Altitude and Flight Speed
When planning missions, enter the size of the plants into the SLANTRANGE Flight Calculator available in BTC Flight Plan and the SLANTRANGE DroneDeploy app to determine appropriate altitude and speed.
For example, if flying a corn field in a population stage with plants that are 5 inches x 2 inches the recommended flight plan looks like:
One fundamental, misunderstood element of remote sensing is resolution. Many have been trained by the consumer electronics industry to think that more megapixels = higher resolution = better data. Ground sample distance (GSD) should be the focus of the conversation in precision agriculture. GSD is defined as the linear dimension of one pixel projected onto the ground, measured in inches or centimeters. For precision measurements of vegetation, the GSD of the system must be smaller than the plants being measured.
The GSD achieved by your SLANTRANGE system is a function of a few factors that must be considered by the operator when planning a mission. The most dominant factor is the altitude at which the flight is conducted. The size of a pixel on the ground, the smallest “sample” in the raw imagery, has a linear relationship with flight altitude above ground. As altitude increases, the GSD increases. If the pixels are much larger than an individual plant, the plant will be smeared together with surrounding bare soil and neighboring plants, making it impossible to accurately identify, count, and measure the plants.
In corn population measurements, for example, plants need to be resolved that may be a few centimeters across. However, for a fully developed canopy, GSD up to 1 meter may be sufficient. Flying at 50 m altitude, SLANTRANGE systems have 2 cm GSD, or about 3/4". Flying at 25 m they have 1 cm GSD.
Rules of Thumb
Population maps are typically flown between 20 and 40 meters altitude above ground using 2p or 3p sensors, and between 20 and 30 meters using a SLANTRANGE 1p sensor. Fully developed canopies may be flown up to 120 m.
The Table is provided as a reference between GSD and flight altitude. For best results, enter the plant size in the Flight Calculator and follow the recommendations for altitude and speed.
Ground Sample Distance (GSD) as a function of altitude above ground
|1p – GSD (m)||0.019||0.023||0.026||0.034||0.045||0.068||0.090|
|2i, 2p, 3p – GSD (m)||0.010||0.012||0.014||0.018||0.024||0.036||0.048|
The speed of the aircraft over the ground can cause image blurring especially in low altitude, high resolution images. When an image is being captured, each pixel is exposed to a particular portion of the image. If the image is in focus and the image is still with respect to the sensor, the scene will be in sharp contrast. If the aircraft is moving quickly, and the scene moves a significant portion of a pixel or more while the image is being exposed, the content of the pixel will become a smear of that pixel’s information and the information from an adjacent pixel. When this happens, the image is blurry. For example, if you plan a flight altitude for 2 cm GSD, but the aircraft travels 5 cm during the exposure time of the image, you will introduce blur into your images, effectively causing your GSD to be much larger than desired.
Rules of Thumb
Population maps are typically flown at 8 to 12 m/s ground speed. Fully developed canopy may be flown up to 20 m/s ground speed.
The Figures illustrate the effects of motion blur on an image. The upper image shows sharp contrast where individual plants in rows can be seen. The lower image is the same scene with significant motion blur, where individual plants cannot be seen. Max ground speeds for given altitudes for each SLANTRANGE sensor are listed in sections Flight Metrics for 1p, 2p, and 3p.
High resolution image of early stage corn for crop counting with sharp contrast
Same image with motion blur.