Some notes on using Litchi

[OzoneVibe]

@milkmanchris

Probably needed the centre chimney as the POI on the way out and way back

Inserting an extra waypoint between 2 and 3 can help (using the insert button on the WP2 popup).

The way the drone interprets the information isn’t quite as you might think from using Litchi.
This is complex … I’ll try and explain …

The Mavic api knows not a thing about POIs … it only knows waypoints.

So - progressing between WP2 and WP3, it’s not trying to focus on POI1.

What it’s doing is using the camera heading and tilt at WP2 (to focus on POI1) and progressing those to what is set at WP3 (to focus at POI1) … at a constant rate of angular change between the two.

Now - we all know that as you approach something that’s a long way off, the angle to it changes very little initially, but as we pass close by that angle changes really fast … and this is NOT how the MP works.

In fact, I’d probably insert a WP after WP2, and then insert another one after that new one … and might do the same after 5 (which would be 7 after the two additions - creating 8), and then insert another after 5. So - a mirror to the approaching idea but in the departing sense.

An IMPORTANT note re using the “insert” … since I see you are using heights above ground.

The newly inserted WPs are an exact half-way point between the two … in distance and in height. BUT (CAUTION!) it’s added as an absolute height, not an above ground height.

A quick example …

[OzoneVibe]

This may explain a little better … a picture paints a thousand words, etc.

What your POI will create is the 5° for WP1, and 90° for WP2.

At the mid-point what you need is 20°.

But the Mavic will turn at a constant rate so that, at the mid-point, it will be at 42.5° …. half way between 5° and 90° … and not pointing at the POI.

Edit: … and, my inserting method … you can see they keep the POI more central.

[OzoneVibe]

Oh - whilst I’m doing explanatory diagrams for Litchi - and in case anyone doesn’t see the potential danger of “inserting waypoints when using height above ground mode” …

Assume the 4 black waypoints exist and are set to x feet above the ground … and you realise you need to get over that hill and need a waypoint (green).
If you select the waypoint before the hill and click insert, you’ll get the real mid-point (red), and it won’t clear the hill.

[joe.k]

What say you want to go from way point 1 to 2 at 40kms/h and 2 to 3 at 30kms/h.
Which waypoints would you set with what speeds.
Dumb question ?

[OzoneVibe]

The speed set at a waypoint sets the speed for the following leg.

[joe.k]

Thanks .

[OzoneVibe]

This applies to camera pitch, too, as I was reminded today when I forgot. Duh!

See text above for explanation.

[OzoneVibe]

Another thing I discovered yesterday … if you’re contemplating significant distances … there is a limit as to how far apart waypoints can be.

Annoyingly, at the planning stage (phone app or web ui) it doesn’t warn you. It’s only when you go to upload the mission to the drone that you get a message and it won’t upload.

Yes, you can “quickly insert an additional waypoint” in the leg that’s too long … but beware of all of the issues from doing this without thinking it through.
The inserted waypoint will adopt the default set in your settings, it won’t (necessarily) relate to the two it sits between correctly.
Check ALL the settings of an inserted waypoint to ensure its doing what you would want at the mid point of the leg.

So, how far between two waypoints is to far?
Specifically I don’t know.
What I do know is 3km is too far, and 1.5km is OK.

[OzoneVibe]

Can we try and keep this thread to useful notes on using, and the pitfalls of using, Litchi … and any other general discussions over on …

Litchi chit-chat … What’s Litchi? Should I use Litchi?

Thanks!

[OzoneVibe]

@OzoneVibe

Believe it or not, the Spitbank Fort off Southsea is about 30m out of position on Google Earth.

OK - that had been my assessment by flying over it, back in December, and deciding for myself exactly where it was … because my orbiting video showed it not to be where Google Earth (Litchi Mission Hub) was telling me it was.

Then - following @milkmanchris reminding me of MaritimeTraffic … I investigated further - and things are strange.

This is what Spitbank Fort looks like ….

( me! )

On that you can clearly see the lighthouse, and you can clearly see the name that helps to orient the fort in the Google Earth (Litchi Mission Hub) images in a tic.

MaritimeTraffic has the lighthouse that’s on top of Spitbank Fort :

If there’s one thing that I know one can really trust in the UK- and that’s the positioning data provided by Trinity House of Lithghouses. So - that Lat./Long. will be correct!

The MaritimeTraffic “map” suggests that the lighthouse is on the south-west side of the fort.

Google Earth shows the correct orientation of the fort (I know - I’ve flown it!)

… and WP1 is the position using the coordinates from MaritimeTraffic (Trinity House) … again, showing it to be on the south-west side.

BUT … WP2 is where the red-and-white lighthouse is actually is located. (Refer to my pic above and the lettering for orientation.)

Those two positions are 26.8m (close to my 30 … considering I was 100m up when I did an accurate position recce) and, in Google Earth, you can also see a second “image” of the fort that’s a bit further to the east.

So - some things in Google Earth may be 30m (or more?) out … so, take care when flying real cosy Litchi flights. A recce flight higher/wider of any objects might be a good policy!

Note: That the MartimeTraffic map also suggests that the lighthouse is on the south-west of the fort would suggest that the underlying map data they use is also Google based.

[Sunseeker]

Hi, I have completed my first Litchi mission, it was just a small basic one and that went ok, now I prepared a 2nd which I inserted a POI but unable to get the WP’s to point at POI as you can see from screen shot it is set to point at POI one but they all just follow the flight path, I have saved and opened on all devices and still shows the same.

Also when I export as KML 3d nothing happens?

I have browsed through this thread but I don’t think anything covers this issue.

So do you have any ideas am I missing something?

Cheers

[OzoneVibe]

Probably mission settings ….

[Sunseeker]

Thanks sorted just a simple setting change and all WP’s point to POI

[OzoneVibe]

Mission Take-off Location / WP Heights / "Relative to Ground"

Something that many Litchi users don’t fully appreciate is that the Waypoint information sent to your drone by Litchi have their heights set relative to take-off … even if you use the “relative to ground” in the planning.

For both Waypoints and POIs, the DJI API knows only about latitude, longitude, height relative to takeoff (and speed, heading, camera tilt, etc for WPs … but that’s not relevant to these notes).
So - when planning “relative to ground”, it’s important to understand that your drone has, and will have, absolutely no knowledge of ground heights.

What Litchi does is turns all mission info into those three values - latitude, longitude and height relative to take-off before sending it to the drone.

So, if you’re setting up a mission at home, try and set WP1 immediately above where you anticipate to take off. This not only acts as a really useful reminder as to where that was, when you get to the location, this also minimises the chance of any WP altitude errors compared to those you intended.

If you get to the flying location and find you’ll be taking off from somewhere different, try to find somewhere that’s at the same height as you’d planned … otherwise, find somewhere higher than you’d planned to take off - since all WP heights are relative to take-off, all heights will be elevated compared to the initial plan made at home - which is less likely to cause a problem as them all being lower than planned.

Consider a mission that’s planned to fly at 50ft above the land that’s adjacent to a cliff - and the plan was to take off from the top of the cliff.
You get there and find you have no access to the land above the cliff and decide to take off from the beach at the bottom of that 100ft cliff …. I think you’ll see the problem with everything being relative to take-off!

If you are setting a mission’s WPs in heights relative to the ground - things get a lot more complicated if you actually take off from a different location. If your mission has large altitude variations due to the terrain, taking off from an unplanned location really is best avoided until you fully understand the ramifications.
(This one bends my brain inside out every time it happens … especially since my drone-phone doesn’t (usually) have data for Litchi to access Google’s data to do all the re-calculations … and I’ve been flying in places that don’t have phone data in any case.).