> In between, mapmakers figure, the error is swamped by the imprecision of mapping and GPS equipment. Future maps may be updated at a rate closer to real-time. “We have the technology now with GPS to be able to make those slight adjustments on a more frequent basis,” Craun said.
A French startup operates an impressive technical solution in this direction:
> new GNSS (Global Navigation Satellite Systems) augmentation services based on the PPP-CNES technology (Precise Point Positioning).
> Absolute positioning accuracy of 4 cm (2D-95%), everywhere in the world, on the ground, on the sea and in the air, all the time, with only one receiver, without any nearby GNSS reference station: Precision, Availability and Integrity
> Depending on the level of service subscribed: Absolute positioning precision of 2, 4, 10, 50 or 80 cm (2D-95%)
> Compatible with GPS, GLONASS, BEIDOU and GALILEO constellations and mono and multi-frequencies receivers
I attended a meetup were they explained part of it. They have models for each of the aspects that can deteriorate precision and update all the parameters of all models in real time based on continuous measurements.
At a point during the development of the solution they had an unexplained discrepancy and tracked it down to some satellites of a common network being classifiable into two clusters of two different latencies (like, in the nanosecond range or even smaller). They contacted the network owners, listing the two clusters and asked "what is the difference between these and those satellites?".
The owners were a little confused. Something like "Well, inside each satellite, the cable from the antenna to the processing unit is a high performance transmission cable of tightly controlled specification, one inside each satellite. They happened to have been supplied in two batches with supposedly strictly identical behaviour up to the precision available at the time. And you sent us the exact listing of satellites matching batches A and B. But how on earth have you got this list? Were you spying into our internal archives?"
Answer: "no, we just have a model so precise that we could calibrate difference in transmission delays in your satellites from our receptor network on the ground".
4cm accuracy is almost enough for precision agriculture. You usually want a year-over-year positioning accuracy of better than 2.5cm for your combine harvesters to mange issues of soil compactification. This is about the same order of magnitude as continental drift speeds, which can be up to 10cm/year. The usual solution to this is to use a correction signal generated by a nearby reference station with a well known position that can be distributed over the internet: https://en.wikipedia.org/wiki/Real-time_kinematic
Compared with this, the new approach you describe may have the disadvantage (at least for applications that care about positions relative to the surface and not the core of the earth) that it does not use a reference station that is fixed to the continental plate it is on (so that continental drift cancels out implicitly), and users have to handle it explicitly. Or do they offer a drift corrected service?
That's similar performance to NASA's Gipsy solver and a couple other PPP solutions.
The practical problem is PPP falls apart when there's any sort of multipath or noise, with will be the case for any receiver not in a completely open field. A single nearby tree is often the difference between 4cm accuracy and a meter.
Trimble has a similar service, called PP-RTX.[1] There are various online PPP services that can give up to CM accuracy in post-processing, but near-real-time CM accuracy is still a new (and unexpected, to me) thing.
As for updates, the NGS have plate models that allow you to get the shift in coordinates at any given location[2] between two moments in time. Those models are in large part based on GNSS signal recorders called continuously operating reference stations[3] (CORS) which give actual movement at locations throughout the states.
For example, Bar Harbor, Maine, is moving at -0.0013 m/y north, 0.0020 m/y east, and -0.0009 m/y in elevation relative to the North American plate, and 0.0071 m/y north, -0.0153 m/y east, and 0.0001 m/y in elevation relative to the world[4].
That's incredible technology - I wasn't aware it was even theoretically possible to obtain such a resolution with data available to civilians.
But what are some use cases that would benefit from that level of service? For example the highest tier is a 4 cm resolution with a convergence interval of 5 minutes.
I first really understood the importance of geodetic datums whilst in the Antarctic a few years ago. It’s generally treated as “nobody lives there, who gives a crap”, which means that all datums are significantly wrong there - meters out for both elevation and position, all over the place. Generally folks just pick the least worst - WGS 84.
This also highlighted the inaccuracies of satellite imaging, particularly when it comes to uninhabited islands far from other land - Franklin Island, for instance, is over a mile from where it appears to be on google earth - and on marine charts. The upshot was that we had to stand off a few miles from shore, and had a long journey to shore by dinghy, as at that point the captain was understandably unwilling to trust the charts at all. You’d think we’d have the whole world accurately mapped out by now - but there are still seemingly some significant mistakes in commonly accepted points of truth.
Reminds me of Sandy Island (Isla Arenosa), an island which was "discovered" in the 19th century, but as late as 2012 was "undiscovered" as the island didn't actually exist. https://en.wikipedia.org/wiki/Sandy_Island,_New_Caledonia
Maybe there is still time to become an explorer, which I dreamed of as a child.
You’d be amazed just how much is still out there, unknown or forgotten. I’ve got around a fair bit over the years, and I’ve seen some incredible things that exist on no map, no online resource, in some cases not even in the minds of locals. I’m currently living in northern Portugal, which, as part of Western Europe, you’d think would be pretty well explored - but no. The local medieval castle, which is shockingly obvious sat atop a volcanic plug, was not recognised or documented until 1979 - and only in the 1990s did someone realise that this place was that place referred to in various historical documents. The locals knew about it, of course, but it was unremarkable to them. When we moved here, we wanted an old building to restore, somewhere surrounded by nature. We talked to realtors, to locals, young and old, nobody knew of such a place. I pored over satellite imagery, chose some spots to go for a ramble. Discovered a medieval watermill - a series of them, in fact. Talked to locals again, to the council -
nobody had a clue they existed, and the land was public land - and they agreed to sell me one of them. Since we started work we’ve had a little trickle of people wandering down to say hello, and unanimously they’ve all lived there all of their lives, and they’re all agog when they see that this place exists. Exploring the land around the mill has been a blast - mines, from prehistoric to medieval, various ruins, water management channels, terraces, roads, all sorts - and nobody knew any of it was there - and the wildlife is like living in a zoo. Tortoises, snakes, foxes, deer, big damn lizards, eagles, you name it - and every time I wander up a random track I find something new, even if I’ve walked that way before.
There’s a huge amount around, everywhere, to explore - you just have to go where people aren’t, and observe.
The region is Tras Os Montes (“Beyond the Mountains”), and is a rural backwater - mountainous terrain, and going 10km as the crow flies by paved road is 40km. Most of the roads here are unpaved trackway.
The nearest paved road is about 5km by track from the mill - the bottom km was impassible when we found the place, but nothing a day with a bulldozer and a chainsaw couldn’t fix - it looked like about 60 years of growth. The track down the valley is steep and winding, and looks like it goes nowhere. Despite being at the bottom of a deep, steep valley, we get an almost full day of light, as the mill is on the north bank of a large inverted U bend in the river - the sun rises over the valley to our left, grazes the hills to the south, and sets over the valley to the right. Someone in the 1300s really thought about where to put the structure, as 50m either way along the bank gets shaded way more.
Right now we’re having to walk in and out, as we had prodigious rain in December, which both highlighted our flood risk (overtopped the roof!) and made a steep, poorly drained section of the road collapse - I’ve spent the past week teaching myself how to make a pitched stone road - nice Neolithic technique, and means all the materials I need are within spitting distance of the road - rock, gravel.
The next mill, which is about 1.5km downstream of us, has no road or track, just a steep path.
I am still scratching my head over how they built them - the lower courses of stones in the walls must weigh five tonnes apiece - huge boulders, moved purely with human and animal power. After seeing the river go crazy last month, I understand why - the force was sufficient to uproot large trees, to wash away the car, to completely scout the earth to bare rock - but the mill couldn’t have cared less. It’s good timing, insofar as we hadn’t done too much work yet, and now we know we need to ensure everything structural in there can deal with immersion, and everything else needs to be portable. Constraint, sure, but an interesting one.
While shovelling muck there yesterday (temporarily renting a house after being flooded out) I heard a loud plop over the roar of the weir - and saw two otters hunting crayfish in the mill pond - I could see them flipping rocks on the bottom and grabbing them, as the flood has washed away sediments and organics, leaving the water crystal clear. This is the stuff that makes me persevere!
Yes - it’s brilliant, but it’s not for the faint of heart. We started with walls, threw a roof on in a few weeks this summer - only needs to last a few seasons as we want to extend upwards slightly. our water is coming from a spring about 300m away, 60m vertically up - gravity feeds through filters into a tank, going to do reverse osmosis this year so we can drink it - just cooking and washing right now. Power is currently a solar array, and I intend to add hydro and possibly wind to the mix - hydro is going to be tricky, as the river is ridiculously variable, as are the streams nearby.
It has been an intense few months, just getting the place habitable - and when winter arrived life turned into a cycle of gathering firewood and plugging holes in masonry with rags - and then the whole place flooded in December, so the last six weeks have been muck-shovelling, hosing, repairing and reclaiming chattels which got flooded - we moved everything up to a shed with its floor level 5m vertically above the mill’s floor, which is 4m above the normal river level - still flooded - almost a 10m surge! Thankfully, it was only 75cm deep in there, and stopped 5cm short of trashing the solar batteries, inverter, etc., but got all the power tools and electronics (pcs, tv, NAS, routers, etc.) - which miraculously all still work after a wash and dry, barring CMOS batteries. Worst flood in centuries, apparently. We waded out in the darkness, in torrential rain, in thigh deep swirling muddy water, as the river had already burst its bank onto our track, the cats screaming bloody murder in their bag, and walked the 5km up to the car that I’d had the foresight to park up by the paved road... had nightmares for weeks - not something I care to ever repeat. Good we did evac, even if late, or we’d have probably ended up hypothermic and/or drowned.
Anyway. Ultimately, we want to use technology and automation to live off grid without reverting to medieval peasants - and the last four months have underscored that necessity, as I don’t think I can sustain this level of physical activity indefinitely. Today, after my conference calls, I’m heading back down there to keep on rebuilding the road, in the bloody freezing rain - but I have to get it done, as it’s about to be a blocker on getting anything else done. I comfort myself with the probable lie that I’ll be lying in a hammock listening to the birds and the river in a few months, and this will all be a distant memory!
The pros definitely outweigh the cons. We’ve met one other guy in this area who is doing the whole off grid thing - and he’s been doing it for years, and it’s still an incessant struggle - from components of his solar system failing (Chinese crud. Sorted him out with victron gear), to a donkey getting in and eating his crops, to his water ending up poisonous (Christmas vomit buddies), to his road also washing away - but he also wouldn’t trade it for the world - it’s the feeling of freedom and agency that is the attraction - nobody is doing these things to you - either it’s your own mistake, from which to learn, or just nature being a harsh mistress. Also, it’s really goddamned nice to just walk out of the front door in the morning, straight into wilderness. I start each day at home sitting on a rock with a coffee, just observing. Even with the stressors, I’m happier than I ever was behind a desk.
I see, your story matches those that I've read online. I'm in a very stressful job field and if I can't make it till pension I'm considering doing the same. Thanks a lot!
That's very interesting, thank you for sharing that story, makes me feel that information about the world is still young and there is yet things we have discovered. Thank you!
I have a similar experience, in Central Portugal: we found an undiscovered/undocumented site with neolithic ruins. Nothing major or too well-conserved, but still, it's quite stunning to realise that trekking in the summer in the woods almost in your backyard, and less than 10km from a city of 100,000, you can find such a thing :)
Not yet - we’ve only been here about four months, and we’re still discovering stuff all the time. I am considering doing an archaeology summer camp this or next year, however, and am chatting to a lecturer friend about the prospect. It’s a fascinating site - there are traces of a ford, which is interesting, as the bridge (3km downstream) was built in the 1200s, and presumably replaced the ford. I think the majority of what we’re seeing is ~1200 - ~1880s, but there is a cave way up a hillside that I went for a ferret down that was suspiciously man-sized, and had an antler poking out through the mud on the floor, at which point I decided to leave it be until someone who knows more than me looks at it - as I suspect it was an antler pick, which would place the mine in the Neolithic at latest.
There were also Romans in this neck of the woods, but I’ve seen nothing that has said Roman to me yet. Either way, the site has evidently seen quite a bit of activity over the ages, only falling into disuse in the early 20th century - I found a taxation tag embedded in a gnarly old olive tree that identified it as being planted in 1914, and it looked like it hadn’t been pruned ever, meaning someone probably planted it and then got the Spanish flu.
I've sometimes wondered how surveyors deal with a similar, but more extreme situation. Many properties, at least in the northeastern United States, are defined by "metes and bounds". A combination of landmarks, distances, and compass bearings. Newer properties metes and bounds are often based on the older metes and bounds as an old farm is split up, and can be hundreds of years old. But magnetic North moves by dozens of miles each year, and has shifted from Russia to Canada and back, moving by hundreds of miles.
The closer you are to the poles the more inaccurate compass readings will become. Based on the alignment of old stone walls[0], they think over the last few hundred years magnetic north has drifted by around 7 degrees in the northeast US, should be worse in England. When the property description uses a bound of a road, a stream, or other landmark, then the rest is all based on compass readings and distances of up to miles, that can make a huge difference.
I believe finding that your house is on your neighbors property is solved by "adverse possession" laws (sometimes referred to as squatters rights?) where if you act like you own something, and everyone else acts the same way for long enough time, it doesn't matter what the land records say, that is your property. It's a way of cleaning up mistakes.
Oddly, since GPS values will necessarily shift as the Earth's surface moves around, I think the old metes and bounds can be more accurate over time as long as it is based on true North instead of magnetic North.
Short answer, surveyors don't use absolute coordinates, only relative ones from locally known points, output as distance and bearing. The actual coordinates "cancel out" of differential GPS, which also makes it much more accurate (and precise) than standalone GPS. Differential GPS (and newer VRS) gets sub-centimeter horizontal accuracy, and has been available for over 20 years.
Same goes for North. You try to get it close, but it's the relative angles between bearings that actually matter. So it doesn't matter if magnetic North changes or if there was an error initially measuring it.
For legal descriptions of boundary surveys, "what's on the ground" is what controls. Meaning an iron pipe set as a corner doesn't "mark" a property corner, but defines it (unless it's an offset, but it's still the defining feature). The legal description is more a treasure map than some idealized abstract legal "truth". This especially applies to metes and bounds.
GPS coordinates are generally not used for boundary surveys, except maybe to drive to the job site.
As for neighbors: the only people who can decide where a property boundary falls are the adjoining land owners. It's sort of zen, but the boundary is whatever you agree it is. It fits with the "local first" philosophy. In the absence of agreement, prescriptive acquisition (or judicial delineation) is the general term for when the courts have to become involved in a dispute (because the judge prescribes). "Adverse possession" is a type of positive defense to claim prescriptive acquisition when the party has no good-faith claim to the title. It's what most people think of as squatter's rights. Philosophically, it's not for cleaning up mistakes, but returning idle or abandoned real estate back into active commerce. "Good-faith possession" is if you think you have a legal claim to the property, like if you bought a bad title, or a property owner sold the same lot twice, and has a much lower burden of proof and required period of possession (generally ~30yrs adverse vs ~10yrs good-faith) Judicial delineation is more for bad boundary descriptions or cleaning up mistakes, but there's no need for judges if the neighbors can come to an agreement on their own. They just have to file an amended plat with the conveyances at the courthouse.
I'm not a surveyor, but was the IT guy & supervisor for a city's two-engineer surveying team as we went GIS...
Basically you fudge it. Precision losses, errors, and local ground movements swamp continental drift. As you noted, some of the original sources go back over 100 years. A huge, huge amount of the surveyor's time was spent cleaning up data. One of the steps was to map the vectors around a plat. There was always some kind of closure error when you get back to the beginning position. The surveyor's job was to tweak the points to resolve the error. Kind of like auto body work; he used a lot of skill and experience in making the most seamless repair possible. (The most important part of my job was running interference for him, nobody understood why it took so long to "just digitize some maps".)
In general the original surveying records remain the source of truth, and the maps we produced with the tweaked coordinates were working documents. To be technically correct, another map would have to go back to the original documents and re-apply corrections. IIRC for some really egregious problems there were ways for the surveyor to amend the official truth.
Automating or re-implementing legacy systems is always a horribly messy data-cleansing exercise, but I think this was the most mind-numbing, tedious, endless slog I've ever been involved with.
Even if the surveying continues to be both local and continuously observed, and the shapes of properties are changing over time, the current system works just fine.
Properties change, and their values change, and owners adapt. Really this is no different in a practical sense than having a house on an eroding beach, or (sigh...) in a sinking city.
I think I have a property defined like that (in Rhinebeck, NY). The deed is based on static landmarks, the only directional mention is from stone landmark to landmark (ie, walk North 120 feet to a landmark set 4 feet from Paper Street). No reliance on magnetic north. Allegedly when the village was laid out they sunk stone landmarks at the intersections of the original properties.
Global coordinates in general are a lot more complicated than many people realize. For example, how exactly is GPS "elevation" determined? If you're standing on a beach at mean tide, is the elevation from a GPS receiver guaranteed to be zero everywhere on the planet? Here's a good starter on that one:
ESRI is a major US company in the Geographic Information Systems, mainly for government and business.
>Coordinate systems, map projections, and geographic (datum) transformations
>Every dataset has a coordinate system, which is used to integrate it with other geographic datasets within a common coordinate framework. Coordinate systems enable you to integrate datasets to perform various integrated analytical operations, such as overlaying data layers from disparate sources and coordinate systems. Geographic, projected, and vertical coordinate systems, as well as geographic (datum) transformations, are discussed in this topic.
Here is another nice anecdote about sea level not being sea level: https://www.science20.com/news_articles/what_happens_bridge_...
(The article doesn't actually mention what happend, according to german wikipedia, the error was spotted early and the moderate costs for the correction efforts were covered by the architects' insurance policy.)
What's "mean tide"? Wouldn't it be different around the world depending on the position of the moon (and the Sun..) relative to your location? The lowest tide (neap tide) is when the effect of the Sun and the moon partially cancel one another out, and the highest tide (spring tide) is when the Sun and Moon line up and add their effects on the tidal range.
That's why you use the mean tide which averages over all the possible positions of the sun and the moon, not the momentary tide which, like many computer problems, depends on the time of the day and the phase of the moon.
I don't know why this was downvoted. Sea level does indeed suck as a baseline.
Traditional vertical datums like NAVD88 get their baseline zero elevation from an averaging of mean sea level at a small number (~5) of tidal stations around the area of interest.
The reason why multiple stations are used is that mean sea level is different from place to place. This is because of ocean currents and (I think though I can't say for certain) different levels of gravity at different points on earth's surface.
This is most clearly demonstrated with chart datum, which is a local elevation datum particular to a single maritime chart (eg a single harbor). Chart datum is the mean low tide for the area in question (useful to know if you don't want your craft to run aground...). The point of mentioning this is that: there are different chart datums along a single seaboard. Mean sea level is local.
The current practice is to use geoids which are based on the force of gravity. Einstein described geoids as surfaces on which clocks run at a constant rate. This is analogous to mean sea level on a waterworld in free space, eg sea level without the confounding factors of currents, weather, climate, or astronomical objects.
Yes, in the US (and presumably some other countries, I don't know of any offhand).
Many other countries such as Canada use Lowest Astronomical Tide. The difference is actually important to know at times when boating between the countries.
Average bulk plate motion is about one inch a year. Fastest plate boundaries move about four times that. So on the the surface this would be well within the error of mobile device GPS.
A major exception would be major geomorphic events like magnitude 7+ earthquake (10s of feet), landslide, river jumping its banks (like before the Mississippi was channelized). Then maps and property titles may need to account for this.
In practice, plates are not rigid masses. There are internal differential lateral motions of millimeters a year, or in most cases an order of magnitude slower than plate tectonics. There is a branch of geophysics called dynamic geodesy which teases such precise measurements from more coarse GNSS (GPS is one of six national GNSS). Such motions study pre-earthquake fault strain, pre-eruption swelling of volcanoes, man-made or natural injection/extraction of various subsurface fluids.
Is there any plan or agreement for how national borders should respond to plate tectonics? I guess one answer could be "they don't" and it would be possible for a town to gradually move into a different country, or a country to move ever more into the ocean, but that doesn't seem particularly satisfactory.
Borders are defined based on things like rivers, mountain tops, and physical markers. Which inherently move with the plates, so plate movements are effectively irrelevant.
As to something moving out to sea, the land everyone cares about is floating due to lower density, if it moves a mile in ~15,000 years it’s not going to noticeably change in anyone’s lifetime. And chances are the border will have changed several times before that happens.
Rivers don't move with the plates, well, they do, but they also move in addition. I'm not an expert but you can see the California-Arizona border along the Colorado river doesn't line up with the current course in a few places:
If a river straddles a flat line you might get a kink, but that’s normal in rivers. Generally they are surprising resilient, consider the rather inaccurately name new river basically ignore a 480 million year old mountain range that’s in it’s path. https://en.wikipedia.org/wiki/New_River_(Kanawha_River_tribu...
I'm really holding on borders becoming irrelevant in a time far shorter than 15000 years. Humanity must be united into a single entity and explore the stars together.
interesting to see both "it won't change in your lifetime and anyway it is a giant natural feature that moves with the grounds" and "for every known lake in the world, I found a real example of this problem" under the same reply ! Both are insufficiently general ..
Precision request in, and precision model out.. the expectation and means of the questioner does matter.. A national boundary in many places is quite adequately described with "1 meter resolution" because.. the whole nation is what is bounded ! However, to a active combatant, ask how the border is defined.. not even close to one meter.. the enemy is in our territory!
On the Earth Sciences side, there are sudden events that can move Earth more than a meter, and there are tectonically active areas which moves in different directions, in different and relevant amounts, often, every year.
Does a lake in the middle of nowhere, move a portion of a meter in a lifetime? The answer is, who is asking and why, because the intensity of the precision requested is related to the intensity of the inquiry by the inquirer. Water moves constantly, you fool! It is the stability that is more a question.
Last point in this pointless response on mapping.. Math and numbers are a fine tool, and remarkably useful, but the living, cycling, dripping world we live in, will not be revealed solely through better tables of numbers (as tempting as that is to YNews readers). Two hundred years ago this world was teeming with life of all sorts, and now, pesticides, pavement and poaching have seriously changed that, with no end in sight. Please act accordingly.
Could this also be another reason for starlink? Elon needs better positional data to develop better autopilot without lidar? Having 1000s of points of reference in the sky could create a better gps?
CSAC (chip scale atomic clock) seems to be well below $2k, I think they could swallow that. Army would probably be interested in such case. Also, TIL that GPS satellites are not geostationary.
Plate shift is why it is important when recording coordinates to also record the time of the measurement, because not only do the plates move, they deform.
Some related things about tectonic plate motions might be of interest to the HN crowd:
The surface of the Earth is tesselated into irregular tectonic plates that (mostly) move rigidly, with little internal deformation. Plate boundaries are geological faults (dislocations where the crust on one side of the fault slides past the other side), but there are also faults within the plate interiors, especially near the boundaries, that accommodate strain within the tectonic plates in 'deforming zones'. The rigid interiors of plates generally have strain rates at the nanostrain/year level, i.e. a few parts per billion or less per year. This means that the relative deformation (not plate motion) between say Florida and Denver is on the order of 1 mm/yr.
Plate motions are pretty constant through time, such that the velocities measured over 10 year scales with GPS and other geodetic techniques are quite comparable to velocities estimated over the past ~5 million years by looking at longer-term proxies such as the bar-code-like pattern of magnetic anomalies on the seafloor produced at divergent plate boundaries[1]. Within about 20 km of an active fault, or 200 km from a subduction zone, the instantaneous velocities have a lot of variation related to earthquakes, though.
The horizontal velocity of any point on the Earth's surface can be described as a rotation around another point on the Earth's surface, with some angular velocity; this is sort of a corollary or application of Euler's fixed point / rotation theorem[2]: the point is where the rotation axis intersects the Earth's surface (so there are two antipodal points, actually).
Because tectonic plates are mostly pretty rigid, the motion of plates on the Earth's surface can be modeled as a rotation around a single point, even when the plate is huge like the Pacific plate, i.e. almost a hemisphere. However as stated in the OP, there is no 'true' reference frame, so plate velocities are usually described relative to another plate as a reference frame, or just considering a pair of plates. We'd say that relative to the Pacific, the North American plate rotates around a point near Hudson's Bay, or equivalently, that the Pacific-North America rotation pole is near Hudson's Bay. To go from N. Am. relative to Pacific, to Pacific relative to N. Am, flip the sign on the angular velocity.
Therefore there is a bit of shared heritage between some aspects of graphics programming and tectonic geodesy, with respect to the mathematics of rotations, etc. However in practice the math is probably different (I'm not sure because I don't do graphics or game programming). For a good intro to the math used in this aspect of plate tectonics, see here[3].
In terms of the rates of the velocities, relative plate motion rates between adjacent plates are typically 1-10s of cm/yr. The fastest on earth is near Tonga and Vanuatu in the southwestern Pacific (north of NZ), at about 25 cm/yr.
A French startup operates an impressive technical solution in this direction:
> new GNSS (Global Navigation Satellite Systems) augmentation services based on the PPP-CNES technology (Precise Point Positioning).
> Absolute positioning accuracy of 4 cm (2D-95%), everywhere in the world, on the ground, on the sea and in the air, all the time, with only one receiver, without any nearby GNSS reference station: Precision, Availability and Integrity
> Depending on the level of service subscribed: Absolute positioning precision of 2, 4, 10, 50 or 80 cm (2D-95%)
> Compatible with GPS, GLONASS, BEIDOU and GALILEO constellations and mono and multi-frequencies receivers
I attended a meetup were they explained part of it. They have models for each of the aspects that can deteriorate precision and update all the parameters of all models in real time based on continuous measurements.
At a point during the development of the solution they had an unexplained discrepancy and tracked it down to some satellites of a common network being classifiable into two clusters of two different latencies (like, in the nanosecond range or even smaller). They contacted the network owners, listing the two clusters and asked "what is the difference between these and those satellites?".
The owners were a little confused. Something like "Well, inside each satellite, the cable from the antenna to the processing unit is a high performance transmission cable of tightly controlled specification, one inside each satellite. They happened to have been supplied in two batches with supposedly strictly identical behaviour up to the precision available at the time. And you sent us the exact listing of satellites matching batches A and B. But how on earth have you got this list? Were you spying into our internal archives?"
Answer: "no, we just have a model so precise that we could calibrate difference in transmission delays in your satellites from our receptor network on the ground".
Here's the startup Web site : http://www.geoflex.fr/?lang=en
Compared with this, the new approach you describe may have the disadvantage (at least for applications that care about positions relative to the surface and not the core of the earth) that it does not use a reference station that is fixed to the continental plate it is on (so that continental drift cancels out implicitly), and users have to handle it explicitly. Or do they offer a drift corrected service?
The practical problem is PPP falls apart when there's any sort of multipath or noise, with will be the case for any receiver not in a completely open field. A single nearby tree is often the difference between 4cm accuracy and a meter.
As for updates, the NGS have plate models that allow you to get the shift in coordinates at any given location[2] between two moments in time. Those models are in large part based on GNSS signal recorders called continuously operating reference stations[3] (CORS) which give actual movement at locations throughout the states.
For example, Bar Harbor, Maine, is moving at -0.0013 m/y north, 0.0020 m/y east, and -0.0009 m/y in elevation relative to the North American plate, and 0.0071 m/y north, -0.0153 m/y east, and 0.0001 m/y in elevation relative to the world[4].
The NGS is a remarkably good agency. See https://www.ngs.noaa.gov/TOOLS/ for more of their work.
[1] https://trimblertx.com/ [2] https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml [3] https://www.ngs.noaa.gov/CORS/ [4] https://www.ngs.noaa.gov/cgi-cors/CorsSidebarSelect.prl?site...
But what are some use cases that would benefit from that level of service? For example the highest tier is a 4 cm resolution with a convergence interval of 5 minutes.
The slides of the meetup are on https://fr.slideshare.net/LaurentDunys/meetup-voiture-connec... . Geoflex slides are in English, pages 22-53.
This also highlighted the inaccuracies of satellite imaging, particularly when it comes to uninhabited islands far from other land - Franklin Island, for instance, is over a mile from where it appears to be on google earth - and on marine charts. The upshot was that we had to stand off a few miles from shore, and had a long journey to shore by dinghy, as at that point the captain was understandably unwilling to trust the charts at all. You’d think we’d have the whole world accurately mapped out by now - but there are still seemingly some significant mistakes in commonly accepted points of truth.
Maybe there is still time to become an explorer, which I dreamed of as a child.
There’s a huge amount around, everywhere, to explore - you just have to go where people aren’t, and observe.
Do you have any ballpark figures as in how remote those watermills are? Like.. next road is 10km? 50km? Are there even roads to your property?
The nearest paved road is about 5km by track from the mill - the bottom km was impassible when we found the place, but nothing a day with a bulldozer and a chainsaw couldn’t fix - it looked like about 60 years of growth. The track down the valley is steep and winding, and looks like it goes nowhere. Despite being at the bottom of a deep, steep valley, we get an almost full day of light, as the mill is on the north bank of a large inverted U bend in the river - the sun rises over the valley to our left, grazes the hills to the south, and sets over the valley to the right. Someone in the 1300s really thought about where to put the structure, as 50m either way along the bank gets shaded way more.
Right now we’re having to walk in and out, as we had prodigious rain in December, which both highlighted our flood risk (overtopped the roof!) and made a steep, poorly drained section of the road collapse - I’ve spent the past week teaching myself how to make a pitched stone road - nice Neolithic technique, and means all the materials I need are within spitting distance of the road - rock, gravel.
The next mill, which is about 1.5km downstream of us, has no road or track, just a steep path.
I am still scratching my head over how they built them - the lower courses of stones in the walls must weigh five tonnes apiece - huge boulders, moved purely with human and animal power. After seeing the river go crazy last month, I understand why - the force was sufficient to uproot large trees, to wash away the car, to completely scout the earth to bare rock - but the mill couldn’t have cared less. It’s good timing, insofar as we hadn’t done too much work yet, and now we know we need to ensure everything structural in there can deal with immersion, and everything else needs to be portable. Constraint, sure, but an interesting one.
While shovelling muck there yesterday (temporarily renting a house after being flooded out) I heard a loud plop over the roar of the weir - and saw two otters hunting crayfish in the mill pond - I could see them flipping rocks on the bottom and grabbing them, as the flood has washed away sediments and organics, leaving the water crystal clear. This is the stuff that makes me persevere!
It has been an intense few months, just getting the place habitable - and when winter arrived life turned into a cycle of gathering firewood and plugging holes in masonry with rags - and then the whole place flooded in December, so the last six weeks have been muck-shovelling, hosing, repairing and reclaiming chattels which got flooded - we moved everything up to a shed with its floor level 5m vertically above the mill’s floor, which is 4m above the normal river level - still flooded - almost a 10m surge! Thankfully, it was only 75cm deep in there, and stopped 5cm short of trashing the solar batteries, inverter, etc., but got all the power tools and electronics (pcs, tv, NAS, routers, etc.) - which miraculously all still work after a wash and dry, barring CMOS batteries. Worst flood in centuries, apparently. We waded out in the darkness, in torrential rain, in thigh deep swirling muddy water, as the river had already burst its bank onto our track, the cats screaming bloody murder in their bag, and walked the 5km up to the car that I’d had the foresight to park up by the paved road... had nightmares for weeks - not something I care to ever repeat. Good we did evac, even if late, or we’d have probably ended up hypothermic and/or drowned.
Anyway. Ultimately, we want to use technology and automation to live off grid without reverting to medieval peasants - and the last four months have underscored that necessity, as I don’t think I can sustain this level of physical activity indefinitely. Today, after my conference calls, I’m heading back down there to keep on rebuilding the road, in the bloody freezing rain - but I have to get it done, as it’s about to be a blocker on getting anything else done. I comfort myself with the probable lie that I’ll be lying in a hammock listening to the birds and the river in a few months, and this will all be a distant memory!
The pros definitely outweigh the cons. We’ve met one other guy in this area who is doing the whole off grid thing - and he’s been doing it for years, and it’s still an incessant struggle - from components of his solar system failing (Chinese crud. Sorted him out with victron gear), to a donkey getting in and eating his crops, to his water ending up poisonous (Christmas vomit buddies), to his road also washing away - but he also wouldn’t trade it for the world - it’s the feeling of freedom and agency that is the attraction - nobody is doing these things to you - either it’s your own mistake, from which to learn, or just nature being a harsh mistress. Also, it’s really goddamned nice to just walk out of the front door in the morning, straight into wilderness. I start each day at home sitting on a rock with a coffee, just observing. Even with the stressors, I’m happier than I ever was behind a desk.
There were also Romans in this neck of the woods, but I’ve seen nothing that has said Roman to me yet. Either way, the site has evidently seen quite a bit of activity over the ages, only falling into disuse in the early 20th century - I found a taxation tag embedded in a gnarly old olive tree that identified it as being planted in 1914, and it looked like it hadn’t been pruned ever, meaning someone probably planted it and then got the Spanish flu.
The closer you are to the poles the more inaccurate compass readings will become. Based on the alignment of old stone walls[0], they think over the last few hundred years magnetic north has drifted by around 7 degrees in the northeast US, should be worse in England. When the property description uses a bound of a road, a stream, or other landmark, then the rest is all based on compass readings and distances of up to miles, that can make a huge difference.
I believe finding that your house is on your neighbors property is solved by "adverse possession" laws (sometimes referred to as squatters rights?) where if you act like you own something, and everyone else acts the same way for long enough time, it doesn't matter what the land records say, that is your property. It's a way of cleaning up mistakes.
Oddly, since GPS values will necessarily shift as the Earth's surface moves around, I think the old metes and bounds can be more accurate over time as long as it is based on true North instead of magnetic North.
[0] http://theconversation.com/old-stone-walls-record-the-changi...
Same goes for North. You try to get it close, but it's the relative angles between bearings that actually matter. So it doesn't matter if magnetic North changes or if there was an error initially measuring it.
For legal descriptions of boundary surveys, "what's on the ground" is what controls. Meaning an iron pipe set as a corner doesn't "mark" a property corner, but defines it (unless it's an offset, but it's still the defining feature). The legal description is more a treasure map than some idealized abstract legal "truth". This especially applies to metes and bounds.
GPS coordinates are generally not used for boundary surveys, except maybe to drive to the job site.
As for neighbors: the only people who can decide where a property boundary falls are the adjoining land owners. It's sort of zen, but the boundary is whatever you agree it is. It fits with the "local first" philosophy. In the absence of agreement, prescriptive acquisition (or judicial delineation) is the general term for when the courts have to become involved in a dispute (because the judge prescribes). "Adverse possession" is a type of positive defense to claim prescriptive acquisition when the party has no good-faith claim to the title. It's what most people think of as squatter's rights. Philosophically, it's not for cleaning up mistakes, but returning idle or abandoned real estate back into active commerce. "Good-faith possession" is if you think you have a legal claim to the property, like if you bought a bad title, or a property owner sold the same lot twice, and has a much lower burden of proof and required period of possession (generally ~30yrs adverse vs ~10yrs good-faith) Judicial delineation is more for bad boundary descriptions or cleaning up mistakes, but there's no need for judges if the neighbors can come to an agreement on their own. They just have to file an amended plat with the conveyances at the courthouse.
Basically you fudge it. Precision losses, errors, and local ground movements swamp continental drift. As you noted, some of the original sources go back over 100 years. A huge, huge amount of the surveyor's time was spent cleaning up data. One of the steps was to map the vectors around a plat. There was always some kind of closure error when you get back to the beginning position. The surveyor's job was to tweak the points to resolve the error. Kind of like auto body work; he used a lot of skill and experience in making the most seamless repair possible. (The most important part of my job was running interference for him, nobody understood why it took so long to "just digitize some maps".)
In general the original surveying records remain the source of truth, and the maps we produced with the tweaked coordinates were working documents. To be technically correct, another map would have to go back to the original documents and re-apply corrections. IIRC for some really egregious problems there were ways for the surveyor to amend the official truth.
Automating or re-implementing legacy systems is always a horribly messy data-cleansing exercise, but I think this was the most mind-numbing, tedious, endless slog I've ever been involved with.
Properties change, and their values change, and owners adapt. Really this is no different in a practical sense than having a house on an eroding beach, or (sigh...) in a sinking city.
You thought OpenStreetMap data uses the WGS84 datum? No it doesn't! https://news.ycombinator.com/item?id=20460596
https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/
Global coordinates in general are a lot more complicated than many people realize. For example, how exactly is GPS "elevation" determined? If you're standing on a beach at mean tide, is the elevation from a GPS receiver guaranteed to be zero everywhere on the planet? Here's a good starter on that one:
https://eos-gnss.com/elevation-for-beginners
>Coordinate systems, map projections, and geographic (datum) transformations
>Every dataset has a coordinate system, which is used to integrate it with other geographic datasets within a common coordinate framework. Coordinate systems enable you to integrate datasets to perform various integrated analytical operations, such as overlaying data layers from disparate sources and coordinate systems. Geographic, projected, and vertical coordinate systems, as well as geographic (datum) transformations, are discussed in this topic.
http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b72...
>US datum modernization coming in 2022
https://community.esri.com/groups/coordinate-reference-syste...
What's "mean tide"? Wouldn't it be different around the world depending on the position of the moon (and the Sun..) relative to your location? The lowest tide (neap tide) is when the effect of the Sun and the moon partially cancel one another out, and the highest tide (spring tide) is when the Sun and Moon line up and add their effects on the tidal range.
Sea level sucks as a baseline.
Traditional vertical datums like NAVD88 get their baseline zero elevation from an averaging of mean sea level at a small number (~5) of tidal stations around the area of interest.
The reason why multiple stations are used is that mean sea level is different from place to place. This is because of ocean currents and (I think though I can't say for certain) different levels of gravity at different points on earth's surface.
This is most clearly demonstrated with chart datum, which is a local elevation datum particular to a single maritime chart (eg a single harbor). Chart datum is the mean low tide for the area in question (useful to know if you don't want your craft to run aground...). The point of mentioning this is that: there are different chart datums along a single seaboard. Mean sea level is local.
The current practice is to use geoids which are based on the force of gravity. Einstein described geoids as surfaces on which clocks run at a constant rate. This is analogous to mean sea level on a waterworld in free space, eg sea level without the confounding factors of currents, weather, climate, or astronomical objects.
That's the standard used on nautical charts and also used for tide charts for the depth of the tides each day.
Many other countries such as Canada use Lowest Astronomical Tide. The difference is actually important to know at times when boating between the countries.
See https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_ph...
A major exception would be major geomorphic events like magnitude 7+ earthquake (10s of feet), landslide, river jumping its banks (like before the Mississippi was channelized). Then maps and property titles may need to account for this.
In practice, plates are not rigid masses. There are internal differential lateral motions of millimeters a year, or in most cases an order of magnitude slower than plate tectonics. There is a branch of geophysics called dynamic geodesy which teases such precise measurements from more coarse GNSS (GPS is one of six national GNSS). Such motions study pre-earthquake fault strain, pre-eruption swelling of volcanoes, man-made or natural injection/extraction of various subsurface fluids.
As to something moving out to sea, the land everyone cares about is floating due to lower density, if it moves a mile in ~15,000 years it’s not going to noticeably change in anyone’s lifetime. And chances are the border will have changed several times before that happens.
https://www.google.com/maps/@33.9012327,-114.5227135,14.5z
Similarly, with the US-Mexico border further along the river
https://www.google.com/maps/@32.6117771,-114.7983232,14z
And it's not like rivers and mountains can be depended on to stay in one place. Just ask Carter Lake, Iowa.
Precision request in, and precision model out.. the expectation and means of the questioner does matter.. A national boundary in many places is quite adequately described with "1 meter resolution" because.. the whole nation is what is bounded ! However, to a active combatant, ask how the border is defined.. not even close to one meter.. the enemy is in our territory!
On the Earth Sciences side, there are sudden events that can move Earth more than a meter, and there are tectonically active areas which moves in different directions, in different and relevant amounts, often, every year.
Does a lake in the middle of nowhere, move a portion of a meter in a lifetime? The answer is, who is asking and why, because the intensity of the precision requested is related to the intensity of the inquiry by the inquirer. Water moves constantly, you fool! It is the stability that is more a question.
Last point in this pointless response on mapping.. Math and numbers are a fine tool, and remarkably useful, but the living, cycling, dripping world we live in, will not be revealed solely through better tables of numbers (as tempting as that is to YNews readers). Two hundred years ago this world was teeming with life of all sorts, and now, pesticides, pavement and poaching have seriously changed that, with no end in sight. Please act accordingly.
Are Starlink satellites similarly equipped?
The surface of the Earth is tesselated into irregular tectonic plates that (mostly) move rigidly, with little internal deformation. Plate boundaries are geological faults (dislocations where the crust on one side of the fault slides past the other side), but there are also faults within the plate interiors, especially near the boundaries, that accommodate strain within the tectonic plates in 'deforming zones'. The rigid interiors of plates generally have strain rates at the nanostrain/year level, i.e. a few parts per billion or less per year. This means that the relative deformation (not plate motion) between say Florida and Denver is on the order of 1 mm/yr.
Plate motions are pretty constant through time, such that the velocities measured over 10 year scales with GPS and other geodetic techniques are quite comparable to velocities estimated over the past ~5 million years by looking at longer-term proxies such as the bar-code-like pattern of magnetic anomalies on the seafloor produced at divergent plate boundaries[1]. Within about 20 km of an active fault, or 200 km from a subduction zone, the instantaneous velocities have a lot of variation related to earthquakes, though.
The horizontal velocity of any point on the Earth's surface can be described as a rotation around another point on the Earth's surface, with some angular velocity; this is sort of a corollary or application of Euler's fixed point / rotation theorem[2]: the point is where the rotation axis intersects the Earth's surface (so there are two antipodal points, actually).
Because tectonic plates are mostly pretty rigid, the motion of plates on the Earth's surface can be modeled as a rotation around a single point, even when the plate is huge like the Pacific plate, i.e. almost a hemisphere. However as stated in the OP, there is no 'true' reference frame, so plate velocities are usually described relative to another plate as a reference frame, or just considering a pair of plates. We'd say that relative to the Pacific, the North American plate rotates around a point near Hudson's Bay, or equivalently, that the Pacific-North America rotation pole is near Hudson's Bay. To go from N. Am. relative to Pacific, to Pacific relative to N. Am, flip the sign on the angular velocity.
Therefore there is a bit of shared heritage between some aspects of graphics programming and tectonic geodesy, with respect to the mathematics of rotations, etc. However in practice the math is probably different (I'm not sure because I don't do graphics or game programming). For a good intro to the math used in this aspect of plate tectonics, see here[3].
In terms of the rates of the velocities, relative plate motion rates between adjacent plates are typically 1-10s of cm/yr. The fastest on earth is near Tonga and Vanuatu in the southwestern Pacific (north of NZ), at about 25 cm/yr.
[1]: https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-pol...
[2]: https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem
[3]: https://websites.pmc.ucsc.edu/~thorne/EART118/Lecture_PDF/le...