I am flying waypoint missions where the height above the ground needs to be accurately controlled / known. I can achieve this using the ‘Above Ground’ altitude mode and uploading a DEM in the Litchi planner when the takeoff location is directly below the first waypoint (or when the ground below the first waypoint is at the same altitude above sea level as the takeoff location.
However often my missions are in areas with steep and challenging terrain, and there is no guarantee that it will be possible to get to the first waypoint, or find another takeoff location that is at the same elevation as the ground beneath the first waypoint.
From looking through the forum, it seems like many other people running into this issue.
I understand that drones measure their height using barometric pressure, and therefore they can only measure height relative to the location of take-off (i,e. the homepoint). However, in this scenario, when a DEM is available and GPS is reliable, it is still possible to accurately estimate height above the ground at any point in the mission, for any takeoff location, as follows:
- Use the DEM to get the altitude above sea level at the moment of takeoff (let’s call this DEM_takeoff). This cannot be determined until the homepoint has been set, i.e. it is not known at the mission planning stage.
- The altitude above sea level of the drone at any waypoint can then be calculated as:
altitude_waypoint = DEM_takeoff + height_waypoint
where height_waypoint is the relative height of the drone above the takeoff location (as measured and reported by the drone using it’s barometer). - The clearance (distance) of the drone from the ground at any waypoint is given by:
clearance_waypoint = altitude_waypoint - DEM_waypoint
where DEM_waypoint is the altitude of the ground above sea level at the waypoint, as determined from the DEM.
(I have used the terms ‘altitude’, ‘height’ and ‘clearance’ above to differentiate between the 3 different ways of referring to how high up the drone is)
In the waypoint mission (where ‘Above ground’ altitude mode has been selected), the user has indicated what clearance_waypoint should be at every waypoint. So, substituting and rearranging equations (2) and (3) above, to fly the mission in the way the user intended, Litchi needs to tell the drone to fly so that:
height_waypoint = clearance_waypoint + DEM_waypoint - DEM_takeoff
So my suggestion / request is: change the Litchi app so that it uses the DEM to determine the altitude above sea level at the homepoint (at the moment of takeoff), and then calculates what the drone-measured height needs to be at each waypoint, to achieve the ground clearance that the user said they wanted.
For backwards compatibility, this could be implemented as a new mode / option that can be selected in the mission planner.