Route survey, New to forum

> The question i have is this: What is considered to be among the "best" practices for establishing control networks for route surveys?

The one that works.

Welcome :hi5: to

> Hi everybody, newbie here. I just got my Illinois SIT this spring
Congrats and welcome.
> What is considered to be among the "best" practices for establishing control networks for route surveys?
I create a large network control system utilizing long GPS static sessions on established known control, found monuments, new monuments, established benchmarks then I typically incorporate CORS stations and any conventionally run data when post processing my final baselines in a Least Squares solution constraining to whatever the project requirements are. IE. Local ground, State Plane, LDP ect.

> ..... Typically a scattering of control points are set via GPS and more are set while traversing and preforming topographical surveys for design work. ... I believe there is lots of room for improvement in the accuracy of the control networks.

We feed on details here. What kind of GPS work are you doing. Static, RTK, OPUS ect.
Closed Traverse's?

> As I am getting a list of recommendations prepared for the owners on what i think should be done to improve accuracy, I would like to know what others think the best practices for establishing a control network for this type of survey work.

Sometimes the best procedures are not just taken from a textbook but more importantly are practices and procedures taken from experience, trial and error, textbooks, budgets and contractual obligations. Keep that in mind when you are crafting your list of "What I think"

Keep up the enthusiasm. Im sure you'll get some more responses.;-)

Welcome to the forum, EZ!! Glad to have ya here. :clap: :good: :bye:

Welcome. In general we set a pair of intervisible points at the beginning, at the end and at regular (more or less) points along the route. The spacing of the points depending on the length, the availability of open locations, and their access to the route. Static sessions can be accomplished while reconning the route before actually beginning the field work. Quad sheets and aerial photos (google Earth etc.) can be invaluable in preselecting the control point locations. Good luck and we look forward to hearing you have passed the LS.

Andy

Mostly what Andy said.

In my experience there is an emphasis on RTK because it is supposedly "fast."

I prefer to do static on selected points in network fashion (CORS stations are helpful for this) while traversing. Then later combine everything into one simultaneous adjustment. This can be done just as fast as RTKing a bunch of points and the results will be better.

I'm kind of a step-child with this but, OPUS is great. Put everything on the grid. Use OPUS to constrain your network to. For beginning and ending points, use static only for traverses. Set enough along the way to check in and adjust to. Leave it all on the grid or finally blow it up at the end and don't worry about it.

I use OPUS to make all of my network ties and I stay on the grid. I traverse on the surface and use the raw data reduction in Carlson to use a scale factor calculated at each set up. I have great closures with this method. I've had crap ones when I took off of RTK points.

At any rate, there is hundreds of ways to skin the cat. Don't try and build a better mouse trap unless you can do it better and cheaper, otherwise, keep on keeping on.

Note that OPUS is intended to be a check; not a final solution.

Cliff

A check to what? Surely you're not implying that connection via OPUS is less than the passive network of monuments out there right now.

I have zero problems building networks and connecting to the grid via OPUS (for horizontal work) and then moving over and connecting the two data sets via OPUS as well. The MOST that I've ever seen when checking between OPUS solutions is 0.1' and that was made from a RTK shot to an OPUS point derived by ultra-rapid ephemeris. Normally it's around 0.02' which is more than close enough for all things in the real surveying world but construction layout. It's perfect for getting to the grid, especially when you live in an area where NGS monuments are like finding hen's teeth and getting more scarce by the day.

If it were not good enough, then why is the NGS only using the CORS stations and no longer maintaining the passive network?

GPS Pairs Are Inadequate

Beginning and ending a traverse on monument pairs is acceptable when those monument pairs are long established and higher order geodetic monuments. NGS monuments were more precise than necessary for the route survey and would generally not be Least Squares adjusted along with the field observations. What you start with are 2 positions and a distance check.

In order to come near that precision with GPS the minimum starting and ending points should be triples, because all the GPS points must be a part of the Least Squares adjustment. I also suggest triples along the way, how far apart depends on terrain and the straighness of the route. The maximum range between triple sets should not encompass a larger range than can be accomodated with a single combined scale factor. What you start with are 3 positions, 2 distances and an angle. That is the redundancy that LS requires.

Paul in PA

Congrats on your LSI and good luck in the future.

As far as the route surveying and accuracy issue. Keep in mind surveying /engineering economy. Why are you doing what you're doing? Your goal is to provide a control network of sufficient accurracy and precision to adequately control the work. After that, your burning thru someone's money for no reason.

Paul

That is just asinine! Be it GPS pairs or two nails driven in the ground for a simple loop, two points are fine. I'd recommend static pairs and static pairs along the way. Route surveys don't need to be to the nats rear-end. A pair to correct bearings into along the way is PLENTY good for 3 to 5 miles of route survey.

I would like to first thank all of you for your welcome and all of the comments regarding my questions. As I stated in my first post, what is being done now is within the tolerance of the projects and client expectations. That being said however, there is little direction from the top as to how to make our work tighter. So my goal is to help fill that void and provide some suggestions as to what can be improved so that when we need to be on the “gnat rear end” we can be. I am aware that some people will get “ruffled” by someone suggesting changes to regular practices. My personality however, make me take a proactive approach to everything I do, and I can not just wait for someone else (who may or may not ever) to make a decision.

Jered McGrath asked for a little more detail about the current procedure.

Typically, groups of two or three points are established using RTK. These are set at the ends of the route and at intervals along the corridor. The spacing is typically 1600 feet, more or less, depending on obstructions and signal availability. They are occupied at least twice, but only for short durations. From this data a combined scale factor would be derived.

At this point a field crew would be turned loose on the project. As the crews traverse and survey the route, additional control is set using one set of direct and reverse observations. Backsights and foresights are typically in the range of 300 feet. As they check into the control that was set by GPS there is an error that is perpendicular to the overall route of travel, which typically grows as they progress further along the route. I do not know if any adjustment is being made to the traverse from one group of GPS control points to the next. I assume that it is not being done, as I have not seen any corrected point data.

What I plan to suggest is the following:
1. Longer GPS observations, or at the very least more observations of the same method, just to get more data to average with different satellite configurations and positions.
2. Establish GPS controls perpendicular to the route, to serve as angle checks for the backsight. The longer the better, as the only angle checks used now are typically less than 100 feet.
3. Use the longest backsight possible for setting traverse points. Personally I think that the backsight should be closer to a 90 degree angle as opposed to 180 to help reduce the introduction of an angular error while setting additional control.
4. Use multiple sets of direct and reverse observations to set additional control. Again, more data to create stronger averages.
5. If it is not being done already adjustments need to be made to the traverse between GPS control points. That adjusted data then needs to be shared with the field crews.

Aside from those suggestions, is there anything that I am missing? Is there anything that I am just flat out wrong on?

Again, thanks for the input and responses.

Whoa, Kris, Please Retract That Comment

Two GPS points will work mathematically, but their inherent inaccuracy can throw a lot of slop into the solution.

In a closed traverse you return to the same 2 points, 2 positions = bearing and distance, but the positional uncertainty is greatly extended at the far side of the loop.

I will not attempt to teach you Least Squares here, tonight. Most importantly Least Squares requires redundancy. If you care you may cite some sources for your 2 point comments.

Paul in PA

A 1600' spread between points is a bit too far IMHO. I would go a max of 1000'. The crews doing the stake out aren't going to like to go over 500' -- and some don't even like to go that far. It depends. From what I've done in the last several years, we want the concrete to be within a couple of hundreths in elev. That might be hard to achieve with a robot @ 800'. I like to sleep at night....

But hey, that's far better than what I have seen in the past. 1/2 mile between points. In many cases, you couldn't even see the backsight....

Longer backsights are a good idea if readily obtained. At only 100 ft, sideways centering error of 0.01 ft on each end give you 40" angle error.

More data will certainly tighten things up, but the improvement has to be traded against cost. A reasonable adjustment procedure would seem to offer the greatest bang for the buck if not already being done.

>3. Personally I think that the backsight should be closer to a 90 degree angle as opposed to 180

I don't understand this. An angle is an angle. Error tolerance doesn't depend on what angle if you are finding points by angle and distance. Angles near 0 and 180 are only bad where you derive a point from an intersection of lines making that angle.

So it looks like you have a GPS point to check into every 1,600 feet. IMHO, these are closer together than they need to be. RTK may be fast, but it isn't necessarily better.

If it were me, I'd suggest setting static pairs every 10,000 feet or so along the Route. When the crew reaches a pair and checks in, be sure someone (perhaps you) is responsible for adjusting the traverse up to the pair.

In terms of field practice for the conventional traverse, the method I've come to prefer is to close the horizon using a full direct and reverse set for both observations. Then, before the gun is broken down, add up the horizontal angles to be sure they approximate 360°. Our tolerance was typically 10", except in extreme cases where a traverse length may be particularly short, then the angle standard was relaxed a bit.

Then, depending on the checks, be sure the crew occupies the correct point numbers in the collector to "reset" the traverse. I once had a crew call the GPS pair they checked through a separate number than what they actually were. They checked in at around 0.3 - 0.4 if I remember correctly, then kept on truckin'.
The problem didn't show up until they arrived at the next pair 10,000 feet south, and they freaked out because they were off almost due east 3.5 feet. They came back to the office nervous and apologetic, and then I didn't make the situation any better since I couldn't help laughing at them. 🙂