Overlap
Last updated
Last updated
The spacing between flight lines will determine the amount of overlap and will depend on the limitations imposed by your scanning hardware. e.g. LiDAR sensor and camera.
LiDAR overlap is determined by various factors including aircraft/vehicle velocity, recommended scanning height AGL, and LiDAR FOV (field of view). LiDAR overlap between flight lines is applied to ensure a complete coverage of the final LiDAR data set. The main reason for this precaution is to account for the different factors which can affect the coverage on the ground such as:
Roll movements of the aircraft that will shift the swath across the flying direction.
Crosswinds which can cause the aircraft to fly slightly sidewards (crabbing) which narrows the swath.
Changes in altitude above ground which directly affect the swath width
Variation in terrain elevation which converts to change of altitude above ground and therefore affects the swath width as well.
If you are just scanning LiDAR (no imagery), we recommend a 60° - 90° FOV with a minimum of 50% overlap between flight lines. The more comfortable you get with the LiDAR you can begin lowering the overlap from 50% but no lower than 20%, note lowering your overlap will directly affect the total PPSM of your final deliverable.
After calculating the flight plan, you can project and calculate more detailed aspects regarding the LiDAR data by selecting the following from the LiDAR Sensor tab:
Project LiDAR Swath
Project LiDAR Point Density
After you calculate these you can get a visual layer that projects the information overtop of your AOI. You can turn these layers on or off from the Layers tab on the right side of the screen.
If you are using a camera, your flight line spacing is entirely dependent on the desired Ground Sample Distance (GSD), camera focal length, and flight altitude in order to achieve a proper photo overlap.
The recommended photo overlap for orthoimagery is 60 to 75% overlap across (side) and 80% overlap along (forward); 60 to 75% between overlapping flightlines and 80% between consecutive images. With a 60/80 overlap, you can use the photos in Pix4D to reconstruct a 3D surface from the 2D images. If you are not planning on using Pix4D, then we recommend a 40/60 (40% overlap across and 60% overlap along) overlap to be able to do a basic orthomosaic using TerraPhoto, which requires an existing 3D surface (i.e. LiDAR point cloud). Additionally, if you are planning on projecting RGB color onto your point-cloud, we also recommend using a 40/60 (40% overlap across and 60% overlap along) overlap.
The GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value. A GSD of 2 cm means that one pixel in the image represents linearly 2 cm on the ground (2*2 = 4 square centimeters). To calculate your GSD, you can use the Pix4D GSD Calculator provided in an Excel or OpenOffice format.
You can use the Create Flightplan tool found in the integrated WayPoints planner/viewer in SpatialExplorer to calculate your Expected GSD (initial ground sample distance is required).
Once you calculate your GSD, you will have the width and length of a single image footprint on the ground. This will determine the maximum spacing between flight lines.
