This document discusses factors that influence point density in LiDAR scans, including pulse repetition frequency, above ground level, scan angle, revolutions per minute, ground speed, and swath overlap. It explains how each parameter affects point density and coverage and how they must be balanced for different applications. Proper planning of flight parameters and swath layout is needed to efficiently collect accurate point cloud data.

1. Planning Factors for Point
Density
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2. 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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3. Parameters To Point Density
RPM
Scan Angle
Scan Pulse Repetition Frequency
Above Ground Level (AGL) or Altitude
Ground Speed
SWATH Overlap
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4. 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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5. Above Ground Level (AGL)
or Altitude
Light Spreads out and looses power. This is called Beam
Divergence. It is mathematically defined as:
Intensity (𝐼) = 𝑃𝑜𝑤𝑒𝑟 (𝑃)
𝐴𝑟𝑒𝑎 (𝐴)
= 𝑃
4𝜋𝑟2
Point Density Decreases as Distance Increase. This is not
necessarily the altitude but the slant range.
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6. 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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7. 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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8. 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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9. 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.
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10. Putting It All Together
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11. 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)
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12. Point Cloud Density Charts
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