Hello, I hope that this topic hasn't been beat to death.  I did a search and couldn't find the info I'm looking for but may have used incorrect terms.  I have working on starting a UAS program with my employer (Public Works Department) and have received my Part 107.  I am now looking into purchasing our first UAV system.  We would primarily be doing inspections, but I would like to expand as much as I can to encourage continued interest and possible growth of the program.  The obvious place to do that would be with mapping.

  

I am currently looking at craft with thermal imaging capabilities, like the Mavic 3t enterprise line, but I cannot figure out is it make sense for us to spend the extra money to get the RTK module.  I am still researching everything involved with RTK so I know VERY little but it appears to me that the RTK unit on the drone would be of little value without a base station.  Is this assessment correct?  I highly doubt I could talk my employer into an additional purchase of a base unit, at least not at this stage.  I would not be using the drone for true surveying, just 3D site mapping.

  

Should I spend the money on a RTK drone or use that money elsewhere?  Buying something that is not necessary could negatively impact the future expansion of the program.

  

Thanks for taking the time to read.

Hi,
The RTK Fix is required to perform georeference mapping with an accuracy of +/- 5cm. Use the RTK module without a base station will always be better than with the classic GNSS receiver.

There are extremely inexpensive and effective solutions. I describe one here:
Alternative to DRTK2 and NTRIP Provider | DJI FORUM

Count $1400 more for the base station and the RTK module.
Otherwise, if you have access to an NTRIP network, you only need the RTK module 700$, and the provider subscription if applicable.

The question you have to ask yourself is what is your concrete need?

Simple RGB Mapping: Mavic 2 Pro (used since its a discontinued product but still very efficient) / Mavic 3 E / Phantom 4 Pro
Geo-referenced RGB mapping: M300 + P1 or L1 / Phantom 4 RTK / Mavic 3 E / M (Depending on whether you need more zoom or multispectral images) + RTK module + base station or NTRIP provider
Simple Thermal mapping: Mavic 3 T
Geo-referenced Thermal mapping: M300 + H20T / Mavic 3 T + RTK module + base station or NTRIP provider.

In any case, don't forget that you also need photogrammetry software. Agisoft Metashape, PIX4D... Especially not Terra. About 3000$ for perpetual license. Be careful, all perpetual license doesn't mean support for life. Agisoft yes. Pix4D you have to pay about 20% of the licence each year to get update.

For thermal imaging you also need a "decoder" to transform the JPG into TIFF expressing the temperature in Celsius or Farenheit.
There is the DJI thermal SDK kit, free but to finish it yourself, you need some coding skills.
Otherwise there are some paid software that do it. without such software you won't output anything but random RGB rasters.

I'm only talking about DJI drones here, there are other brands including Autel which produces RGB and thermal sensors of equivalent quality. It's up to you to ask the resellers for tests, whether for software or hardware. Make sure the result meets your expectations before you buy.

Do not hesitate to come back here if you need more information. There are more and more active operators on the forum with different profiles and experiences.

First, thank you very much for taking the time to answer.  Very useful information.

I am actually leaning towards the Autel Evo Dual 640T (V3), do you have any experience with this particular UAV?  The EVO and Mavic seem to be very comparable with each other, however, the EVO is not geofenced.

As I understand your reply, I could get a RTK equipped unit and a NTRIP provider to fully make use of the RTK capabilities, and the NTRIP is basically a web based network of base station.  Is this correct?  I'm located in Norther KY, how would I find out if I would have access to a NTRIP provider?  

Lastly, I will continue my research but could you point me in a direction of a comprehensive 'RTK for dummies' resource that might get me further along with my understanding of how this system works (details)?

Thanks again

NDubz Posted at 2-27 11:43
First, thank you very much for taking the time to answer.  Very useful information.

I am actually leaning towards the Autel Evo Dual 640T (V3), do you have any experience with this particular UAV?  The EVO and Mavic seem to be very comparable with each other, however, the EVO is not geofenced.

No I don't have significant enough experience with autel evo to recommend or not. I will just say that I prefer the DJI interface.

What do you mean by geofenced?

Yes NTRIP (Network Transport of RTCM via Internet Protocol) RTCM (Radio Technical Commission for Maritime Services) is a real-time GNSS correction service via an internet connection.
https://www.rtcm.org/

You can try NTRIP with NGS or KYCORS. Both are free if I'm not mistaken.
https://maps.kytc.ky.gov/KYCORS/
https://www.ngs.noaa.gov/CORS/

But you can install your own temporary base station as I explain in the tutorial the link is in #2. Or even set up your own permanent base station. Very effective if you always operate in the same sector.
https://learn.sparkfun.com/tutor ... s-reference-station

Here you will find a fairly detailed explanation.
https://www.ardusimple.com/rtk-explained/

To make it short, it is considered that two GNSS receivers located less than 35 - 30 km from each other, have the same error.
Therefore if you know perfectly the position of one of the receivers (base station), you are able to quantify this error and send it via a network (NTRIP) to the other receiver (Rover) so that it applies this correction calculated by the base station. This is RTK Real-Time-Kinematic.
Please note that the accuracy of the error calculation decreases with distance. The further away you are from the base station, the less accurate it is. 30-35km is good for a "fixed" rover like a surveyor's antenna which is used to pick up points. For a drone moving at a certain speed, this degrades quickly. For photogrammetry, the software is powerful enough to accept fairly large errors. For a LiDAR it's another story.


There is another method, more precise because it consists in removing the travel time of the information via the network. This is the PPK (Post-Processing-Kinematic) The error/correction calculation method is the same as for the RTK. The difference is that we save these corrections in a memory, server, SD card... Then we apply them later to the rover data which are also saved.

LV_Forestry Posted at 2-27 12:35
No I don't have significant enough experience with autel evo to recommend or not. I will just say that I prefer the DJI interface.

What do you mean by geofenced?

I honestly can't thank you enough for the detailed information.

What I mean by geofenced (or geo locked) is that DJI requires you to take the additional step of obtaining approval from them to fly within restricted airspace.  Unfortunately a very large portion of my 'service area' is within the class B airspace of an international airport.  I have nothing bad to say about the way DJI performs.  I personally own one but it is just an extra hassle that I do not want to have to deal with if something comes up that is unplanned.  I have more trouble with DJI approval than FAA approvals.  I honestly wanted to see that DJI just really stood out compared to Autel but the zoom is the only thing that comes to mind as I am typing this.

Thank again, I'm sure I'll have more questions as I dive deeper and it is great to know you are willing to take your time to help.

NDubz Posted at 2-28 03:50
I honestly can't thank you enough for the detailed information.

What I mean by geofenced (or geo locked) is that DJI requires you to take the additional step of obtaining approval from them to fly within restricted airspace.  Unfortunately a very large portion of my 'service area' is within the class B airspace of an international airport.  I have nothing bad to say about the way DJI performs.  I personally own one but it is just an extra hassle that I do not want to have to deal with if something comes up that is unplanned.  I have more trouble with DJI approval than FAA approvals.  I honestly wanted to see that DJI just really stood out compared to Autel but the zoom is the only thing that comes to mind as I am typing this.

Yes it is true that this restriction system is painful. However, you can write to them to find out if it's possible to have an extended license so that you don't have to unlock areas too often. I don't know if this is possible, but it's worth asking or suggesting it to them.

flysafe@dji.com