Using a Digital Elevation Model (DEM) to Ensure Safe, Efficient, and Cost-Effective Drone Missions
For regulatory compliance and safe flight planning, adhering to the permitted maximum flight altitude (Above Ground Level, AGL) is crucial. A reliable Digital Terrain Model (DTM) or Digital Surface Model (DSM) enables drone pilots to carefully plan automated flight paths in their Ground Control Station (GCS) software—whether for Visual Line of Sight (VLOS) or automated Beyond Visual Line of Sight (BVLOS) missions—ensuring that climbs or descents over varying terrain remain within regulatory limits. Elevation data makes it possible to plan and visualize the drone’s flight profile in mission-planning software (such as Mission Planner) before take off. For instance, operators can identify sections of the route where terrain change might inadvertently push the drone above the maximum allowed AGL. By leveraging ground elevation data, you can design paths that follow the natural contours of the terrain or critical infrastructure while remaining within regulatory limits.
Therefore, understanding the nuances of ground elevation is essential. The simple illustration below demonstrates that even if a flight starts at 50 meters AGL, calibrated using the drone’s barometer, a planned horizontal flight path can inadvertently exceed the maximum authorized altitude of 120 meters in the EU, the default limits if no other restrictions apply—simply due to changes in terrain elevation.
Often, while conducting a VLOS mission, accurately gauging the drone’s height above the ground can be challenging. This is because the pilot in command typically relies on barometer and/or GNSS altitude readings, which are calibrated at the departure point and may not account for significant changes in the terrain’s elevation along the flight route.
Drones are equipped with various onboard sensor technologies available today, such as barometers, ultrasonic sensors, radar, and potentially LiDAR (a more expensive option), often combined with GNSS. GNSS (Global Navigation Satellite System) systems, when augmented with RTK (Real-Time Kinematic) technology, provide highly accurate altitude measurements above the ellipsoid (or above Mean Sea Level for orthometric height based on the EGM96 geoid model). While having an accurate horizontal and vertical positioning system relative to the earth's ellipsoid or geoid is valuable, it is not sufficient on its own to ensure the drone maintains a constant AGL (Above Ground Level) altitude during its missions.
Even if you know the exact altitude above MSL with 10 cm accuracy, it is of little use if you lack precise ground-level elevation data relative to MSL, or if the ground elevation data you have is of poor accuracy or resolution.
For inspection missions—such as linear inspections of power lines—knowing the precise ground elevation is crucial to maintaining a constant distance from the power line. By referencing a high-resolution Digital Terrain Model (DTM), the drone’s autopilot can automatically and safely adjust flight paths. The red line illustrates a path without altitude adjustments to account for terrain elevation changes, while the blue path is interpolated with altitude adjustments to follow the terrain. The actual resolution and accuracy required will depend on the mission requirements and terrain, which will determine the appropriate DEM data to use.
Agricultural applications, such as spraying or crop monitoring, often require consistent, low-altitude flights just above the crops. This can typically be achieved using one of the UAV onboard sensor technologies mentioned above. However, when planning specific missions, such as spraying trees at a constant height Above Ground Level (AGL) in hilly terrains with steep slopes, relying solely on a single onboard sensor can make it challenging to maintain a consistent AGL height. For example, in the case of olive trees—olive oil being in high demand nowadays—there is typically a 5-meter gap between each tree, and spraying needs to occur approximately 2 meters above the tree canopy. Relying solely on the drone's sensors for AGL altitude can cause the drone to descend and ascend between consecutive trees, resulting in uneven spraying and potentially triggering issues with the drone’s Detect and Avoid (DAA) system. A mid-resolution (5 meters) to high-resolution DTM (1 meter),coupled with GNSS/RTK can help to ensure uniform spray distribution.
When it comes to obtaining reliable elevation data for drone operations, there are multiple approaches—each with its own advantages and challenges. The choice will largely depend on the complexity of your mission, budget, regulatory requirements, and the terrain you are operating in.
Most standard drone mission-planning software defaults to using the Shuttle Radar Topography Mission (SRTM) data set provided by the US Geological Survey (USGS). These 1-arc second (~30 meters) resolution elevation grids are referenced to the EGM96 geoid model. Pros include broad global coverage (though there may be voids in some areas) and free availability. Cons include lower resolution and accuracy—especially in regions with steep terrain or where very high precision is required.
Some governmental agencies provide higher-resolution or more up-to-date Digital Terrain Models (DTMs) or Digital SurfaceModels (DSMs). Depending on the country, these might be free or paid. They often come in various coordinate reference systems and projections, which might need conversion to match your drone software’s requirements (commonly WGS84/EGM96). Extracting and converting these datasets into a ready-to-use GeoTIFF for your drone’s Ground Control Station (GCS) can be time-consuming, but it can significantly enhance flight-planning accuracy.
When ultimate precision is needed—such as detailed inspections of power lines or critical infrastructure—conducting your own drone survey is a strong option. By collecting photogrammetry data or using a LiDAR sensor, you can create a project-specific DSM with high resolution and accuracy. While this requires additional flights, hardware, and specialized post-processing, it can pay off for high-stakes operations by ensuring you have a custom dataset perfectly aligned with your mission needs.
Several satellite providers offer commercial DSM products in ready-to-use GeoTIFF formats with various resolutions, ranging from 10 meters to 30 centimeters, and corresponding levels of accuracy. Note that higher resolution does not always guarantee higher vertical accuracy. These products can serve as an excellent middle ground between free global datasets and fully customized drone surveys. While considerations such as licensing costs, data accuracy, and resolution requirements are important, commercial satellite DSMs often provide a cost-effective solution for large-scale projects requiring consistent, high-quality terrain data. When the resolution and accuracy align with project requirements, satellite-derived models are frequently more economical and efficient than organizing dedicated drone surveys and managing the extensive data post-processing involved.
Finding the Right Balance - when selecting the most suitable source for elevation data, the best approach often comes down to balancing precision requirements, budget constraints, and mission complexity. For simpler operations over relatively flat terrain—or when high resolution isn’t critical—free global datasets like SRTM can be sufficient. However, in hilly or mountainous areas where accuracy is paramount, public or commercial datasets may offer a more practical compromise between cost and data quality. If your mission demands ultra-precise, up-to-date terrain data—particularly for detailed inspections or operations in complex landscapes—a drone-based survey (photogrammetry or LiDAR) might be worth the investment to ensure the highest level of detail. By evaluating the scale of your project, the regulatory environment, and the specific accuracy needed, you can choose the ideal mix of cost-effectiveness and reliability.
In summary, choosing the right elevation data source hinges on your mission profile, terrain complexity, and accuracy needs. Whether you opt for readily available global data, higher-fidelity public or commercial datasets, or generate your own with a dedicated drone survey, having high-quality elevation data is essential for safe, efficient, and compliant drone flight operations.
If you have questions, insights, or experiences to share, I’d love to hear from you! Feel free to reach out at yvan@stratomaps.com or leave a comment with your feedback!