How do you combined data types (TS/GPS/Level)?
Hello all,
What method do you use to combine your total station data, GNSS data & levelling data for high accuracy control networks?
For example, do you set a couple points with GNSS, then setup your TS on those points using the same data colector? Or maybe start a GNSS control network then shift and rotate your TS network onto which ever two GNSS points fit best in CAD? Or do a Helmerts transformation between the two? Do you hold the elevations gathered from the level run over all else?
To convert from grid to ground, would you choose local a scale point, pull that positions CSF and apply it with a simple scale command in CAD? Any tips and tricks on data management so you don't confuse which system the data is now in?
I've only ever really used a LSA for the last 15 years so I don't have much experience in alternate methodologies.
I have used C3D to handle scale factors between grid & ground and it actually is really slick for that feature. I have a YT video on that if anyone is interested.
I do it all the time for deformation networks. Least squares is critical, and it is very important to properly weight the observations when mixing. We use GNSS vectors (post processed) to provide scale and orientation. Usually just fix one point (stable). Then compare coordinates at other stable points.
Working on one this morning where I am getting 8 mm on one line observed multiple times. Turns out that some of the obs between those two had -29 for the offset when it should have been -40. So having a lot of redundant data and different types of data is great for detecting blunders, even small ones
Well, most places I've worked didn't want the crews scaling data in the field so right away you have a roadblock to mixing GPS and total station data. So yeah, normally I'd set a few control points with GPS, call them ABC, and go about collecting GPS data. Total station data would be collected from these same control points but they'd be named something else like XYZ. Once the GPS data was scaled to ground everything could be combined.
It is a rare occasion when I do not combine GNSS and TS data for a project. I'm using Leica Captivate and Infinity with both GNSS and TS data routinely collected in the same DC. When working in a known coordinate I always let the TS scale the distances to the current instrument points. My current practice is to observe several control points in the planed network multiple times with GNSS (FS and/or RTK). For the first TS setup I will get a quick GNSS observation on the instrument and back site points. These points do not necessarily need to be GNSS points that will be used in the adjustment, although they usually are. They just get the TS on the coordinate system. I then run my TS network observing GNSS on appropriate points. In the office I QC the raw data and process in Infinity. I like to do quick GNSS and TS only adjustments to check their independent quality. All of the data is combined for the final adjustment. This works well in Infinity and you can do the same in Star-Net. The combined adjustment approach ensures the more precise TS observations are weighted more for the relative accuracy while the GNSS observations get you absolute values tied to the fixed control.
Prior to Infinity, LGO did not do such a great job with TS data so I would normally adjust GNSS in LGO, fix the points and then adjust the TS data in LisCAD. Unfortunately, this method distorts the more precise TS observations to fit the GNSS points.
Being a one-person shop, I rarely break out the digital level. If I do, I usually run through the primary control and throw that data into the final adjustment.
As for scaling to ground, I like to avoid it if possible. Oregon has 39 low distortion projections, so it is not much of a problem here, but I also do a fair amount of work in Washington and California where SPC rules. If I need to scale grid to ground, I always do it about 0,0,0 and I always truncate the values to distinguish the ground values from true grid. Remember, friends don't let friends scale without truncating. I know a lot of surveyors like to scale about a point within the project and not truncate to allow for things like aerial imagery to appear to fit. Just letting you know, that down the road some engineer is going to strip your control meta-data. You can set these scaling/truncating parameters up in C3D to allow for the correct inclusion of SPC based imagery and GIS data.
We're a Trimble shop so it's all seamless, GPS or TS are both done with the same data collector(s) and software.
As for scaling to ground, I like to avoid it if possible. Oregon has 39 low distortion projections, so it is not much of a problem here, but I also do a fair amount of work in Washington and California where SPC rules.
There was an NGS zoom meeting a couple weeks ago discussing the new low distortion state plane zones in Idaho. We're going from 3 to 9 if I remember right and someone asked if we'll still need to scale from grid to ground and the NGS guy said yeah probably. Maybe on the next round of state plane tweaking we can get 50 or 100 zones so we can dispense with the scaling...
I've really been enjoying your channel, especially the videos that include some Star*Net. Thanks!
I use Star*Net to combine GPS and Terrestrial data, routinely. Have done so for nearly 30 years. I have also done it in Liscad (Lieca) a few times and in SurvNet (Carlson) once. They work.
I use Oregon's Low Distortion Projections and loath using anything else. If forced to use State Plane I avoid scaling. IMO, if you require me to use SP then you accept the scaling issues.
When forced to scale, my habit has been to scale around point 0,0 and end up with coordinates that are a couple hundred feet off of being SP grid with no truncating. Thankfully, I haven't been obliged to do that for near 20 years.
9 zones in Idaho? I'd prefer to have a few more if I was working there, but you will just have to accept the scaling and convergence that gets you. Others have decided that is good enough. It will be a lot better than SP has been.
Thank you for all the replies so far!
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 (not the end of the world)? Or more importantly, orienting all subsequent TS measurements based off the inversed bearing between two GNSS derived points? Do you have a method to tighten orientation directly in the data collector when using this method? Or do you use TBC to post process it at all? Could you tell me more about your process?
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?
Least squares seems like the obvious winner (in terms of accuracy but obviously takes a bit more time) to me but I am ignorant to other methods so I appreciate all your input.
Very interesting notes about the addition of zones to reduce distortion caused by scale factor. Coming from surveying in BC for the majority of my life where we use UTM with scale factors in the range of 0.9996#### to Florida where the SPC zones are much closer to 1.0, it's already not what I'm used to at all. Hearing that even relatively large construction projects here just ignore grid to ground blew my mind.
What kind of scale factors are you all dealing with in Oregon?
I do like the method of scaling about 0,0 and truncating the numbers for the exact reason you mention Norman. The benefit of having your ground coordinates land relatively close to the right place for approximate geolocating purposes are not worth the possibility of mixing the two up. 2 years ago I was on a water supply tunnel project that had two sides 1.3km apart going under a river. The previous surveyor was working in grid only but the design was in a ground based system scaling about the center of the vertical shaft on one side of the river. Which means the shaft on the other side was built about 43cm in the wrong place. It kept the engineers busy for a while on the redesign.
"What kind of scale factors are you all dealing with in Oregon?"
0.99999 (five nines) or better, generally. Scaling distortions on the order of 0.01 feet/1000 feet. Plus, convergence angles of a couple minutes, not the degree and a half that SP yields.
The Portland Zone that I use is valid for use over the entirety of 3 counties and stretching into a couple of neighboring counties. That number would be higher if a couple more counties in Washington State could be counted. It's not a real restricted area. I should admit that the origin point of the Portland Zone is just feet outside of the limits of my fair city, to which all my work is confined these days. But the scale factors and convergence angles don't grow a lot within the Metro Portland area.
