LiDAR Point Density: How Repetition Rate Determines Your Specification
*Point density is decided before the first scan, by three numbers: pulse repetition rate, platform speed and
scan geometry. This article shows how the three relate, works through the arithmetic with published figures, and
explains why a bare pulse rate is not enough to specify a mapping system.*
In short: along-track spacing is platform speed divided by pulse rate; cross-track spacing is swath width
divided by points per sweep. In a single-beam scanner the cross-track figure is metres while the along-track
figure is millimetres — so the cross-track spacing is what limits the survey, and no increase in repetition rate
fixes it on its own.
What point density actually is
Point density is the number of measurements a LiDAR system records per unit area of ground. It is usually
quoted in points per square metre, and it is the specification that determines what the resulting data can be
used for:
- a few points per square metre gives terrain and canopy structure at a coarse resolution
- tens of points per square metre resolves individual trees, poles and building edges
- hundreds of points per square metre resolves surface detail, road markings and low vegetation structure
Everything in between is a matter of what the survey is for. What matters for this article is that density is
not a property of the laser. It comes out of how the laser is used.
The two spacings
A scanning system lays points down in two directions, and the two are set by different things.
Along the direction of travel, points are spaced by how far the platform moves between pulses:
along-track spacing = platform speed / pulse repetition rate
That is the whole relationship. Nothing else enters it.
Across the scan, points are spaced by how far the beam moves between pulses as it sweeps:
cross-track spacing = swath width / points per sweep points per sweep = pulse repetition rate / sweeps per second
Again, only three quantities: rate, sweep frequency and swath.
Point density is then the reciprocal of the product of the two spacings:
density = 1 / (along-track spacing × cross-track spacing)
Working it through with published figures
Take the four high-repetition models in the ERDI range, which run at 500 Hz to 5 kHz, and put them in a
representative airborne configuration: platform at 5 m/s, scan mirror at 50 sweeps per second, swath 100 m.
| Model | Pulse rate | Points per sweep | Along-track spacing | Cross-track spacing | Density |
|---|---|---|---|---|---|
| ER70 | 500 Hz | 10 | 0.010 m | 10.00 m | ~10 pts/m² |
| ER40 | 1 kHz | 20 | 0.005 m | 5.00 m | ~40 pts/m² |
| ER20 | 2.5 kHz | 50 | 0.002 m | 2.00 m | ~250 pts/m² |
| ER10 | 5 kHz | 100 | 0.001 m | 1.00 m | ~1000 pts/m² |
Density is the reciprocal of the product of the two spacings, over the area swept.
Two things stand out.
The along-track spacing is very fine — millimetres — because a platform moving at metres per second and a
laser pulsing at kilohertz produce points close together in that direction. This is almost never the limiting
factor.
The cross-track spacing is coarse — metres — because a single beam sweeping a 100 m swath has only as many
points across the swath as it fires during one sweep. At 500 Hz and 50 sweeps per second that is ten points
across a hundred metres.
This is the number that decides the survey. The cross-track spacing sets the resolution the data can
support, and along-track density cannot compensate for it. A dataset with 1 mm spacing along the flight line and
10 m across it is a set of parallel lines, not a surface.
Why real airborne systems use more than one beam
The arithmetic above makes clear that a single beam at a few kilohertz cannot produce a dense survey from the
air. Practical airborne LiDAR solves this by firing several beams at once — commonly 16, 32 or more in parallel
— and by scanning each across the swath.
Conceptually, if a system carries sixteen parallel beams and each runs at 5 kHz, the effective pulse rate
across the swath is 80 kHz, and the cross-track spacing falls by a factor of sixteen. The laser's own repetition
rate has not changed; what changed is how many beams are in the air at once.
This is the reason a specification that quotes only a pulse repetition rate is incomplete. The same laser in a
single-beam scanner and in a sixteen-beam scanner produces very different data.
It is also worth knowing before choosing a model: if the architecture is multi-beam, a lower-rate source may
serve perfectly well, and the money is better spent on the beams. The
ER40 at 1 kHz is a reasonable starting point for that reason.
Working backwards from a requirement
In practice the requirement comes first: a point spacing the survey must achieve. Then:
required pulse rate = platform speed / required along-track spacing
At a platform speed of 5 m/s:
| Required spacing | Pulse rate needed |
|---|---|
| 1.0 m | 5 Hz |
| 0.5 m | 10 Hz |
| 0.25 m | 20 Hz |
| 0.1 m | 50 Hz |
At 10 m/s, each of those doubles.
Read against the model table, a requirement of 0.1 m along-track spacing at 5 m/s needs only 50 Hz — well
within even the low-repetition models. What the requirement is really testing is the cross-track direction,
which brings the number of beams and the sweep rate back into the calculation.
The honest summary: **for a single-beam scanner, high repetition rate buys fine along-track spacing that is
usually finer than needed, and does not fix the cross-track spacing that usually matters.** High repetition
rate earns its place in multi-beam architectures, in short-range dense sensing, and in instruments that need
many measurements per second at close range rather than a dense grid from altitude.
The other parameter: pulse width
Repetition rate sets how many points. Pulse width sets how far apart two surfaces can be and still be recorded
as two.
A return from a surface lasts about as long as the pulse. If a second surface lies within half the pulse's
length in distance, the two returns merge into one. The relationship is:
range resolution = c × pulse width / 2
With the pulse widths published for this range:
| Pulse width | Range resolution | Models |
|---|---|---|
| 3.5 ns | 0.52 m | MINI-100 |
| 4 ns | 0.60 m | ER40, ER70 |
| 5 ns | 0.75 m | ER20, ER200, ER300 |
| 6 ns | 0.90 m | ER100, ER400, ER500 |
| 7 ns | 1.05 m | ER800, ER900 |
| 12 ns | 1.80 m | ER2000, ER4000, ER10000 |
| 13 ns | 1.95 m | ER8000 |
Two consequences worth noting.
The short-pulse models are in the high-repetition family. The 4 ns and 5 ns pulses belong to the ER10–ER70
group, not to the high-energy models. If an application needs both long range and fine separation, that is a
tension to resolve rather than a specification to look up. The
pulse width article goes into what the choice costs.
The high-energy models have the longest pulses. Twelve to thirteen nanoseconds is a consequence of
generating more energy in the same cavity. For a survey measuring the ground surface, that is usually
immaterial. For separating a cable from the ground behind it, or a vehicle from the road under it, it is the
difference between one return and two.
What else enters the design
Repetition rate and pulse width are the parameters this article can put numbers on. Three others matter and are
worth fixing at the same stage:
Scan pattern. Raster, oscillating, rotating prism and Risley prism produce different point distributions
from the same laser. The pattern decides whether points fall in a regular grid or along curved traces, which
affects how the data is interpolated.
Beam divergence and footprint. A larger footprint at range spreads the pulse over more ground, which affects
both the achievable resolution and how vegetation is penetrated.
Timing and receiver bandwidth. The detector and timing electronics must be able to resolve the pulse width
being used. A 4 ns pulse needs a receiver chain with the bandwidth to match it, and a time-to-digital converter
with resolution finer than the pulse.
A practical order of work
- State the point spacing the survey must achieve, in both directions.
- Decide the platform speed, which is usually fixed by the aircraft or vehicle.
- Work out the along-track requirement: speed divided by spacing.
- Decide the swath and the number of beams, and work out the cross-track requirement.
- Check whether any target separation is required, which sets the maximum pulse width.
- Only then choose a repetition rate and a model.
- Confirm the receiver bandwidth matches the pulse width.
The models available
The high-repetition family covers 500 Hz to 5 kHz at 10 to 70 µJ, all in a 17×8×7 mm³ package at 6 g, and all
drawing 5 A from a 2 V supply:
| Model | Pulse energy | Repetition rate | Pulse width | Drive pulse width |
|---|---|---|---|---|
| ER10 | ≥10 µJ | 5 kHz | 5 ns | ≤0.3 ms |
| ER20 | ≥20 µJ | 2.5 kHz | 5 ns | ≤0.3 ms |
| ER40 | ≥40 µJ | 1 kHz | 4 ns | ≤0.4 ms |
| ER70 | ≥70 µJ | 500 Hz | 4 ns | ≤0.4 ms |
For single-beam instruments where a lower rate is sufficient, the lower-repetition models run at 1–10 Hz with
pulse energies from 100 µJ to 10 mJ — see the Laser Sources page for the full table.
Next steps
If you can describe the survey — intended point spacing, platform speed, altitude and swath — the
Contact / RFQ form is the quickest route to a recommendation. Datasheets and 3D
models for mechanical integration are published on each model page, and the drive and timing requirements are
on the datasheet for the model concerned.