Litchi's "Above Ground" Option
DJI Drone Altitude Measurement
DJI drones determine their altitude primarily through barometric pressure readings. The drone records the atmospheric pressure at the power-on 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 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.
- No Real-Time Ground Reference: The drone lacks a continuous way to verify its actual height above terrain, making barometric altitude subject to drift over time.
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). They are primarily used for precision landing and obstacle avoidance but do not contribute to the altitude readings displayed in the DJI app.
- 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 Mission Hub is pre-mapped and does not account for real-time changes such as construction, vegetation growth, or obstacles.
Using this information, Litchi estimates above-ground altitude for waypoints under the assumption that the elevation at Waypoint 1 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 used interchangeably.
Above Ground Scenarios
Above Ground NOT checked: 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 Waypoint 1: When the "Above Ground" option is enabled, Litchi utilizes elevation data to compare the ground elevation at each waypoint to that of Waypoint 1, adjusting the altitude accordingly to maintain the specified above-ground height. However, for this feature to function correctly, Waypoint 1 must be located at the same ground elevation as the 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 Waypoint 1.
Above Ground checked and Take-off location NOT the same as Waypoint 1: 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 Waypoint 1, 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 Waypoint 1.
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.