An image acquisition plan is a process of mapping your drone flight path depending on two factors:
1) the aim of your surveying project
2) the type of terrain
To get high-quality results and facilitate map stitching from individual photos, a high overlap between images is a must. So, a good image acquisition plan is one that allows for enough overlap. A bad image acquisition plan, on the other hand, will lead to inaccurate results or a processing failure when stitching imagery, which will ultimately require repeating the survey to capture the images again.
In this section we’ll cover a few different types of image acquisition plans and have a go at identifying the ones best suited for our project.
Image acquisition plan types
General case image acquisition plan: suitable for projects that don't include difficult-to-reconstruct terrains, like forests, fields, snow, and lakes. The flying path in this plan forms a regular grid pattern and the desired image overlap for this plan is at least75% frontal overlap and at least 60%overlap between flying tracks, known as side overlap. The drown should be flown over the terrain at a consistent height.
Dense vegetation image acquisition plan: works for forest and bushland areas. When capturing this terrain it can be tricky to ensure reliable overlap, because tree foliage may have different appearances in overlapping pictures (leaves and branches are not static and have complex geometry). In order to achieve good results, it is recommended to use a grid image acquisition plan, like the general case described above, with an increase in image frontal and side overlap (to 85%) at a higher altitude to minimise perspective distortion.
Flat terrain image acquisition plan: requires the same adjustments as the dense vegetation plan (i.e., increase in overlap and altitude). This is because flat terrain, such as agricultural fields, offers fairly homogeneous visual content, making it difficult to extract common characteristic points between the images needed for stitching.
Special case image acquisition plan: may be used for a wide variety of terrains, such as snow, sand, and water surfaces. Like flat terrain, sand, snow, and water are challenging for imagery reconstruction and stitching because they are so uniform in appearance. High overlap approach works for sand and snow, while for water surfaces, it is important to ensure that each image includes land features as visual anchors. Flying higher may help to include more land features when capturing water surfaces.
Corridor mapping image acquisition plan: used when you're interested in capturing linear areas, like railways, roads, and paths. To ensure high accuracy of the imagery, at least two flight lines over the designated area are required, with relatively high overlap between images: 85% frontal overlap and at least 60% side overlap.
Multiple flights image acquisition plan: when done properly, provides the most accurate outcomes, but is also the most time intensive, as it implies two or more surveys conducted in the same location. When designing multiple flights image acquisition plans, make sure that each plan captures the images with enough overlap and that there is enough overlap between separate image acquisition plans. It is also important to carry out different flights under the same conditions (e.g., sun direction, weather conditions) to simplify imagery stitching. This plan is good for particularly complex and varied terrain types.
Image acquisition plan check-point
Have a go at the following questions to check your understanding of different image acquisition plan types.
You have successfully determined the drone flight route, but have you thought about the optimal altitude? We'll be focusing on that in our next section...