Hey guys, i did a job today and the backsight was .059 out on distance when i set zero to start. I was set up on a GPS magnail backsighting T.1 spike. This is at about 300 shots into a job where one lot and house has been located with 3 monuments so far. The job is a title survey including 3 lots 3 houses. Is this too large of an error? If so, what would you do to remedy the situation? I continued the job from that point with 600 more shots including traverse, control, house corners, etc. Is that ok? Thank you
0.05' between RTK points is typical. You're fine.?ÿ It's not like it's anchor bolt layout. 🙂
As Sirveyr said, that kind of slop can exist in control that has been set with RTK only.?ÿ However, 0.075' is the difference in prism offset between a 30mm offset prism and the 7mm offset of the 360?ø prism with my Topcon PS.?ÿ So make sure you were using the right prism offset.?ÿ?ÿ
Thank you for your replies. However, i wasnt fully clear in explaining the situation. The gps was set up on the mag only to take an elevation. we then sideshot the mn in 3 times from T1 and flopped out so the instrument was on the gps mn and backsighted T1 then proceeded. Im leaving a note on traverse sketch so the office knows. Is that still ok?
There is about a tenth of foot difference between prism shot and prismless shot at a prism on my Sokkia.
It changes from Mini, to Seco, to Retro style prisms.
Dont know what it would be on Leica or Trimple or other.
Depending upon the size of the urban areal and your state BOR requirements.
In New York your requirements may be 1:50,000 or more as they want 3rd decimal accuracy.
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I'd follow up with"the office" personally. Writing notes is good, but it doesn't assure that they get read.?ÿ Your 2nd post makes me think even more that it's a prism offset problem.?ÿ ?ÿ?ÿ
Ive had the prism offset problem in the past. Its definitely not that. All other traverse works. No settings have been changed. Our backsights are often slightly out but we try to keep it under .04. This time i let it slide and im having second thoughts. I did talk to the main map drawer and she said it should be ok but we'll have to see from the boss. That didnt give me much relief. The chief is somewhat an accuracy nazi which is good but at the same time lets a lot of things slide. So im asking you guys, What are your standards for acceptable backsight errors?
It depends on how the setup coordinates were derived by the controller. Sometimes on projects with a lot of control points and redundant observations you can get the occasional large deltas but it all comes out fine in StarNet. If the coordinate came from a backsight on one end of a baseline then you went to the other end I would would expect a sub-hundredth delta. If that's the case then something is out, prism constant or rod bubble out or tribrach out (got dropped or just normal wear and tear).
My last traverse on the job on my desk today was slightly over 9,700 feet long with 8 hubs and setups with an uncorrected misclosure of N47?ø55'45"E, 0.0441 ft.
That is and has been the normal result since the TS came out in the early 80s using tripod, tribrach and tilting target and prism.
My difference between a TS shot and GPS static data has been no more than 0.08ft near or far using one control point and one rover and not over 0.03 using two control points and one rover.
I have had people follow me with robots and report the differences you are reporting.
I was following RTK for part of this survey and was finding monuments out near 0.4ft difference than reported in less than 5yr deed on 22 acres.
2/3rds of the property had descriptions in varas that predated the Farm to Market Highway built in the 1950s and there was little to compare there as none of those rocks were found, yet there was a rebar or pipe or concrete monument at every fence corner.
My normal is not everyone's normal and remembering from college, not everyone of us 14 classmates using the same equipment ever setup and reported the same exact angle to the second for the angle between 3 hubs.
They all were very close but never equal to the same second using a 1sec gun or interpolating using a 5sec or 6sec gun.
When I set on the next hub and check the distance to my BS, if it is over 0.01ft I will check that tribrach and find it is usually not centered on the hub or it has been knocked out and needs adjusting.
Running traverse with a prism pole being held and stacking lath for BS is different, the results depends upon the rod holder on the FS and wind speed at the BS.
0.02
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Dave beat me to it, but I will add again: check your tribrachs.?ÿ Are the legs tight??ÿ This time of year they can loosen up.
Ken
Could be an incorrect state plane scale factor.?ÿ?ÿ
When I use GPS for elevation I do an extra long static, and often hold the GPS elevation over a typical traverse elevation. However 0.059' on distance is unacceptable to me for distance on a traverse or a scaled GPS position.
In other words, "IT IS NOT OK".
Paul in PA
How long was the backsite? Also 900 shots on a house survey, what kind of lot survey are you doing?
The backsight closure in the OP would be acceptable for me when doing they type of survey that is described. However, not knowing how long the backsight is or what its comprised of makes me pause. Also, for the type of work described and moving forward, I would be checking the back sight more often than 300 shots and probably would have checked the setups during the observations.?ÿ
Context is everything as far as your back sight error goes. If all of your side shots are equal or less in distance to your back sight and your locating improvements that don't require a higher degree of accuracy, it's not much of?ÿan issue. Store a shot on the backsight as a check, but fwiw, standard operating procedure for me when it comes to mixing RTK and conventional is to have a third RTK control point that I can check into so I have a better feel?ÿfor the errors in the control. If you're starting a mile long traverse with a 300' backsight and you have .06' of error in the distance starting off, you can count on that the error growing exponentially the farther out you get from where you started and plan accordingly. All this assuming you've eliminated the sources of non random error in the equipment like prism offsets, faulty tribrachs and the like.
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As others have said, it is probably slop in RTK shots with GPS.?ÿ Next time you start with the TS and see this much error in your BS, take a sideshot and store a new point on the BS.?ÿ If you have to occupy your BS point with the TS, use your new point number.?ÿ This keeps the slop out of your traverse control and makes everything check a little tighter.?ÿ Running a rural boundary traverse using plumb bobs and mini prisms you won't see much over 0.02' difference is FS & BS.?ÿ Tribrachs and legs should be much tighter.
