Hi,
I have a drone Mapping job coming up in the city area of downtown Boston Massachusetts USA requiring some GCPs. It is a small job but it is going to require some thinking and planning on my part. I would like to ask any operators out there about best techniques and practices of making and taking GCPs in a city setting. As an example what is the best practice to minimize the effects of buildings blocking or reflecting GNSS signals and yet maintaining an optimal layout for the GCPs? How do deal with and minimize problems with people taking or disturbing my equipment. Are there any best settings for city environment with an Emlid RS2+? I will be on some busy streets so making a GCP on the street itself may be impossible. Any caveats that people have come across operating in the city environment that you came across on the job that were not readily apparent? Thank you for any input and thoughts!
I’m using Metashape but I think the workflow should apply in general.
After the first stage of photo alignment I have the option to do a rigid transformation over control points. I also have the option to adjust camera parameters but I leave it to a later stage.
In urban areas there is plenty of reference points, I simply select something visible from the drone and in reasonably open areas. Manholes, road stripes, anything visible is doable.
I also measure 3-4 points that are not mistakable and evenly distributed (checkerboards, cross signs made with spray).
At this point I do a first rough registration over 3 unmistakable evenly distributed marks.
After that all the other control points are close to the feature, and I register them (with filter photos by markers I see the calculated position).
The trick is if you mark a manhole you cannot be sure which manhole is, but if you perform a first registration with only 3 (well known) points the error between control point and physical point will be in the 20-30 cm range, so you cannot be wrong.
If you add control points this way you refine the registration, and you can also update camera parameters calibration.
The process is very fast and easy, and in a urban environment solutions like checkerboards and similar are not doable.
Massimo
For Reach, as for any GNSS receiver, it may be challenging to obtain a stable FIX solution in urban areas, as it depends on the environmental conditions. But here are some tips for collecting the GCPs reliably.
Collect the points with Averaging instead of Instant collection. It’ll provide Reach with more data to calculate its position accurately.
The PDOP value should be under 2. You can monitor it on the Status screen in Emlid Flow.
The more satellites you have with a high SNR value, the better. Please ensure there are at least 5-7 satellites with an SNR greater than 35.
I hope it helps. But please feel free to share any experience from your project!
Be prepared to post process your control marks. If doing radio RTK make sure you log enough time for post processing. A minimum of 10 minutes, more if control marks are near buildings. Base receiver should be placed with no obstructions, perhaps on a roof if possible and also logging data. Keep your base-rover baselines at a minimum. This will help post processing if enough time is logged at rover. Another static receiver wouldn’t hurt either with a baseline at least as long as your project width. Logging data at all three receivers will give you confidence in your post processing results, IF you are using commercial post processing software as you will have three baselines for each point. This will give you a true mathematical error of your rover points unlike single baseline processors.
As for targets, there are multiple items to use, paint stripe corners, sidewalk seams with a paint mark, manholes as @studiodemarchi.net mentioned, fire hydrants, valve covers, etc.
Be prepared to purchase a business license for the city you are in. I would highly recommend this as first time offenders may have to pay a hefty fine as well as a year long license to go along with it.
If you do all of the above right the first time, it will save you many headaches.
My first recommendation is the use what’s there. Manholes, joints in concrete and striping are all great markers and you don’t have to worry about maintaining the network.
As far as placement you can only do what you can do. Your flight pattern will need to be configured to provide the best views possible. You only need 5-6 images per point to accurately geolocate and that hasn’t been much of an issue in my experience.
Regarding GNSS settings, I dial up my elevation mask to mitigate the “dirty” satellites on the horizon and more elevated paths that may be seriously hindered by structures. Last one I did had to be increased to 30% for me to get a fix.
Hey everybody, Thank you so much for your input. I truly appreciate you taking the time to answer my questions. Just wondering any input about security? With all the “Drone Hysteria” lately I want to avoid any confrontation and theft. Again Thank so much for your thoughts and time.
AirTag or similar in all your cases. Don’t let your base get out of your line of site (another reason to run local NTRIP) and hold your ground against drone Karens. You know the regulations and most of the public does not. Always have your registration and certificate available and be prepared to teach.
