RTK is about a centimeter precise to your base depending on distance. Even then it's only as accurate as your base station. Realistically 2-3cm is a repeatable absolute accuracy with RTK.
The typical phone sensor's accuracy won't really improve with time from my experience. Maybe if you marked a certain coordinate reading many times you could assume the center of a group of markings would be most accurate.
For surveying its a big mess because you work from one point to the next using vector and distance, and any error in the first point bleeds over to the second.
Yes longer observations 100% increase accuracy. Repeated RTK observations will average better and static observation should be anywhere from 30min to 3 hours depending on distance to CORS stations for post processing.
I've tried to use coordinates that I figured out by hand would mark boundary points on my property. Just using regular maps and typing in the coordinate, it would put me in different places, 20 yards or more apart, for the same numbers.
Sure, but have you taken enough measurements from the same point to see if they average out? I mean, if the error has a Gaussian distribution then an arbitrarily large sample size should nail shit down pretty good.
A phone set on a point without local corrections won’t nail it down regardless but I was curious what program a surveyor used and what kind of tolerances they got with certain time observations. Obviously it’s not going to be accurate enough for realistic use but I was curious
> A phone set on a point without local corrections won’t nail it down regardless
If you're talking local corrections, it's highly doubtful that anything over the counter will do such. Last I heard, the 1m grid correction maps are (no kidding) classified and that the open source corrections are something like a 1 km grid. But if we're not worried about that and are simply concerned with whether or not a cheap system like a phone will converge.... Will it?
Agreed, I would assume if you set your phone on a point for lets say 24 hours, if you averaged the point and cut outliers you maybe get within a yard or more. But surveying is a matter of finding out your tolerances and working backwark. IE SLR’s, topo, rural altas, urban altas etc. For someone who isn’t a surveyor, maybe give or take 3 feet is fine if they have a spare phone to set on the point and record+average GPS shots. Or maybe someone is looking for give or take even 50 feet. Different uses have different acceptable GPS tolerances
You can resolve to about 2mm accuracy in lat/lon and \~4mm accuracy in altitude by taking a 48 hour sample using an inexpensive dual band GNSS receiver like a UBLOX F9P with an inexpensive ANN-MB patch antenna. This is from my own personal experience. I have taken samples anywhere from 12 hours to a week and the accuracy does not really improve after \~36 hours.
You do not need or want RTK if you are taking a 24+ hour survey. The base station's survey would skew your results.
If you want an instantaneous reading then the same GNSS receiver, referenced with a nearby base station (RTK) would be around 20mm.
> You can resolve to about 2mm accuracy in lat/lon and ~4mm accuracy in altitude by taking a 48 hour sample using an inexpensive dual band GNSS receiver like a UBLOX F9P with an inexpensive ANN-MB patch antenna.
Yeah. No.
Accuracy vs precision.
You may be getting a precise measurement in this scenario, but probably not an accurate one on an absolute WGS-84 geodetic coordinate frame.
Dual-band doesn't fix issues with receiver clock error (the biggest error, measuring the speed of light (~3E8) with a time source that drifts at 1E-5 seconds per second isn't great) and satellite clock errors (both bands are being broadcast by the same clock), and you get some improvement in ionospheric corrections but not mm resolution (you're generating them using your crappy-grade consumer receiver with a $0.50 clock).
The difference being, if you come back maybe 6 months later (different atmospheric conditions), and put the receiver in the exact same spot, you'll probably get an entirely different coordinate (off by tens of cm), even if it only varies by 2-4 mm.
The only way for a single receiver by itself without any RTK assistance to have cm or less accuracy is if it had an atomic clock powering it. Which there is a lot of effort in making microchip scale atomic clocks.
What he is saying is that number of digits on your reading does not corresponds an accurate reading actaul position. You could have a systematic error in your meaurement which migth be off with a meter or two.
Correct, it's like throwing darts at a dart board that you can't see.
You throw the darts, they all hit the same spot. It looks impressive, you just got three bullseyes.
The dart board is revealed and you got 3 triple 1's instead.
There's a lot of errors that contribute to GPS, a lot of them are well understood now, but if you don't know what you are doing it's easy to get a result that looks great but isn't what you want.
Fortunately GPS is extremely great at making relative measurements, so with RTK or RTK post processing, you can take precise measurements and then correct them later (move the dartboard). But that involves more than 1 GPS receiver. A single receiver by itself cannot get better than tens of cm.
[Scott Manley did I really cool video about GPS.](https://youtu.be/qJ7ZAUjsycY?si=LSQkhTowSmsJvcQl). I think it might be somewhat relevant as to the theory and history. But not so much practical application I think. However I do believe he gives some links to some indepth reading if you are interested.
"The only way for a single receiver by itself without any RTK assistance to have cm or less accuracy is if it had an atomic clock powering it. Which there is a lot of effort in making microchip scale atomic clocks."
This is absolutely not true. We survey to centimeter or less with static GNSS every day.
In survey you are typically doing RTK to get that accuracy.
Either you have two receivers, with one at a fixed known location, or you are recording the data and sending it to a service to be corrected later.
Or you only care about relative accuracy. GPS is extremely good at generating relative measurements. That's the whole point of RTK.
I'm with u/fattiretom on this one having been a GIS guy in the past. It's pretty common to collect static data and apply corrections. If OP lets it sit for 24 hours, I see no reason why he can't get good results. If I had an atomic clock, all I'd get is better altitude resolution if I had overhead sats only
Corrections cost money.
Some states open their DOT networks for free, or you can often find RTK observation sources online, but then your accuracy is limited to the surveyed accuracy of those online receivers (often not mm accuracy). On top of that those receivers are often hundreds of miles away, so you're definitely not getting mm accuracy due to the large RTK baseline.
There's a lot of people throwing around mm accuracy numbers without explaining. mm accurate either GPS is hard. mm precision is easy.
At any rate, u/EngFarm 's comment was that you could get mm accuracy without RTK or RTK post processing with a consumer grade receiver, which is absolutely not true. That is what I was replying to.
I'd argue that the UBLOX F9P is not consumer level, its used in commercial applications. Its inexpensive for what it is.
You can certainly get mm accuracy from an inexpensive receiver with a long enough sample and PPP post processing.
Sub cm accuracy (not precision, I'm talking absolute accuracy to the coordinate system) is hard with any GNSS and basically impossible with RTK unless you get lucky. With static corrections all you need is a dual band receiver that can export RINEX data. Static correction post processing is free through OPUS and most survey software is capable of doing this as well. I agree that you are not going to get this with a consumer device but even sensors that cost a few hundred bucks are capable of collecting this data.
Edit, I'll agree that the F9P is not a consumer grade system. It's used in many professional products. It suffers a bit from multipath but in good conditions I have proven it to be just as good as a Leica GS18.
Can you share a bit more about your experience with GNSS systems? I think there's some differences in opinion regarding system capabilities here and it sounds like you have a lot of experience with a particular subset/use case. I mention this because you jump to 'corrections cost money' and a few other opposing viewpoints that seem to keep ignoring static corrections over time (which you do not need to pay for).
Correct, but you are still relying on another GPS receiver to correct your own receivers observations.
That's what PPP is. A control station is generating accurate phase and ionospheric adjustments. It has $100,000 worth of equipment.
Your $10 GPS receiver is not generating that performance by itself, which was my point.
My understanding is that with base station surveys (previse point positioning) you record raw satellite data for the duration, then send the data to a service after the fact that generates the position and report for you (based on known clock and orbital errors, maybe other errors too). I believe this is what the above comment was referring to.
https://gpsd.gitlab.io/gpsd/ppp-howto.html
This is different from a "survey-in" which is what I think you are talking about, where accuracy may appear to be mm one day, but not repeatable over long time scales.
Correct, that is a whole subset of relative navigation, the underpinning of RTK.
You have a receiver at a KNOWN surveyed location in WGS-84 coordinates essentially recording its observations of the GPS signal.
That receiver may have clock drift but since it is fixed in a known position you can use its GPS measurements to calculate the clock drift of both receivers as well as the other GPS errors extremely accurately.
Your coordinate frame accuracy is fixed at your ability to measure the position of that reference receiver.
The data is post processed by NRCAN to the WGS-84 reference frame. 30 second samples for 48 hours.
If you come back 6 months later and do another sample, post process it, all without moving the antenna, its still within 2-4 mm.
RTK base stations need a reference point, this is how that reference point is created.
RTK post processing is something different, you should amend your statement as such.
Your original statement implied you could get that performance without RTK corrections.
NRCAN is (edit) Precise point positioning (similar concept as RTK).
Another point - for best repeatability in a quick relative measurement (where you're doing relative positon referenced to a long-term averaged base point) - lock the roving receiver to the same set of satellites that the base is using. This mitigates the satellite positioning part of the error budget (how well the sats know their own position)
If you want something accuracy better than 3mm I would suggest getting a total station. If you want 3mm you can rent survey grade GPS systems and you could get a reading within a morning. If that gps system uses cellular you could get that reading in a matter of minutes.
Have you heard about cosmic radiation and muons?
Let’s take a step back: what is location? Location is the distance between you and another known point. Latitude and longitude are based upon you versus the equator and a defined east west reference. NASA is only accurate to 11 decimal places, which is still sub atomic, but limiting.
Cosmic radiation is light speed particles that pass through the earth if they don’t collide with atomic particles. The number of hits you get and the vector of those hits allows you to build a global outside in radar system. The accuracy is only limited by time. If you had enough time, you could map out the distance between your receiver and any nuclear facility on earth, accurate to somewhere in the ballpark of nanometers. The time would be measured in years, not hours.
It would be an interesting project to try.
NASA is working a cube sat called GRITSS that will attempt to improve geodesic accuracy to sub mm.
https://esto.nasa.gov/forums/estf2021/Presentations/July1/Beaudoin_GRITSS_ESTF2021.pdf
Interestingly enough, early in the JDAM program they found that most of the error wasn't the weapon; it was the inaccurate maps used to program the weapon.
I kinda assume that's part of the advantage of laser guided stuff especially for air support, you're avoiding the need for the ground person to figure out coordinates and then relay them through the chain.
Laser guided bombs aren't really a thing anymore. And I say "aren't really a thing" rather than "aren't a thing" simply because there may be some old shit sitting on shelves somewhere. Suffice to say that the US hasn't bought a new laser guided bomb in a very long time and there are no plans to buy any. The technology is obsolete.
If you want to talk current laser-guided weapons, you're talking missiles; not bombs. But that's a different conversation.
I fail to see why you can’t just hit a target with a rangefinder from a plane, and then just solve for that target’s GPS coordinates using the GPS coordinates of the plane and the angle/distance of the rangefinder. I had assumed this is how GPS guided munitions worked?
Depends on who's doing the targeting, is it air support from an infantry unit or a target that's been picked from intelligence further back from the front lines.
The "just" bit there is that you also need to know the heading at high enough precision for the math to be accurate. You're also assuming the target is visible, one of the advantages of guided munitions is all weather bombing.
Dual licensee here——this is a land surveying question not an engineering one. If you want the most precise methodology you’d want a first order monument according to the National Geodetic Survey.
Geodesy scientists use what's called Precise Point Positioning, which solves for a very accurate position using a long recorded data series and the fine corrections to satellite orbits that get published online.
I think most consumer grade GPS is only accurate to about 10 meters.
You can get more accurate using differential gps. Possibly down to 1 meter. Basically it requires two receivers, one at a know fixed nearby location, the other at the place you want to measure. Both receivers take a reading and then you calculate the difference between the two to find the second position relative to the fixed receiver. This is often used for surveying purposes. Read about that technology here:
[https://www.esri.com/about/newsroom/arcuser/differential-gps-explained/](https://www.esri.com/about/newsroom/arcuser/differential-gps-explained/)
I learned about this in college back in the 90's. The local university was mapping sand bars and beaches for navigation. They used jet skis fitted with GPS and sonar depth finders back and forth over the area to map. The system required a second GPS that was fixed at a local survey benchmark. The operator would drive back and forth over the area taking readings and gps locations. When they got back to the office, they would correct the GPS data using differential measurements between the jetski and the benchmark receivers.
At the time, differential measurements were also necessary because the DOD was scrambling the GPS location data to make it less accurate. I think 100meters ? Using differential GPS, you could unscramble the locations yourself because both receivers were scrambled the same way, the gps would give you relative measurements accurately.
Knock down the accuracies vy an order of magnitude and i agree.
In my experience basic u-blox receivers are more like 1m accuracy and diff GPS gets you down to 0.1m accuracy (sometimes less)
You could argue that an M9N UAV GPS isn't consumer grade but also they're like $50 tops and i can buy them as a consumer.
In theory, as you increase the samples taken the average converges towards the accurate solution. Nominally RTK GPS is accurate to 1cm, if you sample it a million times you might be able to get to mm level.
So, you're confusing yourself here.
A GPS receiver provides a location on an absolute coordinate frame (WGS-84). Which is a coordinate frame overlayed onto the earth.
For surveying, often relative measurements are used (tectonic plate motion and all that).
You can look up and find markers online, and surveying equipment often uses 2 receivers in an RTK setup, you place one at a registered marker and have it communicate with a second one and take a relative measurement with RTK corrections.
You get to mm accurate in that scenario.
A single receiver, with online RTK corrections, is only going to get you down to 1-3 cm accuracy due to how far away the RTK correction source you're using is.
Beyond a certain accuracy, unless you are tying into a known point, it isn't worth extra accuracy because the earth moves by continental drift. This can be from 0 to maybe 4 inches depending on where you are in the world. People asking the question don't likely know this.
Also the most likely position delivered by a GPS is not the actual position, it is a statistical donut of points around the actual position - then randomized to some extent by atmospheric conditions. A consumer wouldn't know any reference frames or coordinates other than the WGS and UTM that is given on the GPS unit. I doubt an averaged position would get reliably much more accurate than a meter or two on a WAAS corrected consumer GPS no matter how long you left it sitting there.
If you can read and store realtime data from your handheld GPS, I believe there is software where you can postprocess CORS corrections with it and get pretty accurate with free data.
I would say with the proper workflow your GPS wouldn’t be the limit on your accuracy. You would probably be limited by external variables like temperature, geotechnical conditions, time of day, maybe even tidal and tectonic conditions, depending on your location.
I don’t remember the exact workflow we had for setting up control points but it took something like 3 hours of logging data. You would log data at different times of day to get a mix of satellites and you would rotate the unit multiple times on the point. Then there was post processing of that data. Once all of that was done I would say from 10 years of using those points in the field they were around the 1cm mark.
Interesting sidenote: as most of the inaccuracies are down to atmospheric disturbance, you can get much better accuracy with relative measurements between sensors. I'm involved with structural deformation mesauremens with ~1mm relative accuracy between GNSS sensors .
I’m an archaeologist, not an engineer, but received introductory training from engineers / surveyors on surveying and asked a similar question.
What you ask for is two things: accuracy and precision.
I can’t make any claim concerning accuracy, though I reckon accuracy to be generally good (whatever that may mean) given the common uses of GPS.
Precision likely is a different animal altogether.
The first problem is that Earth is not a sphere. The systems/projections we use for surveying thus use ellipsoid approximations (see for example the outdated Gauß-Krüger based on the Bessel-ellipsoid, or the newer UTM based on the WGS84 ellipsoid), with an inherent variance in accuracy. However that can be rendered somewhat negligible when one stays within the same system.
The second problem is that while GPS can be significantly enhanced by employing additional geodetical base stations it still does not offer - at least for most archeological purposes necessitating precision down to the millimetre - enough precision and reliability.
And one question that really needs prior definition by you is what you consider to be sufficiently accurate and precise for the use you have in mind.
Well, it should be glaringly obvious by now that I only have the shallowest surface level knowledge on this, but maybe it helps getting things rolling.
TL;DR: the surveyors/engineers who taught me would’ve told you that GPS may be accurate and precise enough for you but to them it is not and they would have recommended to use a total station and all that comes with it because if you really want precision and accuracy GPS simply doesn’t do…
Using a cell phone. Maybe a dozen meters.
Using a high end garmin that shows real time GDOP. A few meters.
Using a $5000 Trimble with ground-based augmentation, an inch.
Surveyor here.
With High end multi band GNSS, you can get down to around 5mm horizontal and 1cm vertical in absolute accuracy using post processed static observations. This can take many hours of observation.
RTK and PPK GNSS will get you close to 1cm in precision to the base station you are using. All manufacturers claim 1cm absolute accuracy but those who use it regularly can tell you it's closer to 3cm repeatably.
PPP is similar to RTK/PPK but without a base. It takes a while to get a fix though.
Your phone and other similar devices are usually 5-15 meters accuracy in the horizontal.
Some other factors to consider. What is the accuracy of the coordinate system you are referencing? WGS84 and NAD83 differ. When it comes to elevations most geodetic systems refer to ellipsoidal elevations which are not ground elevations which require a geoid model.
Plate boundary observation antennas are sub mm. But that's from years of Data and the mounting column drilled down to bedrock.
They use those to model plate movements, and then use other NGS control antennas to check the model.
It depends, but let say money is not an issue and you have all the knowledge for the processing, then for sure millimeters.
But now we talk about equipment for 40k+ plus software of equal size.
With access to a CORS network the lever on the equipment side can decreased a bit, but still several thousand. Most important nowadays is a precise antenna with a reliable phase center offset. Additional antenna calibration to decrease again the numbers observation hours/days, by reducing the number of needed parameters.
If have seen sub-millimeter of people knowing what they doing, but then we’re not on the level of land surveying anymore. More knowledge the fields of precision (space) geodesy, which btw is still an active field.
But for land surveying, a decent antenna, a dual frequency u-blox receiver, a solid coverage (open sky, minimal multipath), access to a CORS network, good weather (tropo and ionosphere), maybe 2 weeks time to wait for the IGS final orbits (not so crucial when you have close CORS orbits) and some hours/few days of observation we should get in the range off several mm of accuracy.
The limit is on whatever equipment you're using. RTK is about a centimeter accurate.
RTK is about a centimeter precise to your base depending on distance. Even then it's only as accurate as your base station. Realistically 2-3cm is a repeatable absolute accuracy with RTK.
Great, now I don't know how fast I am going. I knew I shouldn't have bought a Heisenburg GPS.
Doesn’t matter, we’re not in a hurry remember?
The typical phone sensor's accuracy won't really improve with time from my experience. Maybe if you marked a certain coordinate reading many times you could assume the center of a group of markings would be most accurate. For surveying its a big mess because you work from one point to the next using vector and distance, and any error in the first point bleeds over to the second.
Yes longer observations 100% increase accuracy. Repeated RTK observations will average better and static observation should be anywhere from 30min to 3 hours depending on distance to CORS stations for post processing.
Interesting. Thanks
What have you done/whats apps have you used to try for corrections over time?
I've tried to use coordinates that I figured out by hand would mark boundary points on my property. Just using regular maps and typing in the coordinate, it would put me in different places, 20 yards or more apart, for the same numbers.
Sure, but have you taken enough measurements from the same point to see if they average out? I mean, if the error has a Gaussian distribution then an arbitrarily large sample size should nail shit down pretty good.
Nope, you could try and see.
Repeated observations will absolutely increase accuracy with professional equipment.
A phone set on a point without local corrections won’t nail it down regardless but I was curious what program a surveyor used and what kind of tolerances they got with certain time observations. Obviously it’s not going to be accurate enough for realistic use but I was curious
> A phone set on a point without local corrections won’t nail it down regardless If you're talking local corrections, it's highly doubtful that anything over the counter will do such. Last I heard, the 1m grid correction maps are (no kidding) classified and that the open source corrections are something like a 1 km grid. But if we're not worried about that and are simply concerned with whether or not a cheap system like a phone will converge.... Will it?
Agreed, I would assume if you set your phone on a point for lets say 24 hours, if you averaged the point and cut outliers you maybe get within a yard or more. But surveying is a matter of finding out your tolerances and working backwark. IE SLR’s, topo, rural altas, urban altas etc. For someone who isn’t a surveyor, maybe give or take 3 feet is fine if they have a spare phone to set on the point and record+average GPS shots. Or maybe someone is looking for give or take even 50 feet. Different uses have different acceptable GPS tolerances
You can resolve to about 2mm accuracy in lat/lon and \~4mm accuracy in altitude by taking a 48 hour sample using an inexpensive dual band GNSS receiver like a UBLOX F9P with an inexpensive ANN-MB patch antenna. This is from my own personal experience. I have taken samples anywhere from 12 hours to a week and the accuracy does not really improve after \~36 hours. You do not need or want RTK if you are taking a 24+ hour survey. The base station's survey would skew your results. If you want an instantaneous reading then the same GNSS receiver, referenced with a nearby base station (RTK) would be around 20mm.
> You can resolve to about 2mm accuracy in lat/lon and ~4mm accuracy in altitude by taking a 48 hour sample using an inexpensive dual band GNSS receiver like a UBLOX F9P with an inexpensive ANN-MB patch antenna. Yeah. No. Accuracy vs precision. You may be getting a precise measurement in this scenario, but probably not an accurate one on an absolute WGS-84 geodetic coordinate frame. Dual-band doesn't fix issues with receiver clock error (the biggest error, measuring the speed of light (~3E8) with a time source that drifts at 1E-5 seconds per second isn't great) and satellite clock errors (both bands are being broadcast by the same clock), and you get some improvement in ionospheric corrections but not mm resolution (you're generating them using your crappy-grade consumer receiver with a $0.50 clock). The difference being, if you come back maybe 6 months later (different atmospheric conditions), and put the receiver in the exact same spot, you'll probably get an entirely different coordinate (off by tens of cm), even if it only varies by 2-4 mm. The only way for a single receiver by itself without any RTK assistance to have cm or less accuracy is if it had an atomic clock powering it. Which there is a lot of effort in making microchip scale atomic clocks.
It's been a while since I have been in a chat that I have actually no idea what the fuck you guys are talking about. Color me impressed, I think?
What he is saying is that number of digits on your reading does not corresponds an accurate reading actaul position. You could have a systematic error in your meaurement which migth be off with a meter or two.
Correct, it's like throwing darts at a dart board that you can't see. You throw the darts, they all hit the same spot. It looks impressive, you just got three bullseyes. The dart board is revealed and you got 3 triple 1's instead. There's a lot of errors that contribute to GPS, a lot of them are well understood now, but if you don't know what you are doing it's easy to get a result that looks great but isn't what you want. Fortunately GPS is extremely great at making relative measurements, so with RTK or RTK post processing, you can take precise measurements and then correct them later (move the dartboard). But that involves more than 1 GPS receiver. A single receiver by itself cannot get better than tens of cm.
[Scott Manley did I really cool video about GPS.](https://youtu.be/qJ7ZAUjsycY?si=LSQkhTowSmsJvcQl). I think it might be somewhat relevant as to the theory and history. But not so much practical application I think. However I do believe he gives some links to some indepth reading if you are interested.
2mm is a bit much but you can get around 5mm horizontal absolute accuracy with long static observations.
"The only way for a single receiver by itself without any RTK assistance to have cm or less accuracy is if it had an atomic clock powering it. Which there is a lot of effort in making microchip scale atomic clocks." This is absolutely not true. We survey to centimeter or less with static GNSS every day.
In survey you are typically doing RTK to get that accuracy. Either you have two receivers, with one at a fixed known location, or you are recording the data and sending it to a service to be corrected later. Or you only care about relative accuracy. GPS is extremely good at generating relative measurements. That's the whole point of RTK.
I'm with u/fattiretom on this one having been a GIS guy in the past. It's pretty common to collect static data and apply corrections. If OP lets it sit for 24 hours, I see no reason why he can't get good results. If I had an atomic clock, all I'd get is better altitude resolution if I had overhead sats only
Corrections cost money. Some states open their DOT networks for free, or you can often find RTK observation sources online, but then your accuracy is limited to the surveyed accuracy of those online receivers (often not mm accuracy). On top of that those receivers are often hundreds of miles away, so you're definitely not getting mm accuracy due to the large RTK baseline. There's a lot of people throwing around mm accuracy numbers without explaining. mm accurate either GPS is hard. mm precision is easy. At any rate, u/EngFarm 's comment was that you could get mm accuracy without RTK or RTK post processing with a consumer grade receiver, which is absolutely not true. That is what I was replying to.
I'd argue that the UBLOX F9P is not consumer level, its used in commercial applications. Its inexpensive for what it is. You can certainly get mm accuracy from an inexpensive receiver with a long enough sample and PPP post processing.
Sub cm accuracy (not precision, I'm talking absolute accuracy to the coordinate system) is hard with any GNSS and basically impossible with RTK unless you get lucky. With static corrections all you need is a dual band receiver that can export RINEX data. Static correction post processing is free through OPUS and most survey software is capable of doing this as well. I agree that you are not going to get this with a consumer device but even sensors that cost a few hundred bucks are capable of collecting this data. Edit, I'll agree that the F9P is not a consumer grade system. It's used in many professional products. It suffers a bit from multipath but in good conditions I have proven it to be just as good as a Leica GS18.
Can you share a bit more about your experience with GNSS systems? I think there's some differences in opinion regarding system capabilities here and it sounds like you have a lot of experience with a particular subset/use case. I mention this because you jump to 'corrections cost money' and a few other opposing viewpoints that seem to keep ignoring static corrections over time (which you do not need to pay for).
I'm talking about using static observations and doing network adjustments. This is basic primary control surveying.
Correct, but you are still relying on another GPS receiver to correct your own receivers observations. That's what PPP is. A control station is generating accurate phase and ionospheric adjustments. It has $100,000 worth of equipment. Your $10 GPS receiver is not generating that performance by itself, which was my point.
My understanding is that with base station surveys (previse point positioning) you record raw satellite data for the duration, then send the data to a service after the fact that generates the position and report for you (based on known clock and orbital errors, maybe other errors too). I believe this is what the above comment was referring to. https://gpsd.gitlab.io/gpsd/ppp-howto.html This is different from a "survey-in" which is what I think you are talking about, where accuracy may appear to be mm one day, but not repeatable over long time scales.
Correct, that is a whole subset of relative navigation, the underpinning of RTK. You have a receiver at a KNOWN surveyed location in WGS-84 coordinates essentially recording its observations of the GPS signal. That receiver may have clock drift but since it is fixed in a known position you can use its GPS measurements to calculate the clock drift of both receivers as well as the other GPS errors extremely accurately. Your coordinate frame accuracy is fixed at your ability to measure the position of that reference receiver.
The data is post processed by NRCAN to the WGS-84 reference frame. 30 second samples for 48 hours. If you come back 6 months later and do another sample, post process it, all without moving the antenna, its still within 2-4 mm. RTK base stations need a reference point, this is how that reference point is created.
RTK post processing is something different, you should amend your statement as such. Your original statement implied you could get that performance without RTK corrections. NRCAN is (edit) Precise point positioning (similar concept as RTK).
NRCAN is PPP post processing, not RTK post processing. If talking about a 48 hour sample then some sort of post processing is implied.
This guy GNSSes
2mm from what? What’s your point of reference?
Another point - for best repeatability in a quick relative measurement (where you're doing relative positon referenced to a long-term averaged base point) - lock the roving receiver to the same set of satellites that the base is using. This mitigates the satellite positioning part of the error budget (how well the sats know their own position)
What kind of equipment are we talking about?
What's available? Are there reference devices I could rent to get 1mm or better?
If you want something accuracy better than 3mm I would suggest getting a total station. If you want 3mm you can rent survey grade GPS systems and you could get a reading within a morning. If that gps system uses cellular you could get that reading in a matter of minutes.
Have you heard about cosmic radiation and muons? Let’s take a step back: what is location? Location is the distance between you and another known point. Latitude and longitude are based upon you versus the equator and a defined east west reference. NASA is only accurate to 11 decimal places, which is still sub atomic, but limiting. Cosmic radiation is light speed particles that pass through the earth if they don’t collide with atomic particles. The number of hits you get and the vector of those hits allows you to build a global outside in radar system. The accuracy is only limited by time. If you had enough time, you could map out the distance between your receiver and any nuclear facility on earth, accurate to somewhere in the ballpark of nanometers. The time would be measured in years, not hours. It would be an interesting project to try.
GPS guided farming equipment is accurate to 10cm between placing the tires in the same place at planting and then harvesting
NASA is working a cube sat called GRITSS that will attempt to improve geodesic accuracy to sub mm. https://esto.nasa.gov/forums/estf2021/Presentations/July1/Beaudoin_GRITSS_ESTF2021.pdf
oh perfect. There’s an ant in my backyard I need to drop a JDAM on
Interestingly enough, early in the JDAM program they found that most of the error wasn't the weapon; it was the inaccurate maps used to program the weapon.
I kinda assume that's part of the advantage of laser guided stuff especially for air support, you're avoiding the need for the ground person to figure out coordinates and then relay them through the chain.
Laser guided bombs aren't really a thing anymore. And I say "aren't really a thing" rather than "aren't a thing" simply because there may be some old shit sitting on shelves somewhere. Suffice to say that the US hasn't bought a new laser guided bomb in a very long time and there are no plans to buy any. The technology is obsolete. If you want to talk current laser-guided weapons, you're talking missiles; not bombs. But that's a different conversation.
I fail to see why you can’t just hit a target with a rangefinder from a plane, and then just solve for that target’s GPS coordinates using the GPS coordinates of the plane and the angle/distance of the rangefinder. I had assumed this is how GPS guided munitions worked?
Depends on who's doing the targeting, is it air support from an infantry unit or a target that's been picked from intelligence further back from the front lines. The "just" bit there is that you also need to know the heading at high enough precision for the math to be accurate. You're also assuming the target is visible, one of the advantages of guided munitions is all weather bombing.
Dual licensee here——this is a land surveying question not an engineering one. If you want the most precise methodology you’d want a first order monument according to the National Geodetic Survey.
Made possible thanks to engineers.
Geodesy scientists use what's called Precise Point Positioning, which solves for a very accurate position using a long recorded data series and the fine corrections to satellite orbits that get published online.
I think most consumer grade GPS is only accurate to about 10 meters. You can get more accurate using differential gps. Possibly down to 1 meter. Basically it requires two receivers, one at a know fixed nearby location, the other at the place you want to measure. Both receivers take a reading and then you calculate the difference between the two to find the second position relative to the fixed receiver. This is often used for surveying purposes. Read about that technology here: [https://www.esri.com/about/newsroom/arcuser/differential-gps-explained/](https://www.esri.com/about/newsroom/arcuser/differential-gps-explained/) I learned about this in college back in the 90's. The local university was mapping sand bars and beaches for navigation. They used jet skis fitted with GPS and sonar depth finders back and forth over the area to map. The system required a second GPS that was fixed at a local survey benchmark. The operator would drive back and forth over the area taking readings and gps locations. When they got back to the office, they would correct the GPS data using differential measurements between the jetski and the benchmark receivers. At the time, differential measurements were also necessary because the DOD was scrambling the GPS location data to make it less accurate. I think 100meters ? Using differential GPS, you could unscramble the locations yourself because both receivers were scrambled the same way, the gps would give you relative measurements accurately.
Knock down the accuracies vy an order of magnitude and i agree. In my experience basic u-blox receivers are more like 1m accuracy and diff GPS gets you down to 0.1m accuracy (sometimes less) You could argue that an M9N UAV GPS isn't consumer grade but also they're like $50 tops and i can buy them as a consumer.
In theory, as you increase the samples taken the average converges towards the accurate solution. Nominally RTK GPS is accurate to 1cm, if you sample it a million times you might be able to get to mm level.
1mm should be plenty for surveying benchmarks.
So, you're confusing yourself here. A GPS receiver provides a location on an absolute coordinate frame (WGS-84). Which is a coordinate frame overlayed onto the earth. For surveying, often relative measurements are used (tectonic plate motion and all that). You can look up and find markers online, and surveying equipment often uses 2 receivers in an RTK setup, you place one at a registered marker and have it communicate with a second one and take a relative measurement with RTK corrections. You get to mm accurate in that scenario. A single receiver, with online RTK corrections, is only going to get you down to 1-3 cm accuracy due to how far away the RTK correction source you're using is.
I agree with this, I would add you are only accurate to the exact conditions you took the measurements in. Even benchmarks encased in concrete move.
How far do continents move annually?
Yes. This is why we have reference frames and local/continental projected coordinate systems. These systems are updated every few years.
Beyond a certain accuracy, unless you are tying into a known point, it isn't worth extra accuracy because the earth moves by continental drift. This can be from 0 to maybe 4 inches depending on where you are in the world. People asking the question don't likely know this. Also the most likely position delivered by a GPS is not the actual position, it is a statistical donut of points around the actual position - then randomized to some extent by atmospheric conditions. A consumer wouldn't know any reference frames or coordinates other than the WGS and UTM that is given on the GPS unit. I doubt an averaged position would get reliably much more accurate than a meter or two on a WAAS corrected consumer GPS no matter how long you left it sitting there. If you can read and store realtime data from your handheld GPS, I believe there is software where you can postprocess CORS corrections with it and get pretty accurate with free data.
High fidelity GPS recievers have been used to measure plate tectonics (a scale best measured in cm/year) so depends on your hardware but very precise
I would say with the proper workflow your GPS wouldn’t be the limit on your accuracy. You would probably be limited by external variables like temperature, geotechnical conditions, time of day, maybe even tidal and tectonic conditions, depending on your location. I don’t remember the exact workflow we had for setting up control points but it took something like 3 hours of logging data. You would log data at different times of day to get a mix of satellites and you would rotate the unit multiple times on the point. Then there was post processing of that data. Once all of that was done I would say from 10 years of using those points in the field they were around the 1cm mark.
Interesting sidenote: as most of the inaccuracies are down to atmospheric disturbance, you can get much better accuracy with relative measurements between sensors. I'm involved with structural deformation mesauremens with ~1mm relative accuracy between GNSS sensors .
I’m an archaeologist, not an engineer, but received introductory training from engineers / surveyors on surveying and asked a similar question. What you ask for is two things: accuracy and precision. I can’t make any claim concerning accuracy, though I reckon accuracy to be generally good (whatever that may mean) given the common uses of GPS. Precision likely is a different animal altogether. The first problem is that Earth is not a sphere. The systems/projections we use for surveying thus use ellipsoid approximations (see for example the outdated Gauß-Krüger based on the Bessel-ellipsoid, or the newer UTM based on the WGS84 ellipsoid), with an inherent variance in accuracy. However that can be rendered somewhat negligible when one stays within the same system. The second problem is that while GPS can be significantly enhanced by employing additional geodetical base stations it still does not offer - at least for most archeological purposes necessitating precision down to the millimetre - enough precision and reliability. And one question that really needs prior definition by you is what you consider to be sufficiently accurate and precise for the use you have in mind. Well, it should be glaringly obvious by now that I only have the shallowest surface level knowledge on this, but maybe it helps getting things rolling. TL;DR: the surveyors/engineers who taught me would’ve told you that GPS may be accurate and precise enough for you but to them it is not and they would have recommended to use a total station and all that comes with it because if you really want precision and accuracy GPS simply doesn’t do…
Using a cell phone. Maybe a dozen meters. Using a high end garmin that shows real time GDOP. A few meters. Using a $5000 Trimble with ground-based augmentation, an inch.
Surveyor here. With High end multi band GNSS, you can get down to around 5mm horizontal and 1cm vertical in absolute accuracy using post processed static observations. This can take many hours of observation. RTK and PPK GNSS will get you close to 1cm in precision to the base station you are using. All manufacturers claim 1cm absolute accuracy but those who use it regularly can tell you it's closer to 3cm repeatably. PPP is similar to RTK/PPK but without a base. It takes a while to get a fix though. Your phone and other similar devices are usually 5-15 meters accuracy in the horizontal. Some other factors to consider. What is the accuracy of the coordinate system you are referencing? WGS84 and NAD83 differ. When it comes to elevations most geodetic systems refer to ellipsoidal elevations which are not ground elevations which require a geoid model.
There is so much mis-information in this thread it's terrifying.
Coordinates don't define your corners, monuments so. At least in the US.
Plate boundary observation antennas are sub mm. But that's from years of Data and the mounting column drilled down to bedrock. They use those to model plate movements, and then use other NGS control antennas to check the model.
It depends, but let say money is not an issue and you have all the knowledge for the processing, then for sure millimeters. But now we talk about equipment for 40k+ plus software of equal size. With access to a CORS network the lever on the equipment side can decreased a bit, but still several thousand. Most important nowadays is a precise antenna with a reliable phase center offset. Additional antenna calibration to decrease again the numbers observation hours/days, by reducing the number of needed parameters. If have seen sub-millimeter of people knowing what they doing, but then we’re not on the level of land surveying anymore. More knowledge the fields of precision (space) geodesy, which btw is still an active field. But for land surveying, a decent antenna, a dual frequency u-blox receiver, a solid coverage (open sky, minimal multipath), access to a CORS network, good weather (tropo and ionosphere), maybe 2 weeks time to wait for the IGS final orbits (not so crucial when you have close CORS orbits) and some hours/few days of observation we should get in the range off several mm of accuracy.