UAV thermography to reduce mortality of roe deer fawns before harvest
Over recent decades, growing competition in the agricultural sector has encouraged the development of modern harvesting machines whose high efficiency goes hand in hand with increased speed of travel. Working speed of the machinery exceeds 4.1 m/s and the cutter bars reach 14 m or more in length. Although it is very difficult to assess the extent to which wildlife populations are affected by agricultural operations, we are facing an indisputably negative impact of harvesting machinery. For these reasons, the number of animals killed or injured during routine agricultural operations has increased dramatically.
This text documents possible ways to apply UAV equipped with thermal imaging cameras to be used for searching roe deer fawns in the harvest season when the young fawns are most vulnerable.
The experiment game was monitored by LWIR (long-wave infrared, i.e., wavelength of 7–14 m) Workswell WIRIS 2nd gen thermal imaging camera. The selected thermal camera had lens of focal length f = 13 mm (corresponding to FOV 45° × 37°) and micro bolometer resolution of 640 × 512 pixels. The camera was able to record surface temperatures in the range of −25 °C to +550 °C. Accuracy of the recorded temperature stated by the manufacturer is ±2 °C or 2% of the range (the worse of the data applies to the given measurement); thermal sensitivity of the camera is ≤50 mK at 30 °C. The total weight of the thermal camera is approximately 400 grams. The camera has 32 GB built-in memory and HDMI for video output. Video can be transmitted in real time to the ground station via wireless technology, which was used in our case. A correct UAV speed of flight over the monitored site was important. For our purposes, the ideal flight speed ranged from 4 to 6 m/s (4.6 m/s in average). A higher speed limits the user’s ability to scan the images. Due to the limited battery capacity, slower flights would significantly reduce the area that can be scanned. Imaging frequency of the UAV thermal cameras ranges from 9 Hz to 30 Hz. For our purposes, 9 Hz was sufficient.
A key feature of WIRIS is also the ability to connect with GPS technology and accurately store the GPS position of a localized object. Therefore, it is possible to monitor and mark the locations of the roe deer fawns by GPS coordinates under suitable conditions. The flight trajectory can be planned ahead using e.g., DJI Ground Station App (Fig. 3). The application allows estimating the expected flight time, which is very important in relation to limited battery life.
Five-litre plastic containers were randomly placed in the high-grown alfalfa field of acreage about 5 hectares as test items. The containers had an approximate size of 30 cm × 40 cm × 20 cm. The plastic containers were filled with water of 17 to 25 °C, which corresponds to the approximate temperature of a fawn’s surface. The test flights were performed in the early morning (5–7 a.m.) as well as the real search for roe deer fawns. The whole field was standardly explored by the pilot of UAV according to the flight plan (see Fig. 1).
The position of containers was not known to the pilot. For each test flight only one plastic container was placed in the field. The decisive criterion was the successful detection of the test items and the size of the scanned area.
The method of using thermal imaging cameras carried by UAV is expected to significantly reduce mortality of roe deer fawns. Field flights confirmed up to 100% success rate of roe deer fawn detection on the scanned field. As for practical utilization, the effectiveness could be influenced mostly by human factor (loss of attention, tiredness). Also, the fundamental conditions for successful detection were described, i.e., optimum flight altitude of ±40 metres above the terrain, early morning hours with sufficient temperature contrast, and appropriate resolution of the camera.
An undisputed advantage lies in the fact that the method can be applied and performed by gamekeepers before the fodder crops harvest. Therefore, it causes no interruption in the flow of agricultural operations, often precisely planned in relation to climatic conditions and ripeness of the crops.