I am using Leica equipment GPS1200 in a RTK network and TS11 total station and doing my least squares adjustment in CADD Civil 3D. Shouls I be worried about combined scale factors since my distances rarely exceed 600ft? My colleague thinks that not apply it is a fatal mistake and sees this as the cure all. I was under the impression that this was negligible on small sites. I am not doing any boundary work this is just for asbuilt topo within a right of way none of my traverse exceed 3000 feet in total length. Help me put this issue to rest. I am getting closures in the high 30000's.
Yes everything that is quantifiable matters.
Eliminate all systematic errors before adjusting.
Depends ...
and I'm not talking adult diapers.
If you are near sea level and your combined scale factor is in the magnitude of a couple parts per million - don't bother. On the other hand if you're at 5 or 10 thousand feet and far away from the unity line and your combined scale factor is in the magnitude range of parts per 10,000 - then do it.
Depends ...
No I am in washington dc my highest elevations are 350ft
I do appreciate what you are saying but we are trying to be effect and have limited processing resources and I am looking to see if network will be greatly affected by adding scale factors.
Depends ...
Your location within whatever projection you are in may impact your decision. Calculate a project center CSF and see what you find...
I assume that Civil 3D will do an adjustment on the grid. Why apply a combined factor? Why not just work in the actual state plane coordinates and state that fact on the face of your map? If you hold your GPS derived grid coordinates and do not treat them as on the grid and then adjust holding the ground distances from your instrument you will introduce errors into your system. These may be small errors but errors still the same. To make matters worse, your closure will not show these errors if you are doing a closed looped traverse back to the same control azimuth pair. By simply telling Civil 3D that you are on the grid it should individually apply the combined scale factor for each leg of the network. This way you get true state plane coordinates an the errors in your adjustment are based on your network and not reflective of grid to ground distortions. Once you adjust it correctly you can evaluate said distortions and then bring everything down the ground if you think it is necessary.
:good: :good: :good:
Depends ...
I don't know which SPC system DC uses, but I'm guessing it would be MD SPC. And if that is the case, you are going to be almost dead center in the middle of the MD zone. If that is the case I'd bet your CSF correction is minimal - probably along the magnitude of a couple parts per hundred thousand. So, a couple questions come to mind. Is that significant to your project? Are you using field techniques of sufficient precision to justify the adjustment of your data? And finally, since the CSF values are probably trivial, will your regular traverse adjustment essentially accomplish the same thing at the end?
And another possibility, compute your traverse both ways and see if there is any significant difference. It's not a big deal to multiply your distances by the CSF and compute the traverse twice. Maybe an extra half hour? Try it for your own education if nothing else.
One other consideration
Where within your traverse are you tying into known SPC stations. If they are all on one side of your "closed loop" it may be more critical. If the known points are on different sides of your "closed loop" then it may not be as critical and a routine traverse adjustment may take care of small matters.
As always, you need to fully evaluate the given circumstance.
What I think you're saying is that you survey with GPS projected to state coordinate system values, then set up a total station and survey in that system. But, if the Leica is like my Trimble then the data collector already reduces the distance shot by the total station to a state plane distance. So there should be no factor applied in civil 3d, it should be taken care of
That is my basic stance, Thanks for the imput
I haven't worked with Leica in a few years but it would seem to me that if you've gone through the trouble of establishing SPC's with GPS for at least 2-3 of your control points then the hard work is done.
Bring the data into your GPS software, calculate a CSF and enter it into the collector... or do it in excel before you import into C3D.
Thats how we do it with Trimble anyway.
Gregg
One other consideration
Absolutley agree that is what I am trying to get them to understand this is low precission work we are doing it is for small water main replacement. Asbuilt topo we are not even building profiles so are verticals are +-.10 and it is retrofit so our design is +-.25 and one guy is hung up on combined scale factor because he read it in some manual and has never used it or understands its application. We are merely setting a control pair with RTK and running a closed loop traverse for right of way asbuilts for which we use GIS data for ROW information. No boundary work is done. I am trying to tell them we are just splitting hairs forget about combinrd scale fctors unless it is a large site and we do alot of RTK asbuilts within that site then maybe we introduce combine scale factor to maintain consistancey. Not to mention I am working between 50ft to 300ft in elevation we are so close to sea level. I understand the need for precission but this just seems like over kill. And I have to do all my processing in CIVIL 3D it is very limited processing survey data.
By simply telling Civil 3D that you are on the grid it should individually apply the combined scale factor for each leg of the network. This way you get true state plane coordinates an the errors in your adjustment are based on your network and not reflective of grid to ground distortions. Once you adjust it correctly you can evaluate said distortions and then bring everything down the ground if you think it is necessary.
Maybe I'm missing something, but why in the world would you want your cad program to bring down your coordinates or distances to state plane, that should be done as you survey. If the work is all in state plane each distance should be state plane as it's collected and when you download it into cad it is already there, use cad to adjust (again can't imagine why, but if it's all you have...) but not to reduce.
And if you want your project in state plane set it up in your data collector and go to work. It will handle the reductions.
If you are working in SPC then EVERYTHING in state plane, if you are working in ground coordinates then EVERYTHING in ground coordinates, don't mix and match. Makes my head hurt just thinking about it:-O
This is exactly my stance. And what I have been doing. But he has greated such a ferver about this that I was just trying to review my own practice to make sure I was not way off base. Thanks
So he is trying to apply a combined scale factor to the coordinates or distances AFTER the survey?
I reduce distances to mark-to-mark (as well as vertical angles). I then do a least squares 3-D adjustment, which includes GPS, total station obs, and leveling all together. I have no idea what the magnitude of the combined factor is. The adjustment is actually done in the ECEF system, then output as lat/long/height. After the adjustment is done I convert to state plane or UTM as needed. When I give the grid coordinates to a client, I also give them the convergence, grid factor, elevation factor, and combined factor FOR EACH STATION. So, no matter how big or small the network is, it is computed correctly.
On another point...you state you get closures in the 30000's...you will get this whether or not you apply combined factors if the traverse is a closed loop. in fact, your edm could be measuring all distances off by 1 foot and it will still close. If the traverse is open but mathematically closed (i.e. A, BS B, traverse over to C, FS D, where A, B, C, and D are known), then the closure will reflect the accuracy of the distances (assuming the known points are sufficiently accurate).
In the present times where we all have computers and software, I don't know why anyone would adjust or compute a traverse any differently than what I outlined above.
When I started out in surveying, calculators with trig functions were just coming out. I computed many a traverse by first running through to get preliminary coordinates, then computing the combined scale factor for each line using the N,E and elevation from the first run, then running through it again using corrected grid distances. Of course, nowadays that should be very simple, but I have seen some programs that do not do it correctly.
Of course, as you mention, it all depends on what the purpose of your survey is. However, I compute them all the same way to be consistent.
I guess I should have explained that better, but yes now you see what I am dealing with.
Kinda, I think; not really making any sense to me.
When you leave the office before you start your survey the projection for the job should already be set.
If there is a combined scale factor to get to ground, it should already be decided, incorporated into the job and inserted into the data collector. Only adjustments and possibly shifting to a CORS, OUPS, or Bench Mark value should happen when you get back and that should already be planned out before you walk out the door, then all this stuff about combined scale factors, which is really a tiny thing, will vanish. Frankly if you are on SPC with your network and such a low elevation and probably a small grid factor just stay on SPC.
