Hydrographic Survey and Water Sampling Drones: Buyer Guide

Drones are increasingly used on lakes, reservoirs, rivers and coastal waters for two related jobs: measuring water depth (bathymetry) and collecting water samples. They can reach areas that are difficult or slow to access by boat. Both jobs place unusual demands on the aircraft and payload, and the quality of the data depends on details that are easy to overlook. This guide covers what to evaluate and what evidence to request.

Bathymetry with drone-carried echo sounders

Acoustic depth measurement needs the sensor in the water. Drone systems therefore carry an echo sounder that is lowered or towed so that the transducer is submerged while the aircraft flies slowly over the survey lines. Because the transducer must be submerged and has a small dead zone, there is a practical minimum depth that can be measured; ask the supplier to state it.

Points to evaluate:

• Single-beam or multibeam: single-beam sensors measure one point under the transducer; multibeam sensors measure a swath and cover ground faster but are heavier and more complex.
• Frequency: the operating frequency affects range, resolution and performance in debris-laden or vegetated water; field studies report that lower frequencies can penetrate debris-heavy water more effectively. Dual-frequency units give options.
• Positioning and motion: depth readings are only useful with accurate position and attitude data. Ask how the sensor is synchronised with GNSS positioning and how cable swing and sensor tilt are handled.
• Flight behaviour: stable, slow flight at low height over water, reliable altitude holding, and safe behaviour if the cable snags.

Accuracy depends on conditions. Published case studies show larger errors in very shallow zones and when survey lines are spaced too widely, so ask for validation results from conditions similar to yours.

Survey standards

The International Hydrographic Organization's S-44 standard defines survey "orders" with minimum requirements for each intended use. The sixth edition, published in 2020, added an "Exclusive Order" for areas more critical than those covered by earlier orders. A survey is compliant with an order only if it meets all specifications for that order. If your client specifies an S-44 order, ask the supplier which order the system can support and what evidence backs the claim. The standards are voluntary, but they are widely referenced in contracts.

Water sampling designs

Drone water sampling systems generally follow one of three approaches:

• Cable-suspended sampler: the most widely used approach. A container is lowered on a cable and triggered to fill. Light samplers may float rather than sink, so weighting and cable length matter, and the swinging load must be handled by the flight controller.
• Pump-fed onboard container: a pump draws water through a hose into a container carried on the aircraft.
• Landing on the water: some aircraft land to collect samples, saving energy, but this is generally limited to calm conditions.

Depth-targeted sampling is limited by cable or tether length and by how precisely depth is known at the moment of collection. Ask whether the system measures sampler depth and how sampling is triggered.

Sample integrity

A drone sample is only useful if it would stand up to laboratory scrutiny. The ISO 5667 series covers water sampling: Part 1 addresses the design of sampling programmes and techniques, Part 3 covers preservation, handling, transport and storage of samples, and Part 14 covers quality assurance and quality control. Ask suppliers:

• Which materials contact the water, and whether containers are replaceable or cleanable to a laboratory protocol
• How cross-contamination between samples is prevented, for example by purging or separate containers
• Sample volume per container and number of containers per flight
• Whether the system has been compared with conventional sampling methods, and the results

Aircraft considerations near water

• Flight time with the payload, including hover time during sampling
• Wind tolerance while hovering low over water
• Ingress protection rating with its test basis, and corrosion resistance for salt water
• Failsafe behaviour over water, such as return-to-home and low-battery logic
• Optional flotation or recovery aids

What to request from suppliers

• Validation data comparing drone depth measurements with reference surveys
• Minimum and maximum measurable depth for the sounder
• Sampler design documentation, wetted materials and cleaning procedure
• Field test reports from comparable water bodies
• Data formats and processing software compatibility

Next step

If you are sourcing drones or payloads for bathymetric survey or water sampling, send your requirements through our inquiry form and our team will match you with suitable suppliers.