C-DRONE GUIDE · 25 JULY 2026
Drone bathymetry: mapping the bottom of lakes, reservoirs and waterways, method and price
Measuring the depth of a lake, a dam reservoir or a port channel traditionally means sending out a boat fitted with a sonar, point by point, or calling in a diver for the trickiest areas: two methods that are slow, costly to mobilise, and sometimes dangerous on choppy water or in a working port. Bathymetric LiDAR mounted on a drone changes the equation: a green laser, able to penetrate the water column several metres deep in sufficiently clear water, captures the topography of both the bottom AND the banks in a single flight, with no boat and nobody in the water. Flooded quarries, hydroelectric reservoirs, marinas, rivers with unstable banks: here is how a drone bathymetric survey works, what French regulations require (DGAC and Voies navigables de France), and what it costs in 2026.
Published on 25 July 2026, reviewed on 27 July 2026 — regulations in force as of July 2026.
A laser that sees through water: how it works
The principle rests on a topo-bathymetric LiDAR: unlike a standard near-infrared LiDAR, which stops at the water surface, it fires a green laser beam (532 nm), the wavelength that penetrates water best with the least absorption. Part of the signal returns from the surface, another part from the bottom: the sensor captures both in a single pass, while the surrounding banks are surveyed simultaneously with standard near-infrared LiDAR. The result is a continuous 3D model, from dry land down to the bed of the water body, without the artificial cut-off a sonar survey imposes by covering only the submerged area.
The depth reachable depends directly on water turbidity: in clear water, several metres are accessible; in water loaded with sediment or algae, range drops fast, sometimes to under a metre. A study published in 2022 in the journal Sensors by Wang, Xing, He, Yu, Xu and Li evaluated a lightweight UAV-borne bathymetric LiDAR system during a coastal test campaign: the accuracy achieved reached decimetre level on both the water surface and the bottom, with a point density well above that of a classic aircraft-mounted bathymetric LiDAR (see the study on Google Scholar). It is this trade-off — fine accuracy, lightweight deployment, low mobilisation cost — that makes the technology relevant for medium-sized water bodies, out of economic reach of a classic manned aerial campaign.
Flooded quarries, reservoirs, ports, rivers: who needs it
Flooded quarries extracting aggregates regularly need to quantify the volume remaining below the waterline: the bathymetric drone complements dry stockpile cubing with the submerged portion of the deposit, using the same campaign-to-campaign comparison logic. Dam and hydroelectric reservoir operators track the gradual silting that reduces usable storage capacity; the bathymetric survey quantifies that lost volume and complements the visual inspection of the structure described in our guide on drone dam and dyke inspection. Ports and waterway managers use it to prepare dredging: precisely quantifying the volume to extract from a channel or basin avoids paying for dredging that is either oversized or, conversely, insufficient.
Local authorities use it for stormwater retention basins, whose silting must be tracked to guarantee their buffer capacity in a flood, and for the recreational lakes they manage. Finally, on coastlines and unstable riverbanks, bathymetry naturally complements aerial topographic monitoring: the same repeated 3D-model logic detailed in our guide on coastal erosion monitoring, but this time below the waterline.
Two separate authorisations: DGAC and Voies navigables de France
On the aviation side, a bathymetric flight is no different from any other drone survey: open category most of the time (sub-category A2 or A3 depending on proximity to people, maximum height 120 m), operator registered on AlphaTango, a prior check of restricted zones on the Géoportail drone map — the surroundings of dams, ports and certain riverside industrial sites are often among the sensitive zones to check. Professional liability insurance remains the mark of a serious contractor, especially over infrastructure with real stakes such as a hydraulic structure.
But the terrain itself adds a layer rarely found elsewhere: when the flight covers a navigable waterway, a simple declaration to the préfecture is not enough. Voies navigables de France (VNF), which manages the public river domain, must be approached separately for an occupancy authorisation, since flying over the water is treated as temporary occupation of that domain; this request is generally filed one to two months before the flight date, and préfecture approval does not in any way substitute for VNF's — the two procedures are independent and both required. On a private water body (a quarry, a company reservoir), the owner's or concession holder's consent replaces this step, with the same lead time to anticipate as for access to public maritime domain concessions.
The price of a drone bathymetric survey in 2026
Pricing depends on the water body's surface area, its turbidity (which drives the flight time needed to guarantee usable coverage) and the expected deliverable. Orders of magnitude observed in France in 2026:
| Service | Observed price (excl. VAT) |
|---|---|
| One-off bathymetric survey (under 2 ha, clear water) | €1,500 to €3,000 |
| Medium-sized lake or reservoir (2 to 10 ha) | €3,000 to €7,000 |
| Port channel or waterway section (linear) | quoted based on length and VNF procedures |
| Combined bottom + banks 3D model, cubing report | +€500 to €1,500 |
| Annual comparative campaign (silting monitoring) | €4,000 to €10,000/year |
The relevant comparison remains a sonar boat and crew mobilised for several days over the same stretch: the drone covers the area in hours rather than days, with a continuous model of both the bottom and the banks. To put these amounts in context among the site's other services, see our guide how much a drone service costs.
Frequently asked questions about drone bathymetry
What is the maximum depth a bathymetric drone can measure? It depends almost entirely on water clarity: several metres in clear water, much less in turbid or algae-laden water. A preliminary test on site checks this before committing to a full campaign.
Does the drone fully replace a sonar boat? Not in every case: in very turbid water or beyond the laser's range, a sonar mounted on a boat or a surface water drone remains necessary. The aerial drone is most effective on clear water and medium-sized surfaces.
Is VNF approval needed for every water body? No, only for navigable waterways falling under the public river domain. A private water body (a quarry, a company reservoir, a local authority basin) follows a different process, with the site owner's or manager's consent.
Can surveys be compared from one year to the next? Yes, that is the main point for silting monitoring: with careful control (ground control points or an RTK/PPK drone), the differences measured between two campaigns reflect the bottom's real evolution.