Activity Feed › Discussion Forums › Strictly Surveying › Closed Link Traverse Setup, Closure, and Adjustment Refresher Wanted
Tagged: traverse
-
Closed Link Traverse Setup, Closure, and Adjustment Refresher Wanted
OleManRiver replied 2 months, 2 weeks ago 11 Members · 43 Replies
-
Pretty much how the others have responded. We spent more time then I’d like to admit on instituting methods to use RTK observations and repeated observations for control for total station work on projects with similar scope as yours. While it is possible most of the time by averaging there are outliers that will cause a person to question the results of the methods while making assumptions and choices that infringe on the borders of professionalism. The time we spent simply showed us that post processed static on project control is just as timely and cost effective and the residuals are more in line with what control survey expectations are. Then when disagreements in measurements pop up one isn’t left asking is it the control or the traverse?
-
@ Jon Payne
If that was the process, I would expect that the difference in distance between GPS102-GPS101 is indicative of the error in the GPS observations at 101 and 102. It appears you lucked out and got better than should even be expected NRTK GPS positions for 107 and 108 but 101 and 102 are not as good. Can you go back and observe 101 and 102 with your NRTK again? I would be suspicious that 102 is as much as 0.08′ out from the GPS coordinates you have now.
GPS101 and GPS102 have open sky to the east but have some large oak trees to the west. Maybe that’s why their positions aren’t as good as GPS107 and GPS 108. We might be able to observe GPS101 and GPS102 again.
If any of the preceding assumptions about your process of working with the coordinates is correct, then that is a great example of the potential problems with holding the NRTK positions fixed for your solution. Any error in those positions becomes an error your are ‘correcting’ total station work to match – even if the total station work was very good. This would include error in positioning at 107 and 108 which seem to match the total station distance well, but may have provided compensating error to hit the 6″ angular closure, which I was a little surprised to see as that is much better than I expected with holding a NRTK azimuth at each end and running a 5″ total station 3300 feet.
I want to observe these points again using fast static.
MH -
Sorry just seeing this. Was out doing a favor for a friend on a rural site hardly no cell. I checked and saw we are connected so at the top right side of this forum is an icon for mail you can PM through that. I have not looked at the numbers here. But my gut said that once you scale the ground distance to grid your closure will be much better. Since it seems that most of the error is in this direction on your closure. Now it could go the opposite way as I don’t know if you are long or short distance wise and have no idea which cardinal direction this traverse is run in. Also you have a 90 degree turn if I remember correctly so with RTK or NRTK especially you have to look at the big picture to truly determine what’s going on. This is why many who truly understand RTK error and the precision of a total station understand that Least Squares is your best option in determining the closest truth to reality. So I also have no idea where you are in a Transverse Mercator or Lambert. As mentioned above. There is a scale factor for each point in a lambert this changes at the latitude. Then you have and elevation/ellipsoid factor this deals with a scale for your height. So depending on the height above or below the ellipsoid at or with any two points this will effect along with your scale or sometimes called grid factor. These terms you need to study on. Scale goes from ellipsoid to grid and grid to ellipsoid distance. Combined factor we have butchered the definition. If you see this on a NGS data sheet you will see the scale elevation and then combined factor and can check your math for any given point. The combined factor for a point is in the context of ground to grid. Not grid to ground. I have witnessed so many using this incorrecty as we all use the term grid to ground. If we want the grid to go to ground we need the inverse of this on a HP it’s 1/X. On a route survey as mentioned above having a pair at one end and a pair at the other end from NRTK or RTK is a crap shoot. On getting them to close. Especially when the procedure is to do a compass rule. I would prefer static or a fast static. Here on east coast RTK is King as they just want to be on datum and some still battle this issue on linear traverses as a lot still the majority of the time set 2 points with NRTK and run a loop. But they try that on a linear and crews get fussed at and budgets get blown re running a traverse that doesn’t close or they think it doesn’t close as a little error in one set of points to a closure on another end with another set of points.
Here is an example of a situation I was tasked with solving the problem. Which was not a problem. Two 4000 foot linear traverses that crossed each other. Both starting and ending on NRTK pairs in both directions. One closed within a minimum standard the other a little shy of the requirements. The common point in the middle yes you guessed it was not matching hardly at all from two independent compass rule adjustments. Traverse ran twice on both. Still couldn’t be solved. I simply looked at everything on the almighty google showed me areas on both lines where good open skies were. I Simply called crew had them locate those several points and the original points again so redundant measurements. They had also ran levels. I had them turn some rounds at the intersection points to close those crossing lines that were left independent except for one common point. I had also identified a blunder on one line for mean angle sets . Took the CORS station s that surrounded us that were in NGS and looked at the time tables along with the uncertainty and allowed that plus the base line lengths to allow a little float in the NRTK points. The weighting. Performed a least squares with all data. Except for the one blunder the traverse was good. Understanding uncertainty error in the tools and procedures used with any set of tools is most of the battle. Shape is another. Because of money budgets and directives from higher than my pay grade rtk was a tool we needed to use for control. I developed some procedures that was finally excepted that proved to eliminate the re running of traverses so often in the linear lines. But that had to be done using least squares. Least squares does not fix bad data. It just allows for the best possible fit of all tools used together with a confidence to meet a project requirements. You can have two points that you occupy on one end that were set with NRTK or RTK that you shoot the distance to your BS that are flat no error and also on other end but there azimuth could be off so a traverse closure could bust. From one end to another azimuth wise could blow the error budget. Angular wise.
Log in to reply.