I generally agree with all the comments cautioning about using GPS for set out.
I was particularly concerned about my current project, as all I have is GNSS South S82V. I would have preferred some form of total station for some of the work. Previously only used the GNSS for earthworks and road set out with aid of a optical level to confirm heights for pavements.
I have been pleasantly surprised how everything has worked out. Checking points from different localisation points on different days and getting remarkable, seldom differing in position, often within 10-20mm in height.
I kept telling the contractors the best I could set out was the size of the spray paint can base, 65mm diameter circle. And I joke with then that "I don't know why I bother, as they seem to knock out the pegs/nails or ignore them"
I took care to adjust and check the pole, having made my own jig, use a bipod for the critical points and maintain north orientation as an added precaution.
I had a clearly bad set out point when laying out electrical ducts, not a GNSS issue.
One point seemed a little off when doing car park markings, perhaps 50mm. However I set out 15 netball courts with nails in corners and all the other lines and centres, the post sockets set out previously prior to asphalting. The line markers set string lines and checked dimensions and square and only wanted to shift a few nails, all more of less +/- 10mm which is the best I could have hoped for.
On a different tangent, the office vehicle I get to share is a small Subaru Forester 2.0 Diesel. Full size trucks are a rare sight on roads in New Zealand.
rfc, post: 426160, member: 8882 wrote: Don't the cross ties of a trapezoid match?
Maybe a trapezoid but not a rectangle
Mark Mayer, post: 426179, member: 424 wrote: Per Pythagorus, the cross ties of a 40.00' x 65.00' rectangle should measure 76.32'. If your inverses say otherwise we have narrowed the search for the source of your error.
Thanks Mark, that's the key to the cross tie
I always have two bubbles on a stacking pole.
A while back I suspected my GPS pole wasn't true (bubble ie) and I've reverted to the old staff bubble I keep in adjustment and use that.
I can rotate to check as needed.
Any serious work I use pole fixed in alligator clips on a tripod or in a tribrach and 2 minute observations.
Don't mind the wait and results speak for themselves
Richard, post: 426410, member: 833 wrote: A while back I suspected my GPS pole wasn't true (bubble ie) and I've reverted to the old staff bubble I keep in adjustment and use that.
I can rotate to check as needed.
Any serious work I use pole fixed in alligator clips on a tripod or in a tribrach and 2 minute observations.
Don't mind the wait and results speak for themselves
I have got into the habit of always shooting control twice - rotating the pole 180 between observations.
That way you check for both the bubble being in good adjustment and the pole straight. (yes it does happen - we had run of poles where the top section was bent enough to make a difference - took a while to work out what was going on)
Trimble's auto averaging of shots makes this an easy procedure
The horizontal distance may be ok. Were the cuts/fills indicating a big vertical difference at the different stakes? You could check that with a total station.
That being said, I do not layout anything with GPS that has to get a tack set in the hub. I won't layout any formwork, etc. I will use GPS to layout edge of road offsets, rough grade, detention basins, centerline of road, building pad, and other stuff like that. For the tight stuff I use the robot or regular total station.
When using RTK to set control or tie monument I always collect between 1 & 5 minutes worth of data on at least 2 visits separated by 2 or more hours. I know some people swear that they get their redundancy by simply rea-initializing but I'm old school that way. I like to see a different constellation and multipath. When setting control with RTK, static or conventional total station, I never use a rod (unless real rough is okay). Why would I set my compensated total station on a point that was set with a pole and 40' vial?
As for the OP. RTK is good for dirt work, non-gravity flow utilities and EP. Anything hard such as curb or concrete needs to be run in with a gun. I always get a kick out of contractors using RTK for pipe work in the bottom of a 2 m trench with a track hoe sitting directly to the south.
John Putnam, post: 426541, member: 1188 wrote: When using RTK to set control or tie monument I always collect between 1 & 5 minutes worth of data on at least 2 visits separated by 2 or more hours. I know some people swear that they get their redundancy by simply rea-initializing but I'm old school that way. I like to see a different constellation and multipath. When setting control with RTK, static or conventional total station, I never use a rod (unless real rough is okay). Why would I set my compensated total station on a point that was set with a pole and 40' vial?
As for the OP. RTK is good for dirt work, non-gravity flow utilities and EP. Anything hard such as curb or concrete needs to be run in with a gun. I always get a kick out of contractors using RTK for pipe work in the bottom of a 2 m trench with a track hoe sitting directly to the south.
Exactly what I'm saying too....look at the time it is taking to refine your control measurement to an acceptable degree of precision and accuracy and then look at the amount of time that would be spent finding the spot for the tack in the hub using RTK. You're talking several minutes of data spread out over multiple observations vs just where it thinks it is at that second when you are staking something out. It's probably not that far off, but not good enough for structures. I'm sure it's fine for water mains, road signs, stuff like that...
jim.cox, post: 426445, member: 93 wrote: I have got into the habit of always shooting control twice - rotating the pole 180 between observations.
That way you check for both the bubble being in good adjustment and the pole straight. (yes it does happen - we had run of poles where the top section was bent enough to make a difference - took a while to work out what was going on)
Trimble's auto averaging of shots makes this an easy procedure
This is one I did recently; the three shots are two RTK shots and a 15 minute static location. The widest spread is .012'. I rotated the rod and it almost looks like a perfect example of the error in the range pole bubble. I always have two bubbles on the range pole so you have a good indication when one is slipping.
A construction site is usually clear by the time layout is being done, perfect for a total station. It is possible to get acceptable tolerances with RTK but it takes much longer than a total station. It is also possible to get good quick results from RTK 5 times in a row, but the 6th time will screw you.
Oh, and if your boundary corners are really calculated by an engineer you may have bigger problems.
I have used RTK for many construction projects, including building corners, if I have to set grid lines for columns, then I will pull out the total station, but for the most part many contractors are somewhat sloppy. I have watched them work on many projects and have seen multiple instances of pounding spikes right against hubs and putting up batter boards and eye balling over the point. There is a lot more play in construction then a contractor will admit. .02-.04 feet is good enough for just about anything. Yeah you will get comments where surveyors claim setting points within .01 feet. Yeah and everything is bigger in Texas
The biggest concern I have doing layout with RTK is you're always dealing with a moving target, or I should say targets (plural). Aliquot nailed it. 5 times your spot on and the 6th., Murphy's Law kicks in and you're screwed due to a katiwampus satellite constellation. As with GPS and chicks, timing is everything.
Just because I'm paranoid, doesn't mean they aren't out to get me.
kotuku4, post: 426252, member: 8886 wrote: I generally agree with all the comments cautioning about using GPS for set out.
I was particularly concerned about my current project, as all I have is GNSS South S82V. I would have preferred some form of total station for some of the work. Previously only used the GNSS for earthworks and road set out with aid of a optical level to confirm heights for pavements.
I have been pleasantly surprised how everything has worked out. Checking points from different localisation points on different days and getting remarkable, seldom differing in position, often within 10-20mm in height.
I kept telling the contractors the best I could set out was the size of the spray paint can base, 65mm diameter circle. And I joke with then that "I don't know why I bother, as they seem to knock out the pegs/nails or ignore them"
I took care to adjust and check the pole, having made my own jig, use a bipod for the critical points and maintain north orientation as an added precaution.
I had a clearly bad set out point when laying out electrical ducts, not a GNSS issue.
One point seemed a little off when doing car park markings, perhaps 50mm. However I set out 15 netball courts with nails in corners and all the other lines and centres, the post sockets set out previously prior to asphalting. The line markers set string lines and checked dimensions and square and only wanted to shift a few nails, all more of less +/- 10mm which is the best I could have hoped for.
On a different tangent, the office vehicle I get to share is a small Subaru Forester 2.0 Diesel. Full size trucks are a rare sight on roads in New Zealand.
I googled 'Netball' to see what that was all about and this was the first link: http://www.stuff.co.nz/marlborough-express/sport/92091935/new-netball-courts-officially-opened . What are the odds that's your work?
Yes that's the one. I had not see that report though, so that's for the link.
Sent from my m3 note using Tapatalk
billvhill, post: 426607, member: 8398 wrote: I have used RTK for many construction projects, including building corners, if I have to set grid lines for columns, then I will pull out the total station, but for the most part many contractors are somewhat sloppy. I have watched them work on many projects and have seen multiple instances of pounding spikes right against hubs and putting up batter boards and eye balling over the point. There is a lot more play in construction then a contractor will admit. .02-.04 feet is good enough for just about anything. Yeah you will get comments where surveyors claim setting points within .01 feet. Yeah and everything is bigger in Texas
I work directly for a contractor that does heavy construction. (All publicly bid projects, public works, water and wastewater treatments plants, municipal buildings, etc) If I lay out building corners off by 0.02' nobody would say anything. If I was off 0.04' I think they would start asking me about it. That's 1/2" in the carpentry and masonry world. If you did that on both ends and had cumulating error you're talking a whole inch. There's no way that would fly.
I agree on the moving target concept. My preferred method for anything that gets a tack is robot and well-adjusted prism pole on a bipod with a short rod height. We've done some buildings with crazy tolerances and I even went so far as to use a mini-pole with a 0.6' rod height to really tighten up the layout for column lines, etc.
All that said, you can only layout as good as your control (actually slightly worse than it), but that's another topic.
kotuku4, post: 426617, member: 8886 wrote: Yes that's the one. I had not see that report though, so that's for the link.
Sent from my m3 note using Tapatalk
Darn phone, I meant thanks for the link to the report.
leegreen, post: 426159, member: 2332 wrote: The best you can expect from layout RTK is +/- 0.04' under ideal conditions. Add any type of multpath and you got more error.
With RTK you can set a 2"x 2" hub, but you can't set a tack.
I find it hard to believe any Surveyor would set building corners with rtk. This is what gives them a bad reputation. This is why I hear so many Mason's hating layout done by boundary Surveyors. If they all checked the cross ties as the OP did, this practice would stop.
One of our crews was staking out a fairly complicated house recently - total station only, of course. When they were were done the started checking diagonals with a tape. The lead mason told his crew "Look at that! They're checking it!" He told my crew chief that he hadn't seen anyone do that in years. He related several stories of the difficulties he encountered with buildings being laid out using GPS. He said he had told one surveyor that it seemed like almost all his corners were 2" out. The surveyor told him "Sounds about right - that's the best we can do." Almost makes me ashamed to be in the same profession! And this is my competition...
Jim in AZ, post: 426727, member: 249 wrote: .... The surveyor told him "Sounds about right - that's the best we can do." Almost makes me ashamed to be in the same profession! And this is my competition...
That's ridiculous! I guess if they need it better than 2" they better not call that guy again