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Tuesday, June 6th, 2023
Looking for insight on getting ground distances using a GNSS Rover on public or private ntrip RTNs and using state plane coordinate system. What is best flow to set a scale factor in the field prior to data collection? I think there is a way to calculate from an observed point. I assume it saves to the survey file for stake out later. The state does not have county or LDP coordinate systems. Typically seeing 1.5' in a half mile on SPC.
Not to be too cliché, but I'd say the standard surveyor's answer of "it depends" applies here...
It's been several years since I have used Survey Pro, but if I remember correctly there are a variety of options for computing and working with ground/grid values. You can choose to work in grid or ground and apply a single CSF, or apply a unique CSF at each station position.
If you're really seeing 1.5' per half mile (!) and/or your project is larger I would probably want to go with the latter configuration, which is more rigorous than using a single CSF.
You can always change those settings during post-processing and adjustment, prior to computing stake point locations.
First and foremost - using an RTN has absolutely nothing to do with obtaining cartesian aka grid coordinates other than the ease of it. Or at least it shouldn't. That's all done within the data collector and/or office survey software where you should be validating RTN observation to SPC high accuracy ground control. (FBN or CBN NGS stations - BTW you can't get these stations on your FM or AM radio. You have to know how to use NGS data explorer.)
I'm not a Survey Pro user but it took me about 5 minutes to find a user manual on line. It says "– If your job is using a map projection and datum, Survey Pro will automatically calculate the correct scale factor for each station setup so the distance measured on the ground will properly be reduced to the coordinate grid. If you start a GNSS survey, Survey Pro will use the selected projection to calculate grid coordinates from GNSS measurements."
So this is being reduced to the SPC grid if SPC is your selected map projection. In most surveying software there is a option to use SPC and in addition scale to user defined grid. I have seen this option used and abused so many times we developed statewide LDP. But I digress. Countless times I have seen a double whammy scale applied resulting in claims such as yours that there is twice or four times more error then there actually is. Double whammy is when a combined factor is applied to SPC grid when its intent is for it to be applied to distances measured on the ground. When your field software computes your SPC grid for you you need to apply 1/ CSF to get a user defined surface distance. Therein lies the double whammy.
I assume in a GNSS survey each station setup would be each rover location but that's a guess. Hopefully it's not the RTN reference station only.
I also assume there would be a way to select one scale factor for an entire project rather than have one factor for each location. However in theory one factor for each location is correct.
1.5 ft in half a mile is pretty hefty. Here in the Iowa the absolute most we see is 0.3 ft in a small area. So I'm assuming you must be high. (he said with tongue firmly planted in cheek). A quick check using NGS data explorer tells me that in Aspen CO at 7700 ft the difference is 1.15 ft in half a mile you you must be above that.
Look at an NGS horizontal control monument data sheet near the area and elev. you are and the combined scale factor will give you the approximate scale you should be using for your user defined grid representing ground. Remember, if you are going from SPC grid to user defined grid the the formula is 1/CSF.
I had a call from a frustrated party chief in another district one time who was dealing with this. The claim was the RTN was changing his coordinates. After some questioning I discovered his problem. Probably not all of them. He did a field calibration on a few control points, tied in some corners and then moved up the line. When he moved up the line he shot another control point and missed it. He explained to me that since he missed it so far he added it into his calibration and wonder of wonders the next time he shot it it was good. This happened several times the further he went. He was frustrated because when he went back to the beginning he couldn't hit the stuff he had previously tied in under one of the earlier calibrations. Sigh. I had to call his boss and tell him he was unqualified to operate the equipment. That wasn't received very well either.
While 500+ ppm is quite a bit, it's certainly possible here in the Mountain West (especially in West-Central Montana w/ its Single Zone Lambert), it IS encountered more often than some folks might expect. Ski Resorts and and mountainous Mining Properties are the most likely places to encounter "grid-ground" numbers of that magnitude, and ya either have to deal with it, OR NOT. SPC projections are simply a very poor choice for these types of projects, and even customized LOCAL LDPs are not going to be perfect.
Like Norm said above, the RTN/VRS doesn't "work in SPC," it "works" in ECEF XYZ (meters) and it's the Data Collector that converts that to LLH and then to whatever "mapping Projection" that the user selects (UTM, SPC, LDP, whatever).
While generating (designing) a "local" project specific LDP isn't particularly difficult, it does require a reasonable understanding of mapping projections (Transverse Mercator, Lambert, etc.) as well as a basic grasp of geodesy in general. Once you have good projection parameters determined, then you have to understand your Data Collector Software well enough to get those values into a "user defined" Coordinate System that said DC can work with.
Over the last couple of years I have had half a dozen projects in which the UTAH VRS (TURNGPS) has been used along with several LOCAL LDP projections. Both Trimble and Carlson SurveCE based data collectors have been used without any problems. I ASSUME that other DC Software systems can as well.
Again...DOCUMENTATION is critical! And calibration/localization is NOT your friend (except when it is).
Loyal
calibration/localization is NOT your friend (except when it is)
Worth repeating.
Good to see your name Ladd. It's been a while. Are the leaves peaking up your way or is it too late?
This is just one experience of many that led to that expression. That and all the time wasted doing field calibrations when the data needed to put it in the settings is given. Then just validate as you go.
@norm Now that the cold-front has pushed through and the sky has gone gray, the leaves seem to be coming into early bloom. I can only hope for a warm, sunny weekend for a bit of "wind therapy" on the back roads of this tri-State area to take in the sights only Mother Nature can paint. 🙂
I see the words frost and freeze in the forecast. I was hoping one of these years we could skip all that.
One common thing I have seen cause crazy scaling problems is to have the wrong SP Zone set. Such as when you are working in OR(N) zone and you accidently set your dc to OK(N) zone. Or else your have local 5000,5000 coordinates and you have your dc set to a SP Zone, even the appropriate one for your area.
One common thing I have seen cause crazy scaling problems is to have the wrong SP Zone set. Such as when you are working in OR(N) zone and you accidently set your dc to OK(N) zone. Or else your have local 5000,5000 coordinates and you have your dc set to a SP Zone, even the appropriate one for your area.
Not too long ago a new crew leader in one of our other offices started telling his fellow crew members they should always start off their job in a state plane projection, and then calibrate to the assumed 5000/5000 calc points they had been given. 🙄
Had several screwed up jobs that needed to be fixed before we squashed that "procedure". Honestly I think that half of these people just make stuff up as they go.
One common thing I have seen cause crazy scaling problems is to have the wrong SP Zone set. Such as when you are working in OR(N) zone and you accidently set your dc to OK(N) zone. Or else your have local 5000,5000 coordinates and you have your dc set to a SP Zone, even the appropriate one for your area.
Not too long ago a new crew leader in one of our other offices started telling his fellow crew members they should always start off their job in a state plane projection, and then calibrate to the assumed 5000/5000 calc points they had been given. 🙄
Had several screwed up jobs that needed to be fixed before we squashed that "procedure". Honestly I think that half of these people just make stuff up as they go.
Ignorance is like a virus, it spreads and it mutates. Even if procedures start out reasonably good, short cuts here and there degrade things really fast. What appears to "work" okay on small and/or flat projects, will often blow up in yer face when things get bigger (horizontally, vertically, or both). Training, Training, TRAINING! It used to start with a year or two following the dumb end of the chain around BEFORE you even got a chance to touch the "gun," followed by additional YEARS before you were making calculations or running the crew. While some things have gotten much easier, that doesn't mean that the "easy-button" can be employed without any knowledge of what is going on behind the curtain.
Loyal
Ignorance is like a virus, it spreads and it mutates.
True
