I have been asked to propose on providing surveying services to prepare FAA !A Accuracy Certification letters for a bunch of sites that are in urban canyons in a large metropolitan area. I've done many such surveys in rural and suburban environments, with their own challenges, but never any where multi story buildings block the sky and hinder cell phone reception.
I have access to a solid RTN station a few miles away which limits me to GPS only. If I use my own base, I get the benefit of Glonass signals at a much quicker epoch rate. I am concerned that my radio is going to struggle over a two mile radius because of the structures that occupy virtually every square inch of land that isn't public right-of-way.
I guess that I could go test it first, but I avoid downtown like the plague.
I would appreciate any words of wisdom or caveats when working in this environment from anyone that has some thoughts.
Thanks,
JA, PLS SoCal
I only have a very little to say. MISSION PLANNING. And, a Javad LS.
Nate
Rooftops?
Get at least three GPS sessions in the most open areas available in the vicinity and traverse to the points in question?
I will add, do 3 occupations, of each point. This will give you a very good look at it.
Then, come on this forum, and post your results.
🙂
N
1A letters aren't really that tight +-20 ft Hz and +-3ft V, you can always run the elevations in from a good GPS shot..
As much as some people want to do only GPS nowadays, this sounds like a job where a total station would be very useful in addition to GPS.
Can you see the tallest features from outside the urban canyon? If so, set GPS control points outside the area and turn angles to triangulate the location for horizontal and run levels (if there are BM close by) to the building base to determine the vertical. Calculate the height difference from the level run and the angles from your new control. In reality, Google Earth and Google Maps, can probably get you close enough for the horizontal accuracy, the vertical accuracy is the tricky part. Hopefully, there isn't any small antenna that you can only see up close.
Depending on your receivers and their configuration you can use TCP corrections instead of UHF. I also agree that if you are going to try and do it all with GPS I would definitely suggest 3 separate occupations.
I did a job in Baltimore City. We used our Trimble R8 to burn several checks in the morning on known control. Then set our control. Did the same in the afternoon. We could not get a fix half the time. And the worst part is we could have ran traverse and levels and saved a lot of time.
Worst job ever. I love the project managers that just want you to make it work.
John Hamilton, post: 394911, member: 640 wrote: As much as some people want to do only GPS nowadays, this sounds like a job where a total station would be very useful in addition to GPS.
I agree with you 100% John!
The temptation to push GPS/GNSS into questionable situations is something that we ALL are guilty of at one time or another. Like you have said before, a "bad fix" is WORSE than no fix at all (or something to that effect).
Loyal
A bad fix is worse than no fix. Right on, Loyal!
If it was me I think I'd try to gain access to some roof tops with clear unobstructed views of the sky and static in a few intervisible points that could be post processed through OPUS to get some reasonably good elevations and grid coordinates, then go conventional coming off of those points to transfer your control into 'the canyons' to do your 1A work.
Jerry Attrick, post: 394875, member: 1585 wrote: I have been asked to propose on providing surveying services to prepare FAA !A Accuracy Certification letters for a bunch of sites that are in urban canyons in a large metropolitan area. I've done many such surveys in rural and suburban environments, with their own challenges, but never any where multi story buildings block the sky and hinder cell phone reception.
I have access to a solid RTN station a few miles away which limits me to GPS only. If I use my own base, I get the benefit of Glonass signals at a much quicker epoch rate. I am concerned that my radio is going to struggle over a two mile radius because of the structures that occupy virtually every square inch of land that isn't public right-of-way.
I guess that I could go test it first, but I avoid downtown like the plague.
I would appreciate any words of wisdom or caveats when working in this environment from anyone that has some thoughts.
Thanks,
JA, PLS SoCal
RTK is very chancy in urban canyon, RTN is not much better.
I have done some and the best bet is to locate GPS at intersections to get at least 4 viewing lanes. You then post process and because of a probable lack of common satellites GPS to GPS may be way too sloppy. In that case you solve each point independently and tie them together with a field traverse. This means you post process and download information from 3 or more CORS. L1 only receivers are entirely adequate for this project. You might want your L1/L2 receiver away from the site in the quadrant that has no close CORS.
Paul in PA
I do these all the time and it is strange that there are not a lot of rooftop locations because the cell service is basically line of site so that the ground locations would get blocked by the tall buildings. Like Bill93 said, rooftops are the key in that kind of environment. The rooftops that I do the client wants the rooftop elevation, parapet wall elevation and the highest appurtenance. You can drop a tape off the side of the building or make a random setup on the ground and shoot the parapet wall and the ground and figure the height. You will need some good help to watch your equipment because you can not turn your back to it, much less walk away from it.
The biggest problem I have in doing this work is finding a safe place to park and usually leave someone to watch the equipment that you leave in your truck or let them drive around and come back and pick you up.
You can try to find some parks that might have open sky, Google Earth can help in this.
Ed
GPS only isn't going to cut it. With GLONASS you might have a snowball's chance. Plan on traversing from the few half way open locations you can spot.
Thank you all for your input. I was hoping someone had found some techniques for dealing with urban canyons, but it sounds like there might not be any.
I'll put the GPS away for this project. I'll have to bid a real high number to account for the logistics involved.
I really appreciate your thoughts.
JA, PLS SoCal
Loyal, post: 394930, member: 228 wrote: The temptation to push GPS/GNSS into questionable situations is something that we ALL are guilty of at one time or another.
No, not ALL of us. There are surveyors who work under the assumption that the earth is flat. That assumption protects us from that temptation.
Larry Best, post: 395050, member: 763 wrote: No, not ALL of us. There are surveyors who work under the assumption that the earth is flat. That assumption protects us from that temptation.
I don't get it?
Loyal, I think you assume that all surveyors use GPS. Maybe in Wyoming, they do. Plane Surveying assumes a flat earth.
