Drone surveying has become popular in recent years as a fast solution to survey large areas. At the same time, the terrestrial 3D laser scanner (TLS) remains the reference tool for millimetric accuracy in building and infrastructure. The honest question is not "which is better": it is when to use each and when to combine them.
Here we explain real differences in accuracy, timings, legal limitations and typical cases from professional practice in Catalonia.
Two families, two different things
Drone with photogrammetry
A photogrammetric drone flies over the area and takes hundreds of overlapping photographs. In the office, software such as Agisoft Metashape or Pix4D reconstructs a textured 3D model and an orthophoto from those images.
Typical accuracy: 2-5 cm in planimetry, 3-10 cm in altimetry, depending on flight height, GSD (ground sample distance) and the use of Ground Control Points (GCP).
Drone with airborne LiDAR
A drone with a LiDAR sensor (DJI Matrice 300 + L2, for example) emits laser pulses from the air and directly captures a point cloud, without photogrammetric reconstruction. It is better for vegetated areas because the laser penetrates the canopy.
Typical accuracy: 3-5 cm in both planimetry and altimetry.
Terrestrial laser scanner (TLS)
A scanner such as the Trimble X7 is placed on a tripod on the ground and captures millions of points per station with 2 mm accuracy at 10 m. Useful range up to 80 m.
It is the option when millimetric accuracy is required, or when the area is interior, covered or has obstacles a drone cannot bypass.
Direct comparison
| Aspect | Photogrammetric drone | LiDAR drone | Terrestrial scanner (TLS) | |---|---|---|---| | Typical accuracy | 2-5 cm | 3-5 cm | 2 mm | | Coverage | Large areas (ha) | Large areas | Focal area (50-80 m radius) | | Field time | 30-60 min flight | 30-60 min flight | 3-5 min/station × N | | Interiors | No | No | Yes (specific) | | Dense vegetation | Poor (surface only) | Penetrates canopy | Limited to field of view | | Weather | Sensitive (wind, rain) | Sensitive (wind) | Tolerant | | Legal regime | AESA / geographic zone restrictions | Same | No restrictions | | Equipment cost | Medium-high | High | High |
When to use drones
Drones are clearly the best option for:
- Extensive open terrains: rural parcels, large plots, road sections, quarries.
- Volume calculations in large-scale earthworks (cut, fill) where ±5 cm is acceptable.
- Cartography of linear civil works: railway lines, highways, canals.
- Orthophotos for preliminary urbanization studies, heritage documentation, visual site control.
- Areas inaccessible on foot: dangerous slopes, industrial roofs, degraded zones.
Legal limitations in Spain
Worth remembering: drone flights in Spain are regulated by the Spanish Aviation Safety Agency (AESA) and EU Regulation 2019/947. Before flying, you need:
- Operator registered with AESA.
- Pilot certified according to category (Open A1/A3, Specific...).
- Zone verification (airspace, prohibited/restricted areas).
- Prior notification in specific zones.
- In urban areas (most of Barcelona), additional restrictions that in practice limit routine operations.
For a rural plot in the Catalan interior, flying is relatively simple. For central Barcelona, it rarely pays off versus a terrestrial scanner.
When to use the terrestrial laser scanner
TLS is the clear choice when:
- Millimetric accuracy is required: building, retrofit, archaeological documentation, ITE (Spanish Mandatory Building Inspection) facades.
- There are interiors or enclosed spaces (warehouses, workshops, process plants).
- The area is small or focal: an urban plot, a roundabout, a 50-100 m section.
- Flying is not possible: dense urban areas, interior spaces, adverse weather.
- Real colour with high quality is required: the Trimble X7 HDR 360° camera produces photographic texture of the cloud.
We develop this further in topographic survey with 3D scanner.
When to combine drone + terrestrial scanner
On medium and large projects, combining both techniques is the norm:
- Drone to cover the general extent with orthophoto and digital terrain model.
- Terrestrial scanner for focal points that require accuracy: accesses, singular elements, facades, intersections with infrastructure.
- Common GNSS that ties both surveys to official coordinates.
Real examples where this pays off:
- Urbanization of an area: drone for the general volumetry + TLS for party walls and conflict points with existing buildings.
- Retrofit of a heritage complex: drone for the surroundings and the roof + TLS for detailed facades and interiors.
- Linear works (road, canal): drone for the alignment + TLS for singular structures (bridges, crossings, chambers).
Georeferencing: the glue that binds everything
Neither the drone nor the TLS produces official coordinates on their own. For the survey to be usable with the cadastre, ICGC or engineering projects, GNSS georeferencing is required.
Our standard workflow:
- GNSS RTK on permanent or improvised targets on the ground, in ETRS89 / UTM 31N.
- Drone flight (if applicable) with the GCPs marked.
- Terrestrial scanning registered on the same targets.
- Combined processing in the same coordinate system.
Result: a unified, georeferenced survey, ready to integrate with official cartography.
Honesty about drones
To finish, something worth saying clearly:
- Drones do not replace the terrestrial scanner in accuracy or detail.
- Administrative management (AESA, NOTAM, zones) can eat up more time than the flight itself on small projects.
- Catalan weather (wind, rain) cancels more flights than the industry publishes.
- The real total cost (operator + pilot + processing + administrative hours) is not always lower than a TLS day.
On small urban projects, drones rarely pay off. On large open projects, they are irreplaceable. On medium projects, the combination is optimal.
Frequently asked questions
Do you have your own drone or do you subcontract? Depending on the project. We have trusted partners who operate drones with the necessary certifications and insurance. When a project requires a flight, we coordinate it end-to-end.
Can you do topographic surveying with a drone only for small projects? It depends on the accuracy required. For a preliminary study with ±5 cm it may suffice. For a construction project on that terrain, it almost always needs to be complemented with TLS.
What about heavily vegetated terrains? LiDAR drones are better because the laser partially penetrates the canopy. The photogrammetric drone only sees the canopy top. For accuracy under dense vegetation, TLS on the ground with targeted clearance is required.
Can you produce an orthophoto of an industrial roof? Yes, with a drone. Useful for roof inspection, verification of available surface for photovoltaic, detection of visible pathologies. The accuracy is usually sufficient for those uses.
How much does it cost to combine both techniques? More than using just one, but generally less than trying to cover everything with TLS if the surface is large. We quote each project according to scope.
Do you have a topographic project and don't know which method is best? Tell us the area, required accuracy and the type of deliverable. We will tell you which method pays off and how much it would cost. Let's talk.
Cover image: European Southern Observatory (ESO) · Wikimedia Commons · CC BY 4.0
