Why Does Litchi...?
Waypoint Missions
When a POI is associated with a waypoint, Litchi sets the appropriate heading at that waypoint to point at the POI. In a mission where one POI is associated with two adjacent waypoints, the heading at each waypoint is set to point at the POI. This is easily seen in Litchi's Mission Hub. As the drone flies from the first waypoint to the next, the heading is linearly interpolated from the value defined at the first waypoint to the value defined at the next waypoint. This works well for cases where the POI is a similar distance from both waypoints. However, in certain cases, this method yields less than perfect results.
In extreme cases, where two waypoints are associated with the same POI and one waypoint is much closer to that POI than the other, the interpolated heading will cause the view of the drone to wander off of the POI as the drone travels from one waypoint to the next. In the photo to the right, the drone will correctly point at the POI at waypoints 1 and 2. However, midway between the two waypoints, the interpolated heading will not point at the POI. The solution is doing either of the following:
- Position waypoints pointing to the same POI roughly equal distances from that POI.
- Insert additional waypoints between waypoints 1 and 2 to help keep the focus on the POI
There is an exception to what is described above. "Hotpoint Mode" is used if all of the following conditions are met:
- The drone has native waypoints supported in the firmware (Mavic 2 Pro/Zoom and older).
- The mission has only one POI.
- All waypoints in the mission are associated with that one POI.
If the above three conditions are true, the DJI firmware employs something called "Hotpoint Mode" where the firmware does all of the heading calculations to keep the drone pointed at the POI. References:
When a POI is associated with a waypoint and the Gimbal Pitch is set to "Focus POI", Litchi sets the appropriate gimbal pitch angles at that waypoint to point at the POI. In a mission where one POI is associated with two adjacent waypoints, the gimbal pitch angle at each waypoint is set to point at the POI. This is easily seen in Litchi's Mission Hub. As the drone flies from the first waypoint to the next, the pitch angle is linearly interpolated from the value defined at the first waypoint to the value defined at the next waypoint. This works well for most cases. However, in extreme cases where the path between waypoints goes over the POI being tracked, the POI will not be tracked properly.
In this case, where two waypoints are associated with the same POI and the path between these waypoints goes over the POI, the interpolated gimbal pitch will not point down while over the POI between the two waypoints. In the photo to the right, the drone will correctly point at the POI at waypoints 1 and 2 where, in this example the gimbal pitch angle at each waypoint is -36 degrees. However, midway between the two waypoints, the interpolated gimbal pitch angle will still be -36 degrees and not point down at the POI. The solution is to do the following:
- Insert an additional waypoint between waypoints 1 and 2 near the POI to keep the focus on the POI.
There is an exception to what is described above. "Hotpoint Mode" is used if all of the following conditions are met:
- The drone has native waypoints supported in the firmware (Mavic 2 Pro/Zoom and older).
- The mission has only one POI.
- All waypoints in the mission are associated with that one POI.
If the above three conditions are true, the DJI firmware employs something called "Hotpoint Mode" where the firmware does all of the gimbal pitch calculations to keep the drone pointed at the POI. References:
There are two choices for "Path Mode" in Litchi's waypoint settings:
- Straight Lines
- Curved Turns
When "Path Mode" is set to "Straight Lines", the mission can be divided up into multiple straight-line segments from one waypoint to the next. As the drone executes a straight-line mission, it pauses momentarily at the end of each line segment. When using straight line segments, it is trivial to trigger the execution of special "waypoint actions" at the beginning or end of each segment.
When "Path Mode" is set to "Curved Turns", the waypoints become control points of a B-Spline. In general, a B-Spline does not pass through its control points. As the drone executes a curved-turn mission, it does not pause at each waypoint. The use of curved turns does not provide an easy way to define a point on the path where an action should be executed. While it would be possible to continuously check the distance between the drone's current location and each waypoint as the mission is executed to see if that distance is at a minimum, there are too many cases where that logic would fail so it is not practical to do so.
It is not enough to set the curve size to zero at a waypoint. Even with a curve size of zero, the entire flight path is still constructed using a B-Spline with no easy way to detect when a location on the spline (flight path) is at a control point (waypoint).
Waypoint missions are created by specifying the GPS location of two or more waypoints. While a GPS location can be specified with great precision, the accuracy of flying to that GPS location is something less than perfect. Simply stating the accuracy for GPS can be difficult because there are many different types of GPS and many things that affect its accuracy. This is probably why one finds so many different GPS accuracy values quoted on-line.
Smartphones tend to use assisted GPS (A-GPS) which combines traditional GPS signals with cellular network data to increase the location accuracy. Therefore, smartphone position accuracy is generally better than that of devices using standard consumer GPS.
Like many consumer devices, DJI drones use what is called "Consumer GPS". Depending on your source, the accuracy of consumer GPS implementations is anywhere between ± 3 to 8 meters (10 - 26 feet). However, DJI states the horizontal positioning accuracy of their older drones to be around ± 1.5 m (Mavic 2, Air 2s) and their newer drones to be ± 0.5 m (Mini 3, Mini 4, Mavic 3). I have flown literally 100s of waypoint missions and DJI's stated accuracy corresponds well to what I have experienced when flying and repeating my waypoint missions. I have never experienced a waypoint mission to be anywhere near 8 meters (26 feet) off of the drone's intended course.
Web sites with good explanations of GPS
Mission Hub
DJI drones determine their height by comparing the barometric pressure at their current elevation to what it was at the take-off elevation. As a result, the height displayed in the app is relative to the take-off location as opposed to relative to the ground under the drone.
Above Ground NOT checked: When the "above ground" option is NOT checked, all waypoints heights are relative to the take-off location. If you take-off from a valley and fly over a hill, the waypoint height may be set to 100 feet but the drone height above the hill may only be 50 feet.
Above Ground checked and Take-off location same as Waypoint 1: When the "above ground" option is checked, Litchi will use elevation data to compare the ground elevation at each waypoint to the ground elevation at waypoint 1 and adjust as necessary so that the specified height is the "above ground" height. However, it is important to know that this will only work if waypoint 1 is at a location with the same ground elevation as the take-off location. This is because all height calculations in the drone are relative to take-off location. Ground elevation data is only used to compute relative differences in elevation from waypoint 1.
Above Ground checked and Take-off location NOT the same as Waypoint 1: If the "above ground" option is checked, and the height of waypoint 1 is configured to be 100 ft (for example), you will notice that Litchi displays the height as "100 (100)". However, if you take-off from a location where the ground elevation is 40 ft lower (for example) than that of the ground elevation below waypoint 1, the drone will still rise to 100 ft above the take-off location, But that will only be 60 ft above the ground at waypoint 1. In fact, the entire mission's height will be 40 ft lower than expected. This is why it is important to take-off from a location that is the same elevation as the ground under waypoint 1.