Community Forums

Share:

Static GPS control and on the ground traverse  

Page 1 / 4

Avatar
Posts: 8
Member
(@somesurveyguy)
5+ posts
Joined: 2 weeks ago

Greetings all! First let me say if this is in the wrong forum I apologize, this is my first post here.

Background: I got back in to survey last fall after a near decade hiatus. The field stuff came back quick, re-learning CAD and C3D having only used LDD with Carlson Surv-CAD extension a decade prior took a bit, but I'm up and running. I do primarily base plan topo to support our design staff. A lot of bridge work, roadway projects, site plans for developers, etc, and I get a handful of small residential work in the door each month. 

My current issue: We won a large (about 4.5 miles of topo including the side streets) contract to create a base plan for a section of road in a neighboring town. This job will eventually become a state funded project (hope to get the design contract as well) and as such needs to be to their standards. The state has provided us with 6 state plan coordinate control points along the project using static GPS. There are 2 at the beginning, 2 at the end, and 2 in the middle. I am using a tp12, AllegroMX with SurvCE, and Leica circle prisms and new trimax legs. Gun was calibrated at the end of last season, I take good care of it, and run check and adjust pretty regularly. 

So I started at the north end of the job occupying the first 2 points (300'+/- apart, checked to under 0.01 vert and horiz) and began laying out my main traverse. My intent was to traverse the length of the road, tie into their GPS control, balance the error, and then all do all of my topo.

So, when I arrived at the middle set of GPS points and checked my coordinates versus theirs, I found myself about 1.33' off horizontally, and 0.05' vertically. When i finally got to the other end of the project, I am now 3.65 off H and 0.13' off V. 

This was pretty disappointing as I am turning quad angles, and the most error I ever saw when occupying a newly set point was 0.009 and that was for vertical, nothing more than 0.004 for horizontal. 

The thing is, as the crow flies, I'm really not that bad. The calc from first to last GPS point is 13298.7899', and my actual measurement  is 13298.6387 (-0.1512). First point to mid point calc is 6128.8909, on the ground measured 6128.8229 (-.0680). So it would seem that the majority of my error is in the angles turned. When I draw a line from the first to last gps point, and a line from the first and last points of my traverse, I am about 0.0059 (d.mm.ss) to the right of where I should be according to their gps control. Holding either end point and rotating through the other end puts me about 0.45 out in the middle, which won't work if they use my coordinates for layout years down the road when this thing will get built.

I had 59 legs on the traverse, and I was off 59 seconds to the right, so I went in and reduced each angle in my traverse by a second in the .fbk file, which got me about half way to where I needed to be. I again edited the .fbk by reducing each angle by another second, that leaves me off 0.07' at the far end of the run, but still out 0.52 in the middle.

I also checked my direct reverse report and tried editing my original raw .fbk by the standard deviation numbers (highest SD was 4 seconds on a few legs >300ft)  for each turned angle, and again I'm only about halfway back to where I should have landed by their coordinates. Not sure if I should do this again (2 standard deviations) and see how that ends up, or if I'm just faking it to make it. At least in this case I am distributing known error along the appropriate legs? Idk. I just don't want to spend all summer knocking this topo out and end up with a junk survey... 

The best result I have had is breaking the .fbk into 2. The first runs from the 2 notherly points to the middle, and then on the second I basically started over, occupying the known gps coordinates and running down to the end. Rotating those 2 separate runs by standard hold one end and rotate through the other puts me under 0.10 off max for any one point, but doing this I'm also just twisting the traverse to fit the coordinates, not balancing the error through each leg?

So, is there a way to hold multiple known points along an open traverse with equal weight and distribute the error throughout the traverse? I've tried using least squares in c3d and carlson x-port, and that gets me dead nuts on either end, but I'm still out of whack in the middle. I can't seem to find any method in either that lets me specify known points I'd like to close on and then point it to where I've calculated those points to be based on my traverse. 

If I had to do it again I would probably start at one end, work to the middle, then go to the other end, work back to the middle, and just have 2 separate traverses. At least then I'm just trying to hold 3 points for each and not 6 for one long as traverse.

So yeah, sorry for the novel, but I need help!. I've googled every term i can think of and come up with nothing of this nature. The PLS I'm working under is great for property line rotation questions I have, but he's been out of the field for a long time, so he can't really help me with this one. I'd rather not reach out to the state and admit that I'm not exactly sure what I'm doing, but if I have to I will swallow my pride and do so.   

Thanks for reading and for any help you can offer!

LP

37 Replies
Bill93
Posts: 5461
Member
(@bill93)
5,000+ posts
Joined: 9 years ago

Starting with an azimuth on one end pair of points is sure to leave you off at the other end, and I wouldn't adjust each and every angle to make up for that.  Holding end points and figuring out what to do in the middle would be much better.

A mis-fit of 0.45 in 6128.8 is 1 : 13620 and probably within the distortion of a state plane coordinate system, depending on where you are relative to the center of the zone and which orientation your road has relative to the TM or Lambert.  If that isn't good enough, then the work should be done with a low distortion projection customized to the project.

Reply
1 Reply
Avatar
Member
(@somesurveyguy)
Joined: 2 weeks ago

5+ posts
Posts: 8

Thanks Bill. I expected some error, but I also, probably foolishly, expected to hit a little closer their coordinates. I hadn't really thought about the  1:xx number, I think I just balked at a delta of 0.45. I'll consult with the state surveyor using this least squares adjustment and see if they find that satisfactory for the purpose of this project. Thanks again!

Reply
NorthernSurveyor
Posts: 443
Member
(@northernsurveyor)
250+ posts
Joined: 9 years ago

A properly weighted Least Squares Adjustment would probably be best suited for adjustments.  Bill93 is correct too, that the SPC has its own distortion values in the projection.  

Reply
3 Replies
Avatar
Member
(@somesurveyguy)
Joined: 2 weeks ago

5+ posts
Posts: 8

Thanks @NorthernSurveyor

I've not used least squares before, but using Carlson X-port i came up with the error I described above. But what makes for a "properly weighted" least squares adjustment? X-port just asks me what the closing point is, so I put in the last number of my traverse and entered the coordinates from the states static gps survey, and it calculated new coordinates for the traverse. 

 

Is there another utility you would recommend that I can use that would allow me to weigh all 6 gps points equally or to varying degrees?

 

Thanks 

Reply
NorthernSurveyor
Member
(@northernsurveyor)
Joined: 9 years ago

250+ posts
Posts: 443

A least-squares adjustment solves for a solution where it adjusts all of the measurements the "least".   It is a rigorous fit of your measurements and the estimated standard errors for each measurement to control, although even your control can be weighted for its estimated accuracy.  However, most will hold fixed the control values for the adjustments.   Weighting involves setting reasonable estimates of your measurement inaccuracies (GPS or conventional), centering errors, HI measurement errors, sighting errors (if using optical instruments).   Its been around a long time, and is generally the most reliable and accurate adjustment.  The more redundant measurements you have, like cross ties, or repeat measurements, the more the adjustment shines in the ability to determine the correct geometry of your points.   There is a lot of information on this board, or on the interweb that can give you more information.   Easy to use and relatively in-expensive programs like StarNet are available for everyone, and some have a free trial use.   Most all of the GPS processing suites have LSA included in the package.   

Reply
Avatar
Member
(@dave-lindell)
Joined: 9 years ago

500+ posts
Posts: 897

"Least squares" by its very name implies that the sum of the squares of the residuals is minimized.

It does not adjust the measurements "the least".

It provides the most "most probable" value for every given measurement.

Reply
Nate The Surveyor
Posts: 8678
(@nate-the-surveyor)
5,000+ posts
Joined: 9 years ago

Bill above is right.

Another thing comes to mind. Scale factors. Have you any experience with that? Do you know how their gps was done?

Nate

Reply
7 Replies
Avatar
Member
(@somesurveyguy)
Joined: 2 weeks ago

5+ posts
Posts: 8

Hey Nate. I don't have experience using scale factors or GPS surveys in general. When I left the old company we had done some static setups, but I was just  a field hand babysitting the setup while it cooked.

The state did a static gps survey, with a combined ground to grid scale factor in the area of 0.99998xxxxx  for the 11 points that were adjusted. Adjustment type is plane +height, constraint, 95% confidence level. Since I have no experience in this area I'm not entirely sure what to do with this info. I did look at some videos about to grid to ground conversions in c3d, but I would presume that the state is giving me ground coordinates already?

Thanks for the reply!

Reply
Avatar
Member
(@larry-scott)
Joined: 5 years ago

500+ posts
Posts: 611

You have no experience using state plane (grid factors, elevation factors, 1:xxxx precision, least squares adjustment) and are working a state (tax payer) funded project. Did I read that correctly? 

When did you pass your PLS? Is there a PLS in your office? 

Reply
SPMPLS
Member
(@spmpls)
Joined: 3 years ago

250+ posts
Posts: 252

In the OP he stated that he was working under a PLS who is not able to help him with this.

Reply
Avatar
Member
(@rover83)
Joined: 3 years ago

20+ posts
Posts: 47

I don't fault the OP for trying to figure this out, but I do share Larry's concern. I am curious as to how the firm was able to win the project when the PLS in responsible charge appears to be practicing outside of his/her area of competence.

Control network design, observation and adjustment as well as least squares analysis QC/QA is something that every license should be able to do in their sleep. It's not a specialized area of surveying. One cannot competently perform ALTA/NSPS surveys without a solid understanding of least squares.

This is especially true for state projects. When I worked in AK and did DOT work we had to submit proposals detailing how we would establish control. Which published points we planned on holding, how many intermediate points we would set, as well as procedures for observing and adjusting. Part of the deliverable was a reduction narrative discussing which control we held and why, any discrepancies found and what the remedy was. As well as all the field books (signed and stamped by the crew chief, always a PLS), raw data, processing reports, and adjustment reports...if you threw out a published control point or had a blunder, you had better have an explanation as to why.

All of these things should be SOP for larger projects like this. Which, again, makes me wonder how firms can win projects like this with no experience in what is arguably the most critical piece of the job.

Reply
Avatar
Member
(@larry-scott)
Joined: 5 years ago

500+ posts
Posts: 611

If this contract was won low-bid, if I had bid on it and lost? Yeah, this thread would be entered into the record. 

Reply
Avatar
Member
(@larry-scott)
Joined: 5 years ago

500+ posts
Posts: 611

The PLS is not able. 

So? Go get one that is. 

Reply
Avatar
Member
(@larry-scott)
Joined: 5 years ago

500+ posts
Posts: 611

“...primarily base plan topo to support our design staff. A lot of bridge work,..”

is the E&O insurance up to date? 

Reply
Nate The Surveyor
Posts: 8678
(@nate-the-surveyor)
5,000+ posts
Joined: 9 years ago

Bill above is right.

Another thing comes to mind. Scale factors. Have you any experience with that? Do you know how their gps was done?

Nate

Reply
Page 1 / 4