Hey All,?ÿ
We've been doing our own layout for sometime now, but seeing as how we are just a construction company, we've always worked off of established control. Before I came along, they would use the client's contract surveyor, usually the one who did the boundary work for the site, to establish control points on site to be used for layout and machine control. Most of the time, we would get guys who would come out, set a few points with RTK, and either turn over a CSV from their collector right then, or we would get an email with point names and coordinates from the PLS. When we would check into these points with our equipment, we would see busts in the range of 0.06-0.15' as they were probably set with rapid Topo observations using GNSS.?ÿ
Ideally I'd like define where I'd like to TBM's to be set, then to request a drawing from the licensee showing names, coordinates (both SP and GRID), and the locations of the TBM's that they've set, along with some documentation for how they converted from their working coordinate system to our site's grid. I would like for these points to be established with a total-station ran in a true closed traverse for horizontal, and have a level run from an established benchmark, either a static point or monument, through them for vertical. I know this seems like overkill, but the tighter their control, the tighter our layout. I come from a dimensional control background where I want everything to be as tight as possible.?ÿ
I am trying to create some standard proposal language to be used when requesting bids or proposals for a survey crew to come on site, and establish a network of control benchmarks to be used for layout. I'm wondering if you guys might have suggestions for any specifics things I should include that you may have learned through past experiences.?ÿ
Any thoughts or suggestions??ÿ
They likely?ÿused an RTN or VRS, on a 2-meter pole with no bipod, along with only a few seconds of occupation time. This procedure often?ÿyields HRMS/VRMS values that you have described.?ÿ Think of it this way. If the nearest RTN base was 10 miles from your?ÿproject, then they are measuring radially from that base. Therefore if they measure a local baseline?ÿdistance of 500ft from point A to point B, they actually measured 10 miles from the Base to Point A, and 10 miles from the Base to Point B. The closed triangle would be 20 miles + 500ft.?ÿ With distances that long a couple tenths error is not so bad. Within a distance of 500ft a couple of tenths error, not so good.
Two methods:
1) Static network. For this, I use six static?ÿGNSS receivers and often borrow 2 or 4 more. This way I can occupy?ÿup to 10 controls points simultaneously, with at least 60 minutes of static data. This gives me vectors around the entire project (traverse), along with many cross vectors for a solid Least Squares Adjustment or Check.?ÿ
1) RTK with a local base. Occupy each point from two separate?ÿbase stations. Check the triangle?ÿclosure for each point Least Squares Adjustment.
You need to specify a relative accuracy, and perhaps the procedure, such as static control network?ÿwith a report. For DOT construction projects around here, NYSDOT requires PLS to create a Contract Control Plan with a report of the procedure.?ÿ
I recall on one project them having a base setup near by for sure. On others I can't speak to their techniques. In all honestly, I can't say for sure that GNSS is the way to go for the setting out of the required control. I would like to maintain an accuracy of sub 0.02' or less than a 1/4" over as site that's about 1000x1000' For machine control this is way overkill. But for our conventional layout for civil, things like equipment and anchor bolts need to be kept withing that 1/4" tolerance. Also, we rarely rely on state-plane and mostly work in station/plant grid.
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You're going to have a hard time telling someone how to do their job. You can tell then the results they need to achieve, but leave the procedure up to them.
Just make sure that they know that they need to make it right, on their own dime, if the fail to achieve the required tolerance.
You're going to have a hard time telling someone how to do their job. You can tell then the results they need to achieve, but leave the procedure up to them.
Just make sure that they know that they need to make it right, on their own dime, if the fail to achieve the required tolerance.
This. Specify results.
.....I would like to maintain an accuracy of sub 0.02' or less than a 1/4" over as site that's about 1000x1000' ?ÿ
I may be just possible to achieve 0.02' precision over a small area with GNSS - if the area has excellent open sky- and some specific processes are followed -but?ÿprobably you need to specify that the monuments are interconnected by total station ties, with tripod mounted foresight and backsight targets, each station tied to at least 2 adjacent stations, by multiple observations, and the whole adjusted by least squares.?ÿ The least squares report to show residuals not exceeding your 0.02' limit at the 95% level.
But I think that you need to talk to some surveyors in your area, tell them that you need control that is reliably precise at the 0.02' level, and have them convince you that they can do it, and to specify how they propose accomplish it.?ÿ?ÿ
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You're going to have a hard time telling someone how to do their job. You can tell then the results they need to achieve, but leave the procedure up to them.
Just make sure that they know that they need to make it right, on their own dime, if the fail to achieve the required tolerance.
This. Specify results.
Years ago I had to issue many RFPs for aerial mapping and monument preservation for public projects. Lots of work, it drove me nuts.
somehow I ended up writing letters to know quality firms asking their assistance in developing standard specs... to put them on an even playing field, get the products I needed, etc. Several wrote for me. They assisted with terms and methods that did not flow off my tongue.
I then complied and sorted, and then asked all of them for comments.
It worked well, I got completive pricing, and good quality products... that was a win-win. It made finance and legal happy, the bean counters in the front office loved it too.
Less work for me.
REQUIRE levels, REQUIRE they use a GOOD level. That will weed out a number of guys.
.02' horizontally over a 1000'x1000'?ÿis very tight, you may think you achieve that when you do layout, but you probably don't in a positional tolerance sense. Read up on it and set parameters that are realistic. If you still think a positional tolerance of .02' is possible then put that into your RFP. Work up a company plug in spreadsheet showing how you want the control presented. Say what you need for monuments. It sounds like you have been calling up surveyors at the last minute asking for points and they send out a crew to throw a few in.
I think I can speak for most on this board that if you want good, very tight control, with stable monuments, please by all means ask a surveyor to do that, but explain exactly what you need, be prepared to?ÿPAY for it and give a?ÿreasonable time frame to get what you need.
.02' vertically, no problem with good levels, as long as the site isn't on a mountain side or through a steep canyon. .02' positional tolerance horizontally is very tight. Not to be too fine with the math but think of a .02' tolerance as really being less than .01'.
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'.02' horizontally over a 1000'x1000' is very tight, you may think you achieve that when you do layout, but you probably don't in a positional tolerance sense.'
I sat here and wondered about that, thinking that even if you were to set the control marginally outside that project window, you're still looking at 1:60,000, which isn't impossible.
Then there's the idea of why you need three control points <0.02' if your nearest improvement is say 35' from the nearest boundary or setback line, just pick one of the surveyor's control points and stick with it what does it matter. If the precise bolt location is the 1k^2 window you probably need to be focused on the layout crew.
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Liked that post, Mo. It sounded real.
In the past I have done network control surveys were I had to carry control from a BM nearly 1500' from our work area, run a closed loop through a network of 4-6 TBM's and back to the original benchmark, close within 0.015, then run a least squares. So it's not impossible. We were using Leica TS11 1" guns with PRO5000 tripod mounted bs and fs. We would then come back with LS15 levels an dial the verticals in.?ÿ
My layout crew is dialed in. We are not allowed to set control to come off of though as we are not a licensed firm. So when I have them setup on surveyed control, and they are checking horizontally at 0.08, it's hard to tell a client that their anchor bolts for a few $20M pieces of rotating equipment are withing 0.02' when multiple setups are done from various control points that don't check worth a heap.?ÿ
Anchor bolts. That is a different story. You mentioned GNSS and Machine Control. Control for anchors does not work with GNSS. That is a whole different type of accuracy and procedures. Total station and levels always for anchor bolts.
Trying to imagine whether you're building this from the ground up, or matching an existing facility. If this were an existing facility, requiring the tolerances you mentioned, wouldn't there be existing control on site. Since you used dialed in twice I'm gonna' believe you have a handle on this.
Trying to imagine whether you're building this from the ground up, or matching an existing facility. If this were an existing facility, requiring the tolerances you mentioned, wouldn't there be existing control on site. Since you used dialed in twice I'm gonna' believe you have a handle on this.
I'll say with confidence that I have a handle on the layout side of things, regardless of my choice of words.?ÿ? With that being said, this was not a request for help with how to do tight layout. It was me asking for advice from professional surveyors, or those that have dealt with such, on how to clearly and concisely define what I need from them when tasked with the layout of working control.?ÿ
I would take a look at the specs and contracts used by governmental agencies when contracting survey control for public work projects for guidance on how to phrase the request.?ÿ
It is an interesting question first in terms of how to transfer the legal liability of the control network in whole to another body and also from the practical side of getting a network of sufficient quality to do the work. I think you are on the right track, I would also consider requesting copies of the adjustment reports and raw data.
In my experience there are companies that specialize in establishing control, if I were buying control networks I would seek them out and focus more on expertise than price.?ÿ
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