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Best practise for surveying boundaries?
Unknown Member replied 2 years, 5 months ago 19 Members · 49 Replies
- Posted by: @lukenz
Don’t know, find the local network RTK not anywhere near as good as your own base <1-2km from where you are working, only use the local CORS site for 0.1m work as usually 10km+ from it.
If you’re running with a single-base solution from an RTN station, for sure it’s not going to be as good as a network solution or base-rover. And not all networks are created equal, as you mentioned.
Posted by: @lukenzThat timeseries plot is only 1 sigma too, I’d be doubling at least, doesn’t seem like a very good place to have put a CORS site with that level of movement.
Those are pretty typical values. I doubt you’re going to find a ground-based station on Earth that doesn’t fluctuate relative to the global reference frame by at least a few millimeters per year, some more than others. Most CORS, even the ones that have been operating for decades, have estimated network accuracies (FGDC 95%) of around 1-5mm horizontal with 1-3mm standard deviation. That does vary a bit, but I’ve never seen one with zero values.
It’s also worthwhile to point out that the short-term time series should be evaluated in two ways. First by (a) the “minus” value i.e., comparison of the tracked position versus the published position, which is going to drift more and more the further we get from the epoch date; and (b) the STD or standard deviation values, which gives you an idea of how much fluctuation there is on a daily basis, and thus how repeatable that position is regardless of its drift away from published values.
All that being said, 4D coordinates and time-dependent transformations are going to be critical once we move to an NSRS defined by active stations.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman @mightymoe We can get a decent solution if it is reasonably open sky and we are up out of a hole. We’ve had pretty good success this year on monuments in the trees if I can see blue sky and we are up on a hillside, on a ridge or mountaintop. I would say our repeatability is a little worse than 1 or 2 hundredths but not much.
Three monuments down in a hole with more canopy we are having to tie from a GNSS control pair but two can be done with just one pair within vision and one is going to require a little bit of traversing, easy though since there is a road, no clearing. The fire last year burned out all the underbrush.
I have a survey 4 years ago that is based on an OPUS solution at the south end. This year we are using an OPUS solution near the north end. The OPUS points are three miles apart. We tied in two control points and a monument at the north end of the 4 year old survey and match horizontal about 0.05′ and vertically a little worse, about 8 tenths but the differences if I hold the 4 year old values is about 2 tenths. The two control points were used to tie in a monument which was un-GNSSable 4 years ago but now it’s burned out enough that it can be observed. There was a huge 15′ diameter log there which is now just some charcoal except the root ball lying on it’s side somehow survived.
- Posted by: @rover83
If you’re running with a single-base solution from an RTN station, for sure it’s not going to be as good as a network solution or base-rover. And not all networks are created equal, as you mention
It’s because the NRTK results are poorer than base/rover (in <2km range) I use base/rover if accuracy required otherwise just use the nearest CORS (in 10-20km range) for rougher work. In fact the CORS is the best metal dector I’ve ever had as it is broadcasting correct coordinates for our datum, turn on, set out and dig hole. Makes getting started so simple.
The local CORS is state funded and 4 constellation (as is my gear) whereas NRTK is private and subscription based and in our area only GPS/GLO which is also a factor in not using NRTK.
NRTK here based on other stations 70km+ away where weather can be quite different (and lots of NZ close to coast and weather changes blow through) therefore can’t assume linear interpolation between station and survey location, think we’d need more like 20-40km spacings here which wouldn’t be economic, at least until all cars are relying on it to keep them on the road. Plus a lot of NZ is rural and cellular coverage not good enough for NRTK as latency rates too high.
Also was more talking about the longer time series chart first posted not the daily one where the numbers are pretty tight. Agree that even excellent CORS sites move around in geological and tectonically stable locations at mm level and even cm/dm level over a year which isn’t an issue for day to day work.
We have one half of NZ on one plate moving 50mm north a year compared to the other and also where I am we are geothermally active area so earth’s crust is heaving up and down over the year.
4d coordinates scare me, find some local surveyor struggle with using correct datum let alone getting time correct as well. Be lots of messes to sort out!
It means that CORS points have a velocity. It varies depending where those points are.
Look at a datasheet and see the changes to any HARN or CORS point over time.
Once a project is controlled, it becomes irrelevant what OPUS or CORS is doing. From the moment control is established to the time the project is finished the numbers on the control points are fixed and not changeable unless they are disturbed.
The fire surveys have been interesting. Each time you brush against a limb your pants or shirts get covered with black lines like a sharpie.
@fobos8 there is no measuring tool in this profession that doesn’t include some type of error, whenter it be systematic, compensating or operator errors. The perfect examples are not frequently checking and adjusting instrument calibration, setting the proper ppms & temp or just plain operator errors.
Each piece of euipment has its benefits and detriments. The key is knowing which methods and equipment to use in any given situation or conditions on the work site. Being able to achieve the desired results, no matter what equipment is used making the proper adustments and using the propper settings is what makes us the experts.
One thing that strikes me though is that you advised a potential client that another Surveyor was doing it wrong by using GNSS equipment on boundary work. Without being able to prove that the other person was producing gross errors based on the methods employed, you’ve crossed a certain line of professional ethics based on personal opinion and not fact.
I was trained old school and for my own reasons, I will not allow my VRS RTK kits to be used for certain things, not that the use of it would be wrong but because I need tight vertical information on certain projects that I know will be quickly moving on to design and construction projects. I am pretty confident the those of us owning total stations and GPS/GNSS equipment are well aware of what our desired results are required and which piece of equipment is needed to achieve them.
In the US, the objective is to locate monuments, many set with very old tools, with tools that can report precision adequate for subsequent surveyors to retrace the same monuments. I don’t know about the UK but when US surveyors use high precision tools to change the position of existing monuments, (pincushions) they are 100% wrong and completely illegal
I would add that i know nothing about survey law in UK but I visited the UK some time ago and loved the country and the people!
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