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Tagged: traverse
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Closed Link Traverse Setup, Closure, and Adjustment Refresher Wanted
OleManRiver replied 2 months, 3 weeks ago 11 Members · 43 Replies
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dwooley I’m unsure of the point you are trying to make. To me it seems it does not matter how you express your measurements.The measurements and the error in the measurements remain the same whether you express them by “setting points” or creating coordinate values. The error exists regardless of how the measurements are used.
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“I know nothing about fast static. Is that the same thing as PPK?”
Fast static (aka rapid static) is simply static with short baselines – typically limited to 20 km. With short baselines the assumption is that the upper atmosphere is the same at each receiver, therefore there is no need to occupy for a long time (long time being an hour to several hours) to model the upper atmosphere. This 20 km limit is the same limit that there is on RTK’d vectors for the same basic reason.
With fast static you can get a good solution with only a few minutes observation time, but you are limited to short-ish baselines. Plus, you need software to post-process such baselines and do something with them.
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We always tried to jump the short backsights, that will tighten up the traverse. Haven’t done more than a three leg traverse in 30 years, but when we did do something like that it was always best to jump it, if possible, usually it was possible.
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“dwooley I’m unsure of the point you are trying to make.”
I think you are reading a bit too much into DWs comment. I took it as simply meaning that when staking out you want the backsight you are zeroing on to be as long or longer than the foresights you are staking out. But when control traversing a series of angles there will be error in the angles, which is related to the length of the sights, but which is the backsight and which the foresight is irrelevant.
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<div>A couple of “ifs”</div>
If your GNSS unit is able to collect data for static observations and
If the network you are using also collects the data for its users to have available (quick look at their website seems to indicate they do)
then
You could get a static solution on the two point pairs at each end. This might require a software investment as it seems you might not have the software to post process.
The Kentucky CORS system collects the data and it is available for download to post process in conjunction with your own units. It is very handy to be able to download the surrounding network station data for post processing as they are not all in the national CORS database. I’ve got some Kentucky ones nearby, but the national ones are much further away. So I can get a good static solution from the nearby stations with a shorter observation time.
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The traverse was completed yesterday. The closing azimuth was 6″ less than the record azimuth. The traverse length was 3338′. The error of closure was 0.217′, giving me a precision before any adjustment of 1:15382.
I will add 1″ to each turned angle then recalculate traverse points 3-8 (C101-C108, C for conventional). I have a question about how to do a compass rule adjustment for my closed link traverse. Holding traverse points 1 and 2 (GPS101 and GPS102, GPS for RTK), do I only adjust traverse points 3-8 (C103-C108)?
MH -
The scaling of state plane coordinates varies dependent on your relative position on the grid but will will typically be on the order of 0.1’/1000 feet. You have a misclosure on the order of 0.3′ in a bit over 3000 feet. Do you see the pattern here? Comparing SP grid coordinates to ground measurements this may account for the greater part of your misclosure. If you have not already done so by some means determine the appropriate Combined Scale Factor and use it to scale your measured ground distances. Then recalculate your traverse.
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@ Norman_Oklahoma
Comparing SP grid coordinates to ground measurements this may account for the greater part of your misclosure.
GPS102-GPS101 Grid distance: 629.431′
GPS102-GPS101 Ground distance: 629.515′
GPS107-GPS108 Grid distance: 677.985′
GPS107-GPS108 Ground distance: 677.981′
If you have not already done so by some means determine the appropriate Combined Scale Factor and use it to scale your measured ground distances.
I don’t know anything about this, but I’m learning. Isn’t the grid scale factor calculated by my field software when I create a new job?
Please see the attached file if you want to review my traverse.
MH -
This is should help. This is not exact but should be close enough for a math exercise. Take your state plane starting coords. And all your sudo state plane coords along the route. Go to NGS website for NCAT. Plug them in appropriately units datum and zone. You can do one at a time at the bottom you can get the CF combine factor for each one. Your starting pair and ending pair are true the ones along the route are not exact but the CF will not change that much. Write all those down. Take your raw run between all points as is convert or inverse those pairs get the bearing and distance. Draw it on on paper. Make sure they are running in the forward direction. Set up a table the first pair is known that distance inverses assume is correct grid distance. From your first occupy to the first fore sight you have a ground distance. Take the cf of occupy and cf of fs. Add and divide by 2 that’s the avg cf of that line. Now show ground distance use the cf to convert the ground to grid distance. Hold your bearing do this throughout. After all ground distance are converted to grid. Cf * ground dist = grid. Plug those into your table. Two pairs are already grid distance starting and ending pair I assume. Now it’s just a triangle. Right triangle. Take the sine and cosine of the bearing convert to decides first. Multiply that x’s the grid dist this will give you a delta north and east. Watch your signs neg and positive. Keep those in your table. Once all through. Take your angular error. And the knew deltas north and east we call these latitude and departure simple arithmetic now to get those deltas to your starting coords. And see how they end up on ending coords. The angular error is also in need of adjustment. That’s a quick not exact compass rule. There are some rules to follow like in theory doing a compass rule I usually start with the most westwardly point. This is not exact. I just can’t type every step on the phone lol. But that should get you heading in the general right direction.
This is more of a combination of grid traverse and adjustment together. This assumes that all c/r differences in height of target and instrument have already been accounted for. So we are just adjusting the sudo ground coords along the route. To grid. Then reapplying the angular error aka bearing misclosure to readjust . I would honestly have to sit down and look at everything to have all the steps. This is just in my head.
- This reply was modified 2 months, 3 weeks ago by OleManRiver.
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@ OleManRiver
I’m not sure about private messaging on this new website. Thanks for the info. I’m using this job as a model for similar jobs in the future.
MH -
I think the second example of grid vs ground should be 677.905′ for the grid distance.
The ground to grid formula is Grid Scale x Height Scale x Ground Distance=Grid Distance; or Combined Scale x Ground Distance=Grid Distance.
In Florida near Sea Level it’s quite possible that the Height Scale will vary between 1.00000something and 0.99999something. That depends on if the ellipsoid is below your feet or above your head. No doubt the height factor is only a tiny contribution to the final number.
You’re working at about 120 PPM; the height factor is probably around 5-10PPM.
Your angular closure of 6″ implies that the “error” is all distance as Norman has indicated.
Comparing apples to oranges is not good practice, you need to develop the terrestrial survey coordinates as grid or the GPS as ground before adjusting. However, that being said a compass rule adjustment using grid or ground distances will probably result in the same coordinates because it will automatically shrink the ground distances proportionally for each course. Still not a great idea.
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Question about this:
GPS102-GPS101 Grid distance: 629.431′
GPS102-GPS101 Ground distance: 629.515′
GPS107-GPS108 Grid distance: 677.985′
GPS107-GPS108 Ground distance: 677.981′
To get these values, did you collect the GPS points and start with them in the collector based on just exactly what was collected via NRTK and have your data collector set for grid<->ground.
Then run your traverse by setting up on GPS102 with the total station while holding the NRTK GPS coordinates of 102, backsight GPS101 zero out and hold the NRTK GPS azimuth between the two points and shoot GPS101 as a stored point with the total station.
Traverse through and then occupy 107 holding the total station derived coordinates and shoot 108 with the total station and store another coordinate value based on traversing for 108?
Then inverse between GPS coordinates and inverse between total station collected coordinates to report the distances you provided.
Another whole lot of IFs –
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.
You may have been starting with as much as 0.08′ of linear error due to a bad NRTK position on point 102. If that is the case, you might reduce your linear error to about 0.137′ with a better position on 102 and the closure would be more in line with 1’/24,000′.
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.
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Don’t know if anyone has wrote this yet but adjusting two tenths out of a traverse based on RTK observed control is a fools game if that’s what is occurring here. I had a party chief once who wanted to spend days chasing his tail around a project using RTK observations. He’d still be measuring if we let him. It’s bass ackwards to use a higher precision measuring device or method to run traverse than was used to establish control.
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@ Norm
I appreciate your input, but this is our standard way of doing things. My coworker and I are trying to institute new methods of doing things. How do you suggest we setup a traverse and what methods do you suggest? The project consists of locating property corners on both sides of a road and locating all above ground utilities, sidewalks, and pavement. Locate everything from right-of-way to right-of-way. Elevations will be used when appropriate. No natural ground shots, swales, or centerline of road.
MH -
If you PM me I can walk you through setting up to perform all the correct comps. First you need to get the data on the same system. That’s where scaling ground distance to grid first. Then you will know your true closure. Then the adjustment can be done. It’s not hard. You can google compass rule and get that part figured out for sure. Plenty of YouTube and text book documentation out there.
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What you need is appropriate field and office software that work together seamlessly and a set of processes and procedures. While your current method can work its unfortunately very inefficient and potentially inaccurate. Too many non centralized components (GPS, conventional, scale factor) for my comfort zone.
We do this type of work on a daily basis and never have the issues you’re describing. I’m not trying to criticize you as I understand you can only work with what you have to work with but whomever is in charge of the overall processes and procedures and hardware and software should seriously hear your issues and make changes that will allow for downstream efficiency and consistency.
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“How do you suggest we setup a traverse and what methods do you suggest?”
I do this sort of thing almost daily using StarNet to simultaneously adjust the RTK vectors and the terrestrial traverses. Handles all the scaling and grid calcs. There are other Least Squares packages (including SurvNet, which is included in Carlson Survey) which can also do the job.
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This is a very good point. StarNet survnet. Getting everything on the Same page. Like items. Rtk is less precise but if everything is weighted correctly and all items are on the same system grid or ground then you can achieve the best of both worlds. I still think this small traverse is a good one to compute long hand as that can get some very good understanding before pushing the buttons on software.
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@ OleManRiver
How do you PM someone on this website? I need this weekend to sort out all of this information presented to me.
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@ Jon Payne
To get these values, did you collect the GPS points and start with them in the collector based on just exactly what was collected via NRTK and have your data collector set for grid<->ground.
Yes, but the data collector was not set for grid<->ground. We never use grid<->ground.
Then run your traverse by setting up on GPS102 with the total station while holding the NRTK GPS coordinates of 102, backsight GPS101 zero out and hold the NRTK GPS azimuth between the two points and shoot GPS101 as a stored point with the total station.
Yes. Except I didn’t shoot and store GPS101 using a different point number. Look at the first line of the raw data below.
Traverse through and then occupy 107 holding the total station derived coordinates and shoot 108 with the total station and store another coordinate value based on traversing for 108?
Yes. The other coordinate value for GPS108 is C108.
Then inverse between GPS coordinates and inverse between total station collected coordinates to report the distances you provided.
Yes.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
GPS102(TRAV_PT) to GPS101(TRAV_PT):
HD: 629.431 Az: 181°39’17”
dN: -629.168 dE: -18.177
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
GPS107(TRAV_PT) to GPS108(TRAV_PT):
HD: 677.985 Az: 356°06’30”
dN: 676.422 dE: -46.014
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C107(COGO) to C108(COGO):
HD: 677.981 Az: 356°06’24”
dN: 676.416 dE: -46.033
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Edited raw data:
BD,OPC102,FPC101,AR0.00000,ZE90.00010,SD629.5150,–5/8 IRC O_C_ TRAV PT
FD,OPC102,FPC103,AR269.49010,ZE90.00410,SD593.6000,–5/8 IRC O_C_ TRAV PT
FD,OPC103,FPC104,AR174.12360,ZE90.05050,SD590.5550,–5/8 IRC O_C_ TRAV PT
FD,OPC104,FPC105,AR194.31360,ZE89.50030,SD148.5250,–N+D JWG LB 1
FD,OPC105,FPC106,AR87.42270,ZE90.30340,SD150.2150,–5/8 IRC O_C_ TRAV PT
FD,OPC106,FPC107,AR171.43390,ZE89.50420,SD547.6150,–5/8 IRC O_C_ TRAV PT
FD,OPC107,FPC108,AR176.27480,ZE90.11240,SD677.9850,–5/8 IRC O_C_ TRAV PT
MH
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