Airborne laser scanning costs several times more than a photogrammetric setup. It is justified only where there is a problem photography cannot solve, and there are a few such problems.
How it differs from photogrammetry
Laser scanning measures range directly. A pulse is emitted, the return timed, and a three-dimensional point produced.
Photogrammetry infers depth. From multiple images taken from different positions, requiring surface texture to match against.
First consequence. Laser works on featureless surfaces — sand, snow, uniform flat material. Photogrammetry does not.
Second consequence. Laser works in low light and at night. Photogrammetry needs illumination.
Third and most important. Laser pulses pass through gaps in foliage and reach the ground beneath. Imagery only sees the canopy.
Multiple returns. One pulse can produce several points — canopy top, mid-canopy, ground — which allows vegetation to be separated from terrain.
Photogrammetry retains advantages. Colour and texture, a usable orthophoto, and far lower cost.
Combining both. Many systems carry a scanner and a camera together, giving accurate geometry with colour.
Data volume differs sharply. Laser produces very large point files, and the processing hardware requirement follows from that.
Principal applications
Terrain beneath forest canopy. The most important application and the one with no substitute.
Forestry. Tree height, canopy density, biomass estimation, stem counting.
Corridor survey. Powerlines, pipelines and roads passing through vegetated ground.
Vegetation clearance measurement. Distance from conductors to trees, identifying where cutting is required.
Complex terrain. Cliffs and gorges where imagery reconstructs poorly.
Structural modelling. Complex frameworks, industrial pipework.
Night survey. Where daytime flying is not possible.
Common factor. Every one involves something imagery cannot handle — obscuring vegetation, absent texture, or absent light.
Coastal and riverbank monitoring. Where vegetation obscures the bank profile that has to be measured.
Specifications that matter
Pulse rate. Points per second, which determines achievable point density at a given speed and altitude.
Point density. Points per square metre. The headline quality figure.
Ranging accuracy. Error on an individual measurement. But final accuracy depends far more on the positioning system.
Inertial navigation system. Mandatory and expensive. It determines the position and attitude of the sensor at the instant of each pulse, and its error propagates directly into every point.
Number of returns recorded. More returns means better separation of vegetation from ground.
Operating range. Maximum altitude at which returns remain reliable.
Scan angle. Wider covers more per pass but points at the edges are less accurate.
System weight. Sensor, navigation unit and logger together. Usually heavy enough to require a substantial airframe.
Beam divergence. A wider beam at range averages over a larger footprint, which limits how fine a feature can be resolved.
Operating and processing
Reference station. Needed for the high-precision positioning, either on site or via a network.
Initialisation. The inertial system needs a stationary period and often a specific flight manoeuvre to align. It cannot be skipped.
Overlapping swaths. Between passes, allowing cross-checking and correction of inter-strip discrepancies.
Cross flights. Some workflows require a perpendicular pass for calibration.
Post-processing. Combining scan data with corrected trajectory to build the point cloud.
Strip adjustment. An essential step removing discrepancies between adjacent passes.
Classification. Separating ground, vegetation and structures. Semi-automatic and requiring manual review.
Quality control. Against surveyed check points and by examining strip agreement.
Allow real time for processing. It is substantially heavier than photogrammetry and cannot be compressed by wishing.
Deciding whether to invest
Cost is high. Complete system plus a capable airframe plus processing software.
Volume is required. A few projects a year will not repay it.
Skill is required. Operation and processing are considerably more demanding than photogrammetry. Budget for training time.
Hire the service. For occasional projects. Specialist providers exist.
Partner. Take the work and subcontract the scanning. You retain the client relationship without the capital.
Check imagery first. Many projects are adequately served by photogrammetry at a fraction of the cost. Establish that before concluding you need laser.
The one case that forces it. Ground surface beneath dense canopy. Imagery genuinely cannot do it.
Prices are falling. Waiting is reasonable if the need is not yet pressing.
Consider the market locally. Capability with no client base is an expensive asset sitting idle.
Frequently asked questions
What can laser scanning do that photogrammetry cannot?
Pass through gaps in foliage to measure the ground beneath dense canopy. Imagery only sees the canopy top, so terrain under forest is the one case with no substitute.
Which specification most affects final accuracy?
The inertial navigation system, since it determines the position and attitude of the sensor at each pulse and its error propagates directly into every point.
Which processing step is essential and often overlooked?
Strip adjustment, which removes discrepancies between adjacent flight passes. Without it the point cloud contains systematic offsets between swaths.
How should an operator approach the investment?
Confirm that photogrammetry genuinely cannot serve the work, then hire or subcontract until demand is recurring enough to justify purchase.
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