LiDAR Flight Planning Factors for Point Density

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Planning Factors for Point Density
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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

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Parameters To Point Density
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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.

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Above Ground Level (AGL) or Altitude

•  

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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

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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.

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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.

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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 dont miss
  ground area due to shadows caused from ridgelines
  and other terrain features.

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Putting It All Together
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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?
• This is a systematic method for collecting a
  regular area
• 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)

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Point Cloud Density Charts
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