Litchi altitude question

[Sailingfool59]

Greetings!

I'm just getting going with waypoint mission planning on litchi. My question is when I set the altitude at a waypoint (say 20'), is that 20' above the ground, or is it 20' above the home point? Is altitude determined via the avoidance or sonar on the Phantom 4?

I'm filming a condominium development that is on a hillside. My home point will be the top of the hill overlooking the condos, and I'll decend below the home point for several of the waypoints to get the best possible framing. I'm a little nervous to set the waypoint altitudes at a negative value, as I'm afraid of flying into the ground, but I need the low angle to compose the shot.

Any suggestions?

Thanks in advance,

JC

 

[Meta4]

I'm just getting going with waypoint mission planning on litchi. My question is when I set the altitude at a waypoint (say 20'), is that 20' above the ground, or is it 20' above the home point? Is altitude determined via the avoidance or sonar on the Phantom 4?

The only height your Phantom understands is Home = Zero.
Every other height is related to home.

 

[Sagebrush]

I'd do it this way:

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It works well, but I'd suggest you set it up very conservatively, plan on flying a second or third mission after you've adjusted it, and do it only in line of sight.

SB

 

[matti]

I wish Litchi Mission hub had an option to automatically add/subtract ground elevation to/from the desired AC altitude, so that manual error-prone calculation at each waypoint or a de-tour to Google Eart was not needed.

Google Earth elevation precision:

In many areas Google Earth uses SRTM (Shuttle Radar Topographic Mapping) data, which offers something like 16m vertical accuracy at 90m horizontal grid spacing. Google Earth doesn't use exclusively SRTM data, and some places augment it with higher-resolution datasets.

Individual points are fairly accurate in most cases where the terrain is fairly flat. In mountainous terrain the elevation data is less accurate because the point you are evaluating is not directly on a measured sample point and Google Earth is interpolating between measured points. If you are measuring a road that is in a canyon with steep walls, you can have some variations from point to point.

Google Groups

http://dds.cr.usgs.gov/srtm/version2_1/Documentation/MIL-PDF-89020B.pdf

 

[matti]

BTW in the links above the Google Groups' 2nd reply mentions 30 m vertical accuracy while the PDF link says "The SRTM DTED Level 2 has a system design 16 meter absolute vertical height accuracy, 10 meters relative vertical height accuracy and 20 meter absolute horizontal circular accuracy. All accuracies are at the 90% level."

So is the vertical accuracy about max 16 meters or more?

 

[Sagebrush]

As I stated..."but I'd suggest you set it up very conservatively."

There are lots of reasons to to this. Altitude accuracy of Google Maps, your aircraft's altimeter accuracy, the fact that trees are at least a year older than the image Google is using, and the the presence of unseen power lines or other man-made objects that you haven't noticed in the images or have been added since Google obtained the them.

Start your first mission off slow and high. Keep your left trigger finger on the mode button to cancel the mission.

SB

 

[Sailingfool59]

I'd do it this way:

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It works well, but I'd suggest you set it up very conservatively, plan on flying a second or third mission after you've adjusted it, and do it only in line of sight.

SB

Thanks for the great link! I don't think this technique will work in this location, however, on account of google earth's lack of precision regarding altitude mentioned here in other replies...

 

[Sailingfool59]

Thanks so much! I figured it was true that altitude was relative to the home point. Now that I know more about how litchi works with google, I think the best bet is to pre fly the mission to get the waypoint altitudes in FPV , then build the waypoint mission.

 

[Sagebrush]

Thanks for the great link! I don't think this technique will work in this location, however, on account of google earth's lack of precision regarding altitude mentioned here in other replies...

It works or me in steep, rugged county, but when I'm flying a mission developed this way, my finger is on the trigger so to speak to shut it down and switch over to a manual flight.

The fist time I did one of these I flew it up a steep mountain road (that I could see) at 20' AGL and 3.5 MPH, and then across a steep draw (at 20'), back up the hill and home. Worked like a champ. Would I try it on a flight where I can't see the bird most of the time? Probably not.

SB