Need help understanding altitude numbers in LItchi waypoints

[vindibona1]

No, think @Effects have his thinking mostly correct ... the first number is the height you want the drone to be above ground (150ft) & the second (106ft) tells us that the ground elevation at that waypoint is 44ft lower than the first waypoint in the mission ... meaning that the height reading in Litchi during the execution of the mission will show 106ft at that waypoint ... = 150ft above ground. If the "Above ground" wasn't marked there, the AC would have been 194ft above ground.

Here below an example with 9 waypoints ... all set to 1m height. The mission goes uphill a small hill & I have marked "Above ground". During this mission the AC will be 1m above ground at every waypoint ... but at the last waypoint it's ascended in total to 24,3m above the first waypoint.

If I hadn't marked "Above ground" the AC had slammed into the ground just after the second waypoint.

View attachment 111024

Thanks. I think I see now. If I understand correctly, the mission is flying to the altitude listed as AGL, but adjusting for the rise or dip in the ground level (MSL). In other words, check the "Above Ground Level" box to make rise (or descend) additionally at the rate of terrain variation, flying AGL at the designated altitude. It wasn't clear before. And while I don't have the math down exactly, I think for me is just making sure I stay above obstacles- at least for now by checking AGL and making sure I know how tall stuff in the flight path is.

 

[Camino Ken]

I thought this discussion had run its course but I finally realized why the OP was initially confused. I just checked a previous mission and noticed that when I check the “above ground box” the altitude entered does not change but it adds a second number in parentheses that in my case is higher than the number I initially entered. Also, in my case the ground is actually higher than the takeoff elevation so I am assuming that when the mission is run it will fly to the altitude in parentheses which adds the increase in ground elevation to the initially set altitude.

Also, the little note that pops up says “Ground Elevation: 2710ft (29ft above first waypoint)“ which is confusing because it is wrong. What it means to say is ”29ft above the ground beneath the first waypoint“. If you change the altitude of the first waypoint this number does not change. It really is just telling you the ground change amount.

Very confusing.

 

[Jeff A]

Thanks. I think I see now. If I understand correctly, the mission is flying to the altitude listed as AGL, but adjusting for the rise or dip in the ground level (MSL). In other words, check the "Above Ground Level" box to make rise (or descend) additionally at the rate of terrain variation, flying AGL at the designated altitude. It wasn't clear before. And while I don't have the math down exactly, I think for me is just making sure I stay above obstacles- at least for now by checking AGL and making sure I know how tall stuff in the flight path is.

I would caution the Drone Pilot that this is not an actual "Terrain Follow" which, if I am not mistaken, used to be a feature of the Mavic Pro. If you are going to cross Hill A @ 50 feet AGL, you can set the altitude to do that IF your Waypoint is located over the highest point of Hill A. Let's say you put another Waypoint on Hill C and set your altitude to cross @ 50 feet AGL above Hill C. The problem is, if there is another hill between A & C (Hill B?) and that hill is 75 feet higher than A & C, you will possibly impact the intermediate hill. Just choose your Waypoint locations carefully. I use the Litchi Virtual Mission to review my planning and and use Google Earth to Survey the area to look carefully at the trees and wires. The trees are most always higher than you think.

The first time I used the "Above Ground" feature of Litchi I was filming 3.7 miles of fence in a remote area to ascertain the condition of the fence. It was a beautiful thing to see Litchi work over the uneven terrain that I had to deal with.

 

[Dave Maine]

Note also another way to get into trouble. If you set up your Litchi flight using the Mission Hub and your PC you must actually takeoff from that same location you defined virtually. If you do not, the drone height above ground will be incorrect by the amount your takeoff position altitude differs from your virtual takeoff height.

 

[Peio64270]

Anyway, the best is to verify everything is correct by using VLM before real life flight.

 

[Prismatic]

As I plan an upcoming mission in Litchi I need to understand exactly how high I am flying at every point, ...

Be aware that even the best digital geographic elevation data is rarely exact. The highest accuracy--on a flat landscape--can be 3m (15 ft) or more from the true elevation. In heavy terrain, it can be far worse : I've seen as much as a 40m (130ft) error amidst cliffs and spires.

I have also successfully used an altitude of 1m above ground on a mission, but only because at that location the digital elevation data is--I should say, happens to be--very close to correct. But I discovered that happy fact only through a very cautious preliminary test mission.

As a rule, you can not count on such high accuracy anywhere. If I can't do a test run, I keep it "conservatively high", an elevation that largely depends on the terrain, but is never below 5.5m (20ft). Obviously you must also consider trees, structures, power lines, and any other potential higher-than-grade obstacles.

 

[DanMan32]

You also have to consider the AC's own inaccuracies in altitude. It can drift as much as 10ft during flight.