Determining flight parameters
Before you start taking images, you need to determine and set certain parameters in your mission planner application (Hernandez-Lopez et al., 2013; Mangiameli et al., 2013; Mesas-Carrascosa et al., 2015). Figure 5.7 is an example of a mission planner interface (eMotion) which enables you plan your flights. It is absolutely possible to do all these in the comfort of your home/office before you head to the field. These parameters include: identification and specification of take-off and landing sites, flying height, direction and speed, number of flight lines, overlaps, etc. These parameters have an influence on the eventual images and the overall flight campaign. It is, therefore, important to appreciate what goes into each of these parameters and what you need to watch out for when selecting them.
Figure 5.7. eMotion mission planner for the test flight of a senseFly eBee UAV above experimental fields at Sokoine University of Agriculture, Tanzania (source: STARS AgriSense team).
- Flying height: you need to determine how high you would like your UAV to fly. However, this depends on the intended spatial resolution of the eventual images and the focal length of the camera in use. There is an inverse relationship between spatial resolution and flying height. The higher the flying height, the lower the spatial resolution. For the eBee Canon S110 NIR, for example, a flying height of about 145 m will result in images with a spatial resolution of 5 cm. On the other hand, a flying height of about 290 m will produce 10 cm spatial resolution images. If you require very detailed information that captures, for instance, crop canopy characteristics, then you’d have to set a low flying height in order to get very high spatial resolution images. Note, however, that the extent you can cover reduces with lower flying height. In other words, you can cover a larger extent with a flying height of 290 m than when you use 145 m (assuming the same camera). It is, therefore important to consider this critically before selecting. See more details here.
- Overlaps: In order to ensure the generation of a height model (e.g. digital surface or canopy model), images are acquired with an overlap. The overlap between two successive images on the same flight path is termed forward or longitudinal overlap, while the overlap between two images acquired on adjacent flight paths is known as lateral overlap. As the name suggests, a flight path/line is the path that the UAV follows in acquiring images. Longitudinal and lateral overlaps ensure that a common area is imaged on two successive or adjacent images. This permits the generation of stereo images, from which a height model can be generated. The amount of forward or lateral overlap specified will influence the number of flight lines as well as the number of images per flight line. Larger overlaps will lead to more flight lines and more images per flight line, while smaller overlaps will lead to fewer lines and images per line. Within the STARS project, overlaps of between 70 and 80% were used. These overlaps are recommended if accurate height models are intended to be generated.
- Take-off and landing: take-off and landing sites must be defined as waypoints either inside or outside the mission area. For large sites, it may be important to have the take-off and landing sites located in the middle. Ideally, these areas must be large fields devoid of obstacles such as trees, tree stumps, powerlines, large rocks or boulders, etc. In heavy agricultural areas, row crop fields such as groundnuts may be considered as take-off and landing sites. It is important to consider wind direction when selecting these sites, because the UAV needs to take-off and land against the wind. This enables it to fly more stable in low speed stages. See here for more details.
- Wind Speed: Wind speed and direction can be specified in your mission planning application (e.g. eMotion for eBee). Providing this information enables a better estimation of the time required to fly an area.