How do you combined data types (TS/GPS/Level)?
@bc-surveyor less than 50% of our projects involve TS station use and when they do it's usually on the construction side of things. We always start with GPS and GPS only, sometimes tied out to NGS mons but more often than not it's just an OPUS position.
Nothing we work on ever has a state plane requirement so I'm not at all concerned about being tied out to a single point. It's all relative to the project control once the project control is established. We post process all our data in TBC and establish our grid scale factor(scaled to 0,0, which IMO is the only way to do it) before we ever introduce any TS work, it's much easier that way. And we never truncate coordinates, i think that's an outdated practice and has caused nothing but trouble when I've seen it done. Once our SF is established in the office and field software, we're golden. Set it and forget about it. I've been using that process(with obvious software and hardware advancements) for more than 2 decades with very few issues that weren't man made.
In Civil3d we use the grid scale factor in the transformation tab to essentially set the project to Grid. We do this if we ever need to import GIS or state plane-ish crappy utility data we may receive, but mainly for the geolocation tool. We never keep that set, it's merely used to view things on grid or import data on grid. There is a lot more we do in Cad with the survey database and such but that's a topic for another thread.
> For those that combine data in the data collector (WA-ID Surveyor), are
you at all worried about relying on absolute position from a single GNSS
point
Never, ever, ever rely on a single point!
You need two as an absolute minimum for a check
Even then your error(s) could be around 2Sigma as a worst case scenario
Work the law of averages in your favour - with three points (plus a fourth for a check) the errors start to average out.
A resection may be better than setting up over a single peg
Everything gets aggregated in TBC, to mesh levels with GPS derived verticals, orthometric elevations are changed for the GPS points from the levels and Geoid18 is applied to the ortho elevations to get ellipsoid heights.
Horizontal control comes in many flavors, depending on the project requirements. Spreadsheets defining the elements creating the control is placed in the file.
For those that are using CAD (Bstrand), to me this seems like a better approach than the above as you can extend the baseline used to set orientation beyond just two points that require line of site. But are you not still stuck shifting to one point, rotating to another and do checks from there or do you have a method to “tweak” your results using additional GNSS/TS common point observations? You mention using 3 points, can you expand on your methodology?
Yeah, shift and rotate. This would all be done in TBC and I haven't been deeply involved in that process so I'm not sure what sort of tools TBC has that would allow the user to tweak a combination of GPS and total station data.
I just used 3 points as an example to keep the explanation simple; I always set at least 4 points to build the polygon around the site but even with 4 I think it's unlikely you'd have line of sight to all of the things that needed mapping, so more than likely there would be 5+ control points.
How do you shift and rotate one data set to another using more than two points?
Helmert's Transformation. A least squares best fitting.
How do you shift and rotate one data set to another using more than two points?
You can't, or at least that I know of, but that would be a question for someone with more familiarity with TBC than me. Like you suggested the other points would be used as checks.
ditto
As others have mentioned I used Trimble Access and Trimble Business Center. One data collector one job file all on the known Datum and projection almost always state plane.
In TBC I have had jobs where digital levels along with static and total station data along with RTK . I bring all data into a TBC project on the state plane. If the job requires ground I do my scaling in TBC by finding the best combined factor via looking at the average ellipsoid heights and creating an orientation and scale point. Then I performed least squares on observations. This allows me to have a project coordinate system at the surface or aka averaged surface and also be able to switch between true grid state plane and the project coordinate system. Trimble makes it very easy.
Most companies that I worked with usually set a pair or two with GNSS choose a point and scale those to ground and then use total station or robot to be at ground and do whatever else work. This can be done by scaling from a point or at an orientation of 0,0. I don’t like the whole choose a point and scale as no thought is made in seeing if that one points elevation is close to and average for a site it does work most of the time for being good enough in most scenarios. This would be in theory the same issue of scaling in cad you are using one combined factor for a site. Every points combined factor is unique to the scale the elevation or ellipsoid factor to create a combined factor. I basically try and follow a process in which I get the best possible solution for both worlds. Now again 99% of the time it’s not a big difference doing this .
With today’s technology in all honesty it’s not that hard to just stay on grid all the time as we have to scale something one way or the other ground distance to grid or grid to ground . I can understand back when 99% of the work was done with a total station and all the computations were done long hand to scale the ground distance to grid but the software does all of this for us .
We have lost the understanding of what all is going on today I see so many people use the scale factor in cad from a NGS data sheet which is incorrect I also see those who know we need to use the combined factor from a NGS data sheet but they use it incorrectly as they assume it is for grid to ground but its ground to grid . Here it’s roughly a tenth of a foot per thousand feet so we have a few miles project and in their head they say well we have all this error from one end to another so we must use ground they are not traversing from one end to another so why not stay on grid set points along the project on grid where it is feasible and mitigate the density with total static that also uses grid . Then if you truly need a ground system scale everything back at once for a more efficient project factor.
It is not that hard. Now I do understand the need on certain projects for sure but most here are not going to be affected either way as long as everything is on the same system
I've been using SALSA for about three years, it works great, we wrote our own software to convert Trimble JobXML files and project files from Topcon's GLS 3D Laser Scanner, so we include GPS, Total station, Level and Laser scanner observations. 😉
Generally we use a minimum of three static positions, a maximum of four, two of the GPS antenna's have prisms underneath so we can set all static positions up the day we get there and then resect off the two that have prisms. If it's a long job, we break static GPS obs up into weeks and process them 1 week at a time through AUSPOS.
