LiDAR
LiDAR measures distance by timing laser pulses that bounce back from surfaces, building a 3D point cloud from aircraft, vehicles, drones or phones.
What is LiDAR and how does it measure distance?
LiDAR (light detection and ranging) is a sensing technology that measures distance with laser light. The instrument emits short pulses, times how long each one takes to return from a surface, and converts that time into a distance. Repeating this many thousands of times per second builds a point cloud, a set of 3D coordinates describing the surfaces the beam hit.
On a moving platform, such as an aircraft, a vehicle or a drone, the raw distances are not enough. The system also records its position with a satellite receiver (GNSS) and its orientation with an inertial measurement unit (IMU), so every point can be placed in a real coordinate system. Without these two sources the cloud would have the right shape but no reliable location.
Many systems record several echoes from a single pulse. When a beam passes through a gap in vegetation, one echo may come from leaves and a later one from the ground. That is why airborne surveys can map terrain beneath forest cover.
Which platforms carry LiDAR, and what does each one suit?
LiDAR is defined by the sensing principle, not by where the sensor sits. The platform decides the coverage, density and accuracy you can expect, so the choice follows the question you need to answer.
| Platform | Typical use | Main strength | Main limit |
|---|---|---|---|
| Terrestrial (tripod on the ground) | Buildings, interiors, facades | High detail at close range | Needs many stations for complete coverage |
| Mobile (on a vehicle) | Roads, street frontages, corridors | Fast capture along a route | Coverage limited to what the road allows |
| Drone | Sites, roofs, embankments, quarries | Flexible, and close to the ground | Flight time and airspace rules limit the area |
| Airborne (aircraft) | Regional terrain and national mapping | Covers very large areas in one campaign | Point density is lower than close-range methods |
| Phone or tablet | Quick room or object capture | Cheap and always at hand | Short range, and not survey grade |
Terrestrial and mobile scanners are the tools behind most building surveys, which the laser-scanning article covers as a workflow. Drones carry lighter sensors and fly lower, so they can capture a site, a roof or a slope in a single session. Aircraft-borne systems cover a whole region, and that scale is what makes a national dataset possible.
Phone and tablet lidar is a recent addition. Some Pro-class devices include a short-range depth sensor that builds a rough 3D model of a room or an object. It is useful for orientation and for sharing a quick sketch with a client. It is not a substitute for a measured survey, because its errors are larger and it does not give a verified coordinate system.
How does LiDAR differ from a laser scan in a building survey?
The two terms overlap in everyday speech, but they answer different questions. LiDAR names the measurement principle. A laser scan is the act of capturing a building or site with such an instrument, together with the processing that turns the raw cloud into something usable. The same sensor serves both purposes, which is why people mix them up.
In practice a building survey involves planning scan positions, registering the stations into one cloud, removing moving people and stray points, and deciding what to extract. Those steps belong to the scanning workflow. The sensing principle tells you what the instrument can and cannot measure, for example how a dark or glossy surface reflects the beam. Knowing both helps you ask a surveyor the right questions.
How does LiDAR compare with photogrammetry?
Both methods produce 3D geometry, but they capture it differently. LiDAR measures distance directly with light. Photogrammetry infers geometry from overlapping photographs. That difference shapes what each one does well.
| Aspect | LiDAR | Photogrammetry |
|---|---|---|
| How geometry is obtained | Direct distance measurement | Computed from matching points in images |
| Dependence on light | Works in shade and in darkness | Needs even, stable lighting |
| Surface colour and texture | Reflectance values, or colour from an added camera | Realistic texture taken from the photographs |
| Uniform, featureless surfaces | Consistent, because it does not rely on texture | Struggles where the surface has little texture |
Many projects combine the two. A drone may carry a laser sensor and a camera together, so the cloud supplies reliable geometry and the photographs supply the look of a facade. Choose by the question: for dimensional truth on a plain surface, lean on LiDAR, and for appearance or a cheap quick model, photogrammetry may be enough.
Can a national LiDAR terrain model help check a plot before buying?
Yes, for a first look. Slovakia's national airborne laser scanning is run by the Geodesy, Cartography and Cadastre Authority (ÚGKK SR). Its first cycle ran from 2017 to 2023 and produced a seamless digital terrain model, DTM 5.0, covering the whole country. A DTM is a bare-earth surface with buildings and vegetation removed, so it describes the ground you would build on.
The data can be viewed in the MAPKA application, and ZBGIS, the national geographic base run by ÚGKK SR, is the entry point for downloads. Before buying, you can derive contours and slope for the plot from the model, and see how the land falls toward the road, a neighbour or a watercourse. That tells you early whether the site will need retaining walls, a long access drive or significant earthworks, all of which affect cost.
The limits matter. A national model is a regional product, so its detail suits a broad slope check, not a design. It does not show your boundaries, which come from the cadastre, nor the position of underground services. For design, order a polohopis a výškopis from a licensed surveyor (oprávnený geodet), as described in topographic site survey. The national data help you decide whether the plot is worth that expense.
What can LiDAR miss or get wrong?
LiDAR reports what the beam reached, and that is a limit in itself. A few common gaps are worth knowing before you rely on a dataset.
- Hidden surfaces stay hidden. Anything behind a wall, under a floor or inside a duct is absent from the cloud.
- Vegetation changes the ground picture. Dense canopy can leave the ground under-sampled, so check the ground points before trusting a fine contour.
- Glass, water and dark or glossy materials return weak or misleading signals.
- Weather matters. Heavy rain, fog, dust and snow reduce range and quality, and flights are planned for clear conditions.
- Density is not accuracy. A dense cloud can still be offset if the position and orientation were not controlled.
Treat a point cloud as geometry, not as a verified legal document. A cloud can show that a slope is steep, but it cannot establish legal boundaries or ownership.
Frequently asked questions
- Can a phone or tablet scan replace a professional survey?
- No. Phone and tablet lidar gives a quick, rough 3D sketch of a room or an object. It has no verified coordinate system and its errors are larger than survey-grade instruments, so it suits orientation and client discussion. Dimensions used for permits or construction still need a licensed surveyor or a verified scan.
- Does LiDAR see through trees?
- Partly. Airborne pulses can pass through gaps in the canopy and return an echo from the ground, which is how terrain under forest can be mapped. Dense canopy still reduces the number of ground points, so the ground model is less detailed beneath thick cover. A pulse that hits only leaves returns no ground echo at all.
- Is the Slovak national terrain model free to use?
- ÚGKK SR publishes the seamless DTM 5.0 to the public, and it can be viewed in the MAPKA application. The official airborne laser scanning page does not state the download terms for the terrain model itself. Check the terms of provision on geoportal.sk before any commercial or large-scale use.
- What is the difference between a DTM and a DSM?
- A digital surface model (DSM) includes everything the laser hits, including buildings, trees and vehicles. A digital terrain model (DTM) keeps only the bare ground. For a slope check you normally want the DTM, while a DSM helps with roof and vegetation heights.
- Does darkness or bad weather stop LiDAR?
- Darkness does not, because LiDAR supplies its own light. Heavy rain, fog, dust and snow do reduce range and quality, which is why flights and scans are planned for clear weather.