For many years surveyors have described using a process while collecting a point in which they collect a point, then turn the pole 180?ø and collect a second point on the same position and average. The idea is that this will cancel rod leveling uncertainties. I'd like to submit two reasons that this is ill-advised:
1. Phase Center Offsets. Today's GNSS antennas have greatly reduced the phase center offsets, but the offsets are not negligible in many cases. Phase center offsets occur when signals from satellites are received by the antenna at different elevations and from different directions. Think of a circle around a point where the radius point of the circle is not coincident with the point. The offset between the circle and the point will change depending on where you are on the circle. The elevation angle will also affect the offset. While there are models for this, generally the phase center offsets used in the RTK solutions are based on orienting the antenna to North. When the antenna is not oriented to North, the offset anticipated by the software will not be entirely true. Generally the greatest difference is in opposing directions (North vs. South or East vs. West). So some differences observed when turning the pole 180?ø may not be in mis-leveling, but due to the Phase Center Offset. This can be tested by putting the rover on a Tribrach with a rotating center and observing points with the rover oriented in different direction (I'd recommend 45?ø increments) from 0?ø-360?ø. With enough observations, a pattern should begin to form. Note the changes to the horizontal position and the vertical position.
2. Pole mis-leveling can be treated as a constant if the GNSS antenna is consistently oriented the same way for each observation. Assuming that the pole is out of level to the Northeast by 0.10' when the antenna is pointed North, then each point collected will be "off" by 0.10' in the same direction resulting in all surveyed points being accurate relative to each other, even if they are biased by 0.10' from the correction source. This brings up another thought... to reduce possible systematic errors from rover pole leveling errors and base leveling errors, don't set the base up on a control point, but rather locate the project control with the rover, careful to maintain the proper orientation so that the observation on the control point includes the same error canceling technique that observations on other rover points will have.
Great analogy. However you can rotate the pole and still maintain the north direction of antenna. If its not the fixed 2 piece rods. I believe in theory and in practice you are correct. But observing static data for days worth of static I do not see how rtk is accurate enough to see this modeling in every day practice. When a antenna is relatively modeled to perform the antenna calibration it??s about 24 hrs worth of data. When absolute depending on who is doing them then say about the same time frame. Now You have made a great point and most definitely gets the gears turning. I always spin the rod 90 or 270 not 180. On my 2nd observation. I usually do a quick field ck of the bubble first thing of the day. Its not perfect because I barely loosen the bi pod where i can spin the rod 180 to see if bubble is good. Its not as stable as a rod checker but same process. Your way is good as long as we accept the introduction of a systematic error. Now with all these new antennas for rover receivers I admit I have not looked at the APC and offsets like I studied with various geodetic antennas and choke rings. But we are talking very small mm??s . Yes the algorithms have to be thought about as well. This post makes a strong argument for absolute antenna calibration vs relative antenna calibration. But even then absolute is only as good as the geographic location in which it was performed. Got to love the science. NGS has a very smart lady who really dives into the two different calibration techniques for antennas. I do like your post and believe it provides for some sound practices for surveyors to consider. I don??t see the survey community practicing as many standards these days. Use to everyone had some basic field procedures that from one company to the next all used. Now I see a lot looser standards.?ÿ
we use to have a standard that the eye piece while traversing and using forced centering techniques had to all face the same cardinal direction. Not saying it was correct but i worked where it was used. I introduced just the force centering technique and was laughed at and told it was not good surveying practice to use forced centering. But when every day traverses started exceeding what they had done previously there minds were changed. ?ÿI will have to set up and see if i can port a data stream from the receiver and test this. You have noted. It would be very interesting to collect 24 hrs worth of rtk and see if rtk has become that precise. Just have to rig up some way to have it slowly rotate while i sleep.?ÿ
I always enjoy your writing. Makes me think. I learn from you. Keep it up. People like me always looking to learn for sure.?ÿ
Shawn, exactly. Great post. For an important point I always us a bipod and face the rover in the same direction for every shot. Usually north, but a distant mountain is just as good. And I never set the base up on a known point. I try to keep it close to previous base setups, and use the rover to calibrate on a known point. Much more efficient and quite accurate.
And I never set the base up on a known point. I try to keep it close to previous base setups, and use the rover to calibrate on a known point.
OK, this one has me genuinely confused. What is the reason for introducing another unknown point to the dataset every time a base is started? As opposed to simply setting up on an existing point and checking into another existing point?
I'd like to think I'm pretty savvy with statistics and geospatial data, and I can't imagine a scenario in which, already having established (presumably adjusted) points to work with, one would choose to set up close to but not right on a known point. It practically negates one of the most critical pieces of a good survey network - redundancy.
I also really don't get performing a brand new calibration every single setup. Why not just set the coordinate system, whether grid, ground, or local, and run with that?
The argument against rotating the rod and reshooting, namely that there are systematic biases in the receiver, seems to me to be an additional argument in favor of the practice. I'll have to think on this for awhile.?ÿ
What is the quantum of the APC offset for mainstream receivers 1mm? 5mm? https://geodesy.noaa.gov/ANTCAL/ appears to show in low mm from what I can follow.
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I use a sokkia ap41 rotating plummet on legs for RTK GNSS base so I assume my plumbing is 1-2mm of mark as can check by rotating 180?ø at setup time. The pole (2.3m half carbon/aluminium with pins) has 2-3mm of runout (i.e. banana in pole) and I've got the bubble adjusted so its plumb when at lowest setting, so at 2m it's a bit off. So unless I set up legs for every rover point I'm at ~3mm of plumbing error which I consider systematic as can't adjust out (but 180?ø pole differences average out)
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If you are just shooting points for control, setting base up randomly and calibrating and then shooting all points doesn't matter so much if your calibration point was a dud. If you setup random and then get a dud calibration shot and then do a bunch of setout you have other problems. Also if you are setting base up accurately over a single mark and then doing multiple sets of observations you know the differences you are seeing are actual GNSS differences not small calibration differences.
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Count me in the not convinced camp at this point for RTK GNSS and if you are chasing <5mm then it's static and legs with well adjusted tribrachs for me.
Good practice says that you ensure all the gear is correctly adjusted (which means NOT using a bent rod!!). Yes, for control I'd always orientate the antenna in the same direction.
BUT - did you really punch the mark so accurately you had to use a vernier and a punch clamp? and how did you get your head so close to the rod (if you are using a bipod) so as to have your head directly above the bubble...?ÿ and did you call at the opticians on the way to site to get a calibration certificate for each eye? (Don't laugh, try it yourself - it's surprising how many people will set a slightly different angle depending on which eye they are using.)
There are occasions when we really need ultimate accuracies, but generally it's precision within the job rather than accuracy within the wide world which is called for. Part of using best practice is understanding why; and "why" is NEVER because there is a tick-box to fill.
However you can rotate the pole and still maintain the north direction of antenna. If its not the fixed 2 piece rods.
Great point, Mr. River.
What is the reason for introducing another unknown point to the dataset every time a base is started?
If I have the situation straight, observing the control with the rover as opposed to occupying the control with the base exchanges the potential systematic errors in the rover setup and the base setup, and potential systematic errors in the base>rover antenna offsets for the potential error in a vector. The setup errors would all be rover and should cancel if the orientation remains the same, any biases in the base setup are removed from the network as well as any systematic biases in the vector between the base and rover.
@lukenz The apc offset is at the receiver though. When transferred to the ground it becomes something to consider.
@shawn-billings It seems with RTK becoming so easy and quick we often choose this over static because of time savings = money. I am not opposed to using RTK I actually enjoy using this. We all just need to make sure that we pass on to the up and coming that RTK is a method and took that meets certain standards. I believe we are creating this idea that RTK is the best for control to younger surveyors. When Static still is one of the best over certain distances and a good total station and set of tribrachs are better in other situations along with yes one of my favorite tools is the steel tape. ?ÿJust like trig leveling vs running 3 wires. Because of the way we communicate rtk and push it on the crews we need to be cautious of this precedent we are setting. ?ÿI am not one to talk about communication especially in writing. ?ÿI am weak at this. But I try to step back and and read the room. I see so many young people that believe rtk is the best for control and not good enough for anything else but rough staking or topo. We are basically contradicting ourselves in this. As a parent I have learned our kids watch us and do based on what we do more than what we say. Our younger generation is watching us and those with higher knowledge and education wisdom need to desperately make sure we give them all the information to be successful. This forum has a great place in this task. We have so many tools these days. 3D Laser scanners. Drones. Rtk static. Levels digital and automatic. Robots etc. all with different pros and cons. Learning to use the right tool in the right condition to meet the quality requirements and productive requirements is a challenge in itself if we do not educate ourselves thoroughly. One of the things I miss the most is the old standards that use to give you the idea of what was needed to achieve certain accuracies. Like how many sets of angles to achieve X with a 1 second transit. Vs 0.5 second instrument. ?ÿThe rtk guidelines by Mr Henning was a great step in that direction with rtk. For 3d laser scanning i always go the metrology route as I had to use them in those environments. ?ÿTechnology has changed drastically and is changing faster than we can keep up. Procedures and processes we used in the 90??s vs early 2000??s vs 3 years ago is at warp speed. Many of you probably remember it taking 15 minutes to get a fix with rtk. Now the fix before we can grab a role of flagging out of the truck. The first time i grabbed an R10 which is older technology It scared me how fast it fixed. I kept dumping the sats because I could not believe it. Now i can be under a shade tree and it still fixes quicker than the old days. Please please teach your crews that the Hz and Vt precisions on the screen are not how accurate the position is. Geezers. I have seen survey managers run a spreadsheet with those and ask field people why there distance doesn??t check between two rtk points with robot. Within those numbers and ask them to go re observe and make sure the are over the point. Like the robot edm measurement is bad gs the rtk. Lots of things to think about.?ÿ
What is the quantum of the APC offset for mainstream receivers 1mm? 5mm?
I don't have a solid answer for you. We can look to the antenna calibrations and see the small millimeters of offset for signals from different directions and different elevations, but a surveyor is able to see for himself the real-world offsets by setting up an RTK rover on a tripod with a rotating center in a very good location and collecting points at multiple orientations and then comparing the results.
Good practice says that you ensure all the gear is correctly adjusted (which means NOT using a bent rod!!).?ÿ
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I agree!
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and how did you get your head so close to the rod (if you are using a bipod) so as to have your head directly above the bubble...?ÿ and did you call at the opticians on the way to site to get a calibration certificate for each eye? (Don't laugh, try it yourself - it's surprising how many people will set a slightly different angle depending on which eye they are using.)
This would be a good reason to maintain consistency. These would be biases that could cancel out if the process is consistent.
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Agreed, but to be consistent you would have to walk round 180 degrees on the pole before looking at the bubble.
When Static still is one of the best over certain distances
I use RTK a lot. A LOT. A. LOT. And as you mentioned, the rules change fast. I see a lot of rules discussed that were relevant circa 2010 that simply are not anymore. The difference in precision between RTK and static is becoming less and less. Having said that, RTK still requires communication between the base and the rover. There are many applications for precision GNSS positioning that don't require real-time coordinates and could be simplified by removing the need for communications. This is where static shines in my opinion. Having said that, I do agree that static observations will still yield slightly better positions than RTK and do so over much longer distances.
Please please teach your crews that the Hz and Vt precisions on the screen are not how accurate the position is.
Amen. This is so true. I hear from operators all of the time that it must have been a good position because I had low PDOP and good RMS values. RMS values mean nothing to accuracy.
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Hopefully experience and education work together to teach us the limitations of our tools and how to be exploit them to get the right result efficiently. And like you said, hopefully we then take that experience and use it consistently with our crews so that they get the right result efficiently.
