision is made to apply fertilizers to certain areas;
- Early and middle stages of growth: using the NDVI, GNDVI and OSAVI indices, the development of crops is analyzed, the quality of irrigation is assessed, plants affected by diseases and parasites are detected at early stages for their further removal, or spraying pesticides and liquid fertilizers, watering of selected soil areas is added or reduced;
- Harvest preparation: a competent analysis of the NDVI and NDRE indices allows you to start harvesting in a timely manner, preventing overripeness and premature aging of crops.
Working in tandem with DJI Agras MG-1P agricultural drone
The DJI Agras MG-1P octocopter has a 10-liter tank and adjustable nozzles for spraying pesticides, herbicides and liquid fertilizers with a capacity of 2.8–4 ha/h. The efficiency, accuracy and speed of aerial chemical work can be increased if they are carried out only where it is needed. The help, which can hardly be overestimated, is provided in this matter by the analysis of multispectral images and vegetative indices. In order to coordinate, plan and monitor the collaboration of two or more DJI drones, as well as evaluate and archive its results, the use of dedicated DJI Flighthub software is recommended.
Increased yield and profit
The introduction of unmanned technologies is a big step towards the emergence of agricultural enterprises and farms, where automated robots will take over most of the functionality of people. Despite the relatively high cost of the DJI P4 Multispectral drone with the D‑RTK 2 ground station, the investment will pay off in the first year of use due to the increase in quantitative and qualitative crop indicators.
The device was created on the basis of the popular shooting quadcopter, which is also intended for amateurs. This means that P4 Multispectral, thanks to its ease of operation and user-friendly interface, is available for mastering even by a user who does not have experience in operating multicopters.
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