Precision

[Thompsonrh]

I do a lot of drone photography & videography in construction, architecture and realty. Drone positioning is generally more artistic than precision. I was asked recently to take photographs from various heights and bearings for a future construction project Which I have done a number of time before. This is quite a common request but the site manager was a real detail man. So I provided the photographs with a spreadsheet with data taken from the logs (DJI MaVic 2 Pro), including straight down photographs showing the precise position over or between survey pegs. Then he started to quiz me on the precision of the data. I pointed out that the drone is not a survey tool and although it is remarkably precise I am not sure how precise. It’s clear from the map view that it is quite accurately positioned but I cannot state a tolerance on the numbers for lat/long/alt and bearing. Any thoughts please.

 

[Recoveryone]

Sounds like he was trying to figure out how not to pay a survey crew to come out and put in control points. I have not done survey work, but from my understanding (working in the industry) you need accurate control points first to setup from to do any what you could call accurate work with a drone.

 

[R Martin]

This is quite a common request but the site manager was a real detail man. So I provided the photographs with a spreadsheet with data taken from the logs (DJI MaVic 2 Pro), including straight down photographs showing the precise position over or between survey pegs. Then he started to quiz me on the precision of the data.

First thing, they are photographs. They may or may not be geotagged but unless he specifically asked you to provide geoinformation in the contract, then they are just photograph with no implied precision.

If it was precision he wanted, that should have been ironed out before the flight was made. If it was spelled out in the agreement/contract, THEN it would have been up to you to provide some degree of accuracy and proof of accuracy. That would have been a mapping service with precise geoinformation tying the images/orthomosaic to specific points.

 

[Thompsonrh]

I fully understand the business & contractual aspects. I’ve been in the photography business for many years. The question was purely technical and I was curious if anyone had ever stipulated a tolerance on the lat, long, altitude values for their drone or if a manufacturer, specifically DJI for the Mavic 2 Pro had provided answers to such questions.

 

[R Martin]

I fully understand the business & contractual aspects. I’ve been in the photography business for many years. The question was purely technical and I was curious if anyone had ever stipulated a tolerance on the lat, long, altitude values for their drone or if a manufacturer, specifically DJI for the Mavic 2 Pro had provided answers to such questions.

We state 1/2"' horizontal accuracy (and meet or exceed that) but I do this for a living with a capable platform. The Mavic's GPS is alright but without setting control there is no way of gauging accuracy. Based upon my Inspire and field testing, unaided and without control points the DJI GPS unit on board the aircraft was accurate to within about 5 feet with a shift to the northwest. And again, without control points established I doubt you are going to get any better unless you are using an RTK/PPK equipped UAS. Hope that answers your question.

 

[Thompsonrh]

Thanks R. You lost me with RTK/PPK but I see a good article (What’s the difference between PPK and RTK drones, and which one is better? | Wingtra) that will educate me on the subject. If the drone is launched from a point established by a Surveyor and then flown say 30' left and right and say 30' altitude then it is like an aircraft using differential GPS to land at an airport substantially improving the accuracy of GPS. The basic GPS errors are eliminated and then it comes down to the precision of the drone computing its position relative to a known quality. My own sense is that this would be more accurate than ½" but I really have no spec. or info. to support that.
I fly the Mavic 2 Pro most of the time because in Canada its the only drone I have that can be flown in Class C or CZ airspace. However I have 2 Inspire 1 Pro's that I occasionally use in Class G airspace. In fact the Mavic has better video capabilities with over 100 mbps which meets the min requirements for TV. I have been flying drones professionally since 2012 (Phantom 1) as an adjunct to my photography business. Thanks again for your feedback.

 

[R.Perry]

The site I was working on I worked with the surveyors at times, mapping with Drone Deploy, using an Inspire 2 my accuracy was about six inches vertically and six to eight horizontally. In essence it wasn't accurate enough for the surveyors.

 

[Thompsonrh]

Thanks

 

[R Martin]

Thanks R. You lost me with RTK/PPK but I see a good article.

With RTK you are connected to a service and the GPS positions are corrected in real time. I prefer this method as there is less work on the back end and I don't have to post-process the GPS data. You can either subscribe to a service (we use Trimble's) or you can set up your own base station on a surveyed point that you have accurate known values for (we have the ability to do this as well but it is more time consuming).[/QUOTE]

 

[Thompsonrh]

Thanks