Planning Factors for Point Density

What is Point Density Defined as the number of recorded laser returns per square meter 2 Factors The number of returns across the sweep of the laser The number of sweeps we can have along a run

Parameters To Point Density RPM Scan Angle Scan Pulse Repetition Frequency Above Ground Level (AGL) or Altitude Ground Speed SWATH Overlap

Pulse Repetition Frequency Pulse repetition frequency (PRF) is the number of times a pulsed activity occurs every second. It is measured in Hertz The higher the Hertz (Hz) the more pulses and more points along the sweep.

Above Ground Level (AGL) or Altitude

Scan Angle Most people think the maximum range given by a manufacture is the maximum height that can be flown. Since most industrial-class LiDAR is tested on the ground, it is the maximum laser distance. When doing laser scanner the side angle or slant range is always greater than the flight altitude

Revolutions Per Minute (RPM) All current LiDAR systems involve a rotating head. This may be using a mirror or with light emitting diodes spinning about an axis. Assuming the PRF remains constant then the spacing of each pulse is directly affected by the speed in which the laser emitter spins. The lower the head the more points per scan line but less scan lines along the ground track. The faster, the points are more spread out but you increase the number of scan lines. In the case of aerial laser scanning we have a limited FOV so point density becomes a balance between points across the scan line and the number of scan lines per second. Most applications do not require very dense point clouds and a middle setting will suffice. However if you are doing AS-IS modeling or trying to capture objects near the precision limits of the lidar then this becomes much more important.

Ground Speed Once the desired RPM is determined, a lidar specialist can then determine their sweep spacing. The slower the speed over the ground or ground speed the closer the sweep spacing will be. However there is a tradeoff. The slower a drone travels, the less area can be covered. For most applications only a few points per meter is required so flying faster and covering more ground is important.

SWATH Overlap Sweep Width is the last step in the process. It is a function of Above Ground Level (AGL) and the Field of View (FOV). There is a trade off between flying low to obtain a very high point density and flying high to cover more area per SWATH. Next the FOV is usually set to 90 ° for land scanning. You may set it slightly wider if more side scan is desired such as on mountainous terrain. One thing a specialist will want to consider is not picking a FOV too great that the slant range is greater than the maximum distance of the LiDAR. In this case either a lower altitude or smaller FOV must be entered. Once this is determined then SWATH spacing can be determined. Unlike photogrammetry, a LiDAR specialist does not require a lot of overlap, usually 10%. However in mountainous terrain for applications requiring high point density, you may want to increase to ensure you don’t miss ground area due to shadows caused from ridgelines and other terrain features.

Putting It All Together

SWATH Planning SWATH planning is very basic. Once the area is determined and the sweep width is set, a series of overlapping SWATHs can be determined. Why SWATHs? 1.This is a systematic method for collecting a regular area 2.In post processing you will break your data into SWATHs for easy processing Importance of Overhang When we are trying to obtain the highest level of point cloud accuracy it is important to keep the heading constant through each run until we are out of the scan area. Turning creates irregularities in the IMU and the point cloud that are labor intensive to process out. By over-shooting and then turning, we prevent this phenomenon from occurring in the scan area. For more on flight planning see our section on flight automation. (coming soon)

Point Cloud Density Charts