How Does Litchi's "Above Ground" Option Work
DJI Drone Altitude Measurement
DJI drones determine their altitude primarily through barometric pressure readings. The drone records the atmospheric pressure at the power-on (usually the take-off) location and uses this as a reference point for calculating altitude changes. As the drone ascends or descends, it continuously measures pressure variations and converts these into altitude changes based on the standard atmospheric model. However, this method has inherent limitations:
- Relative Measurement: The altitude displayed in the DJI (or Litchi) app is always relative to the power-on (take-off) elevation, not the true ground level beneath the drone.
- Impact of Atmospheric Conditions: Changes in weather, such as temperature and pressure fluctuations, can cause minor inaccuracies in altitude readings during a single flight. To experience this, note the displayed height in the app just before landing. Often it will disagree with the actual height of the drone by a small amount.
- No Real-Time Ground Reference: Drones lacks a continuous way to measure or verify their actual height above the terrain below.
Some DJI drones incorporate additional altitude sensors, such as:
- Ultrasonic and Infrared Sensors: These sensors, often part of the Vision Positioning System (VPS), provide more accurate altitude readings close to the ground (typically up to 30 feet or ~10 meters). However, they are primarily used for precision landing, position holding, or obstacle avoidance and do not contribute to the altitude readings displayed in the DJI or Litchi apps or in waypoint missions heights.
- LiDAR or Time-of-Flight (ToF) Sensors: Available on select enterprise models, these sensors improve altitude accuracy at low altitudes but are not standard in consumer drones.
Litchi's "Above Ground" Option
Litchi provides an "Above Ground" option within its waypoint planning software, allowing users to define waypoints at specific altitudes relative to the terrain rather than the drone's power-on elevation. This is achieved through the integration of digital elevation models (DEM) or online terrain databases, which help estimate ground elevation at each waypoint. However, users must be aware of certain limitations:
- DJI drones do not have real-time terrain awareness: Since DJI drones rely solely on barometric altitude measurements, they cannot dynamically adjust their height based on actual ground elevation changes.
- Litchi's terrain data is static: The elevation data used in the Mission Hub is static and does not account for real-time changes such as construction, vegetation growth, or obstacles.
- The DEM is only available in the Mission Hub: Litchi can only access the elevation API in the Mission Hub while connected to the internet for the purpose of mission planning.
Reference Location
To understand how the above-ground option works, something called the Reference location must be defined. For most waypoint missions, the Reference location is the ground under waypoint 1. However, when using Litchi's Mission Hub to plan a waypoint mission, one is able to specify the anticipated take-off location. If this take-off location is specified, it becomes the Reference location instead of waypoint 1.
Using this information, Litchi estimates above-ground altitude for waypoints under the assumption that the elevation at the reference location matches the elevation at the drone's power-on location. Therefore, it is recommended to set Waypoint 1 at or near the drone's power-on location for optimal accuracy. Since the power-on location is typically the same as the take-off location, these terms are often used interchangeably. The terrain elevation under every other waypoint will be compared to the terrain elevation at the reference location. Any difference will be applied to the specified above-ground height for all waypoints in the mission.
Above Ground Scenarios
Above Ground NOT selected: When the "Above Ground" option is not selected, all waypoint altitudes are referenced relative to the take-off location. For example, if the drone takes off from a valley and flies over a hill with all waypoints set to 100 feet, the drone will maintain an altitude of 100 feet relative to the take-off point. However, due to the change in terrain elevation, the actual height of the drone above the hill may be only 50 feet, as illustrated in the accompanying figure.
Above Ground checked and Take-off location same as Reference Location: When the "Above Ground" option is enabled, Litchi utilizes elevation data to compare the ground elevation at each waypoint to that of the reference location, adjusting the altitude accordingly to maintain the specified above-ground height. However, for this feature to function correctly, the reference location must be located at the same ground elevation as the actual take-off location.
This requirement exists because DJI drones measure altitude relative to the take-off elevation using barometric pressure. Litchi's elevation data is only used to calculate relative differences between waypoints, with all adjustments based on the elevation of the reference location.
Above Ground checked and Take-off location NOT the same as Reference Location: When the "Above Ground" option is enabled and Waypoint 1 is set to an altitude of 100 feet, Litchi will display the height as "100 (100)". However, if the drone takes off from a location where the ground elevation is 40 feet lower than the elevation at the reference location, the drone will still ascend to 100 feet relative to the take-off location. This means the actual altitude above the ground at Waypoint 1 will only be 60 feet instead of the expected 100 feet.
As a result, the entire mission will be 40 feet lower than intended, which could lead to unexpected altitude discrepancies. To ensure accurate waypoint altitudes, it is essential to take off from a location with the same ground elevation as the reference location.
Above Ground Heights are Only Computed at Waypoints: It is important to note that above-ground height calculations are only applied at waypoints. Therefore, when planning a mission in areas with significant elevation changes, waypoints should be strategically placed at both high and low points in the terrain to ensure altitude adjustments accurately reflect the varying ground elevations.