So I spoke with three surveyors who were all running around a large site with GPS rovers who all ganged up on me and claimed that their GPS was extremely accurate and essentially just as good as robotics(conventional). One quoted that his was accurate to the "square root of a centimeter" in an obviously boastful way in front of a group of construction management folks and the other surveyors there nodded their collective heads in unison.....a claim that I knew was untrue but at the time didn't have the energy or desire to discuss.
Without ruffling feathers and explaining it in laymen's terms how would you explain this to the owner and con. management folks when ALL the other surveyors say otherwise?
If it's anything like the network RTK I've seen, set one still, on a tripod, and let the manager watch the elevation bounce up and down by a tenth over a minute or two time span. That's not to say that it's not useful, just that it takes great care and redundancy to get Network RTK down to sloppy total station accuracy.
A picture (or demonstration) is worth a thousand words.
I would ask him to set two points 100 feet apart and get out that old steel tape to check? Shoot the two points with you robot also, but you better know you have had it calibrated and checked correctly. Because many of those same surveyor's you saw running around with their RTK would all get different results with their guns also.
That reminds me of the early 1980s when a surveyor's message was turned into an article in the paper that claimed that with his TS he would not have to cut brush anymore to measure thru the forests.
B-)
ask them what the specs are horiz and vertical on their gps, and how repeatable are the results w/in that centimeter square root
Also, point out that the square root of 1 cm is 1 cm ...
Just smile, agree, pat the biggest shot in the group on the back and say "yeah,whats an inch amongst friends?"
> That reminds me of the early 1980s when a surveyor's message was turned into an article in the paper that claimed that with his TS he would not have to cut brush anymore to measure thru the forests.
>
> B-)
Yes, will still cut brush here, but we did stop carrying chainsaws in the late 80's because they were not getting used anymore. I do not miss the chainsaw one bit!
> Without ruffling feathers and explaining it in laymen's terms how would you explain this to the owner and con. management folks when ALL the other surveyors say otherwise?
“Youth is easily deceived because it is quick to hope.”
? Aristotle
Donald,
I think Pseudo made two good points. I am pretty sure just about every system out there is now specifying the +/- 1 cm horizontal (and Pseudo put it perfect about the square root of 1) but that is horizontal. The other specification is usually -/+ 2 cm vertically which is 0.07 feet. But.... and this is the big elephant in the corner... there is a thing involved called parts per million of the baseline length. So if a surveyor is using Network RTK... I think we should all stop when working at a job site and do a rough calculation on how much error budget in vertical and horizontal you may have according to specifications. So, just looking at vertical 1 ppm in one mile of baseline length = 0.005 ft but in 10 miles that grows to +/- 0.05 ft. So maybe again in dirt work this is still OK. The other elephant in the room is when different geoid models are used on the same site or different local coordinate system (site calibrations) are used.
Depending on if your are just doing dirt work RTK with a RTK base on site is a good tool for dirt work. However, I think anyone using Network RTK they should stop and look at all the specifications if they choose to spit out spec's.
I am not sure how to explain this to the contractor. With Machine Control coming on line more and more and using Network RTK with Machine Control this may just become a magic box syndrome.
The square root of 0.010 is 0.100. Not something I'd brag about.
I didn't read where it said network RTK, just RTK. RTK can be very accurate. I would say that RTK specs can rival standard survey measurements, provided someone knows how to use it. However, I would set a 200 ft baseline, check it with a calibrated tape and ask them to measure it.
It says "Network GPS" in the title of the post.
lol, how on Earth do I miss these things?
I was told by an "expert" yesterday who is merely a flood plain manager that by using GPS all you need is one shot on a benchmark and you are going to be dead accurate for vertical anywhere within 20 miles or more of the benchmark. At least that's the way he remembered it from a seminar on surveying he attended.
> I was told by an "expert" yesterday who is merely a flood plain manager that by using GPS all you need is one shot on a benchmark and you are going to be dead accurate for vertical anywhere within 20 miles or more of the benchmark. At least that's the way he remembered it from a seminar on surveying he attended.
Thanks Mr. Cow., I needed a good laugh today.
"I was told by an "expert" yesterday" and "At least that's the way he remembered it from a seminar" --- a couple of famous quotes to place on the tomb of the profession of land surveying - and I'm sure we can come up with a few more dandy's........
