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GPS Litchi accuracy ?

Discussion in 'General Discussion' started by Jon Sales, Sep 25, 2016.

  1. Jon Sales

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    Hi all, i've been trying to complete a job that requires flying towards a sign on a building. The flight starts from approx 40 metres high and 40 metres distance from the sign and going down to 5 metres high where it stops at around 2 metres from the sign. So quite a steep descent needing the camera to be pointing at the sign throughout the shot.

    I've tried doing this flying manually but can't seem to get it quite right. Reversing the shot would be the easier option but I have someone walking into the building and they would have to walk backwards for this to work. Has anyone had any experience of using Litchi to perfect this type of shot? I've pre-planned the mission on Litchi but i'm concerned that the Inspire will not stop 2 metres from the sign.(I also have to fly over some 10 to 15 metre high trees on the way in). I know GPS accuracy can vary for a whole load of reasons so to mitigate this slightly, would using a Bad Elf pro 2200 instead of the iPad pro GPS improve the accuracy of my planned mission? By the way the location is not somewhere i can pop back and forth to very easily for trial and error testing.

    Many thanks if you can help with some advice on this.

    Jon
     
  2. The Editor

    The Editor Moderator
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    I am sure many will chime in here but you have already hit the nail on the head when you quite rightly say GPS accuracy is not that great (not for civilian GPS anyway). DJI only give horizontal accuracy rated at +/- 2.5m which can easily change depending on HDOP/VDOP at the time of flight etc.
    Personally for this kind of accuracy I would never attempt it using any kind of waypoint flying. You also do not say what speed you will be approaching the sign so wind and braking distance become another factor!

    The best way to approach this (IMO) would be to fly manually and as you already touched on, do it is reverse.
    Have the talent practice, walking backward naturally as they can and do the take over and over again until it's right.
    You will also get a smoother track shot flying backward with a smooth deceleration effect doing it this way.

    Hope that helps. :)
     
  3. Jon Sales

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    Hi Ed,

    many thanks for the feedback. I think you're right but I do have a Bad Elf pro 2200 though and this doesn't use the cell network for
    getting it's position (i believe), so wondered if I could improve my chances of getting better accuracy with this? The speed throughout
    the flight is set to 2.2Kmh so I can speed ramp in post and should give me enough reaction time if I have to abort at any point.
    Walking backwards to make it look as though you were actually walking forwards is quite hard to pull off! :)
     
  4. Eduardo

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    Have you tried Litchi Focus Mode (or even Magic Leash), so you try to handle the flight while Litchi tries to keep the POI focused?

    The flight you described seems to be a 45 degree ramp, you can practice it somewhere else. I wish the App user interface could be a lot better for manual flight, like the option to have big dials showing vertical and horizontal speed (in your case both should be almos equal).
     
  5. Jon Sales

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    Hi Eduardo, thanks for replying. I like the sound of Focus Mode and think that could work well. So basically I only have to worry about maintaining a good 45º slope when coming in towards the sign and Litchi should keep the sign centre of frame.

    I'll give it a go.

    Many thanks. Jon
     
  6. Eduardo

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    Good luck ;)
     
  7. Meta4

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    The GPS in the Inspire which is the only GPS that matters for what you want.
    Even if using the Bad Elf did provide a more accurate position for your Your iPad that would not make any difference to your Inspire.
     
  8. Igor88

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    Litchi is very very accurate in gps flying. Do not draw the path in your ipad but record the WP with "C" button (only 2 in you case).
    First fly a straight horizontal line for some 40-50m there and back (for the best results with gps, compass, baro). Record WP. Dont switch off (change) the battery. Set the desired speed and control it with right stick. You cannot miss.
    If inspire is not descending fast enough, lower your speed.

    Camera. You can record ("C") the camera pitch yaw. You can use POI. And you can control camera manually.
     
    #8 Igor88, Sep 28, 2016
    Last edited: Sep 28, 2016
  9. The Editor

    The Editor Moderator
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    No - GPS accuracy is only as good as GPS (civilian) is.
    This can vary a lot depending on where on the horizon the satellites are and HDOP/VDOP dilution.
    The app has nothing to do with accuracy and should not be relied upon for very close flying.
    GPS can easily be +/- a few meters.
     
  10. Tim Cameron

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    Just for the record agricultural GPS is within +/- inches, rather than meters, but extremely expensive.
    I am not talking drone, talking tractors,Sprayers, etc.
    BTW I agree with the editor, manual flying is the best I can think of. However the new one from DJI seems to be, (read advertised) much more accurate.
    Cheers,
    Z
     
  11. Igor88

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    Here you are. Recorded WP. So waypoints flight.
    very very accurate.

    RC Groups - View Single Post - Litchi vs. Autopilot
     
    #11 Igor88, Sep 29, 2016
    Last edited: Sep 29, 2016
  12. The Editor

    The Editor Moderator
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    You are not understanding how GPS actually works.
    Yes, it might be accurate for several flights but the HDOP/VDOP values will change on any given time or constellation configuration. Depending on how high above the horizon a number of sats are will greatly alter the accuracy.
    There are other factors to take into account but GPS is an aid to flight and should not be relied upon emphatically.
    If
    Incidentally, flying a waypoint mission like that relying on GPS under foliage cover is asking for trouble. Lose a few satellites for a few seconds and your accuracy has dropped to 10's of meters!
     
  13. huppe

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    Wow!!
     
  14. Igor88

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    Yes. You are right. That was unnecessary and rude. I corrected that.
     
    #14 Igor88, Sep 29, 2016
    Last edited: Sep 29, 2016
  15. huppe

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    Ok Igor.Thanks for that.
    As you said this is a forum and I believe that opinions can be made.But I like this forum because of that,despite if you like it(I don't mean this personal;)).But generaly at this forum there is always a kind of respect.That's why I wowed.
    And especially because Editor is one of the people who put a lot of effort here.Sure he's not always right,but he's the last to say he is.
    As for the question from Jon,I use Litchi too,but never trust an app to do the work.It's a help and nothing more.In case you lose satelite's your in trouble with all the bullshit that follows.
    And because Jon will fly near a building and people you have to trust your skills.With any accident there will be media attention.
    So Jon if you can do this job with a cameraman and you do the flying tthat's the best way.Good luck
     
    #15 huppe, Sep 29, 2016
    Last edited: Sep 29, 2016
  16. The Editor

    The Editor Moderator
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    OK, I'm going to let the attitude go because I'm in a mellow mood. However, I have looked over your edited versions of the post and I am glad you decided to change the posted version otherwise you would be looking at a warning for talking to a moderator like that. It wouldn't be tolerated on other forums and it will not on here.
    If I see that a post or thread is factually incorrect I comment on it. I'm afraid your post was factually inaccurate in its information saying that Litchi is accurate in waypoint mode.
    It is only as accurate as the GPS data at the time and this can change.
    Flying under a canopy of trees can and will cause disruption to both Navstar and Glonass reception and this will cause positional inaccuracies which can easily go to several meters.
    If I see a post that is wrong I will always call it since many others who read a post may rely on that information which later causes them to crash or hover out of place or drift into an obstacle. They would then come back and say "But you said Litchi was very accurate and I drifted into a tree"

    Yes, waypoint flying using the Inspire (via any third party app) is reasonably reliable but, the positional accuracy of the Inspire is only +/- 2.5m laterally and +/-0.5 meters vertically (worse in barometric fluctuating conditions)

    I hope that makes the situation clear.
     
  17. Igor88

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    Yes sure, opinions are welcomed. The thing is. I didn't express my opinion. I made a statement (claimed) something. And before I stated the thing – I did tried it. Tested it. Several times, several positions. And I'm confident in my statement. Even show you a video. Now his answer was “NO”, “You don’t know…GPS”. It was not like “I think you are wrong”. It was a statement, not opinion. Saying what? That I don't know what I'm talking about??

    I wasn't aware he is editor. Well. Editor or not. My post is definitely not the “nicest” one. Not even polite one. Written definitely without the respect. So I will delete it. Although he deserves a chance to read it.

    Considering Jon. My guess is, if you are confident to fly near building, city centum, people….. You've already learned how to fly. Those apps are “help” only. But sometimes (many times) you cannot be so precise pilot and fly the route exactly, as those app can.
     
  18. Igor88

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    Yes. You should be careful with third party app. They are not all the same. Behaviour of each app is actually very very different. What is valid for Litchi is not valid for lets say autopilot. etc...
    Everything I said about precision was only in case of Litchi, not any other app.

    and yes. do not fly WP under a canopy of trees. or any other place with poor satelite reception. I did it for the testing reasons only. and do not use any app for work, before you get to know it.
     
    #18 Igor88, Sep 29, 2016
    Last edited: Sep 29, 2016
  19. The Editor

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    Thank you for clearing that up - appreciated.
     
  20. Jon Sales

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    Hi all, many thanks for your comments and sorry that it caused a bit of a Barney! But all good and generally constructive food for thought
    from all.
    I'm happy to report that I nailed the shot this morning and sending to my client now so fingers crossed they feel the same.
    In the end I decided that Eduardo's option of using Litchi with focus would work best. I flew single op and the focus point tracked perfectly
    so i'm very happy, I think this "assisted" single op method is great for getting the trickier shots when it has to be spot on and you're not
    relying totally on the accuracy of the GPS.

    Thank you to the Ed and all the forum members for making this such an invaluable platform for advice and information.
    Hope I can help out others one day too.

    Cheers,

    Jon
     
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