8 ms·

What happens to Google Maps when tectonic plates move?

188 points by dnetesn 7y ago

jzl 7y ago
Cool topic, but that article was a bit long and meandering. Here's a good supplement: https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/ 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 https://eos-gnss.com/elevation-for-beginners
- onion2k 7y ago
  If you're standing on a beach at mean tide... 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.
  - globular-toast 7y ago
    Spring tides have both the highest and lowest tides. Neap tide is where the difference between high and low tide is the smallest.
    - fyfy18 7y ago
      So is the neap tide level zero elevation?
      - fhars 7y ago
        No, neap tide is the tide where the difference between low and high tide level is smallest.
  - fhars 7y ago
    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.
  - wffurr 7y ago
    MLLW is mean low low water. It's the average of the lowest tide each day at that location. That's the standard used on nautical charts and also used for tide charts for the depth of the tides each day.
    - znep 7y ago
      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. See https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_phase https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_ph...
  - ringshall 7y ago
    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.
- fhars 7y ago
  Here is another nice anecdote about sea level not being sea level: https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600 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.)
- Merrill 7y ago
  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. http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b720a5-7339-44b0-8b58-0f5bf2843393.htm http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b72... >US datum modernization coming in 2022 https://community.esri.com/groups/coordinate-reference-systems/blog/2019/02/26/us-datum-modernization-coming-in-2022 https://community.esri.com/groups/coordinate-reference-syste...
Reventlov 7y ago
Related, and featuring openstreetmap whose processes should be more open than google maps: You thought OpenStreetMap data uses the WGS84 datum? No it doesn't! https://news.ycombinator.com/item?id=20460596 https://news.ycombinator.com/item?id=20460596
madaxe_again 7y ago
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.
- capableweb 7y ago
  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 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.
  - madaxe_again 7y ago
    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_mboga_real 7y ago
2020. The world is ending? Here's a great google maps article to take up 10 minutes!
s_gourichon 7y ago
> 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". Here's the startup Web site : http://www.geoflex.fr/?lang=en http://www.geoflex.fr/?lang=en
- wormseed 7y ago
  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.
chaz6 7y ago
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.
cjbillington 7y ago
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.
- Retric 7y ago
  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.
  - whoopdedo 7y ago
    Unless you border spans a fault line, like in Hayward, CA. And it's not like rivers and mountains can be depended on to stay in one place. Just ask Carter Lake, Iowa.
  - Andrew_nenakhov 7y ago
    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.
  - s0rce 7y ago
    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: https://www.google.com/maps/@33.9012327,-114.5227135,14.5z 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 https://www.google.com/maps/@32.6117771,-114.7983232,14z
aristophenes 7y ago
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. [0] http://theconversation.com/old-stone-walls-record-the-changing-location-of-magnetic-north-112827 http://theconversation.com/old-stone-walls-record-the-changi...
- ajross 7y ago
  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.
- epc 7y ago
  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.
taf2 7y ago
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?
- oliveshell 7y ago
  GPS satellites carry multiple synchronized atomic clocks so they can broadcast the stable, precise time signal needed for accurate position-finding. Are Starlink satellites similarly equipped?
  - comboy 7y ago
    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.
OrgNet 7y ago
As the sea rise, does you altitude change?
cossatot 7y ago
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. [1]: https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-polarity/ https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-pol... [2]: https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem [3]: https://websites.pmc.ucsc.edu/~thorne/EART118/Lecture_PDF/lecture20.pdf https://websites.pmc.ucsc.edu/~thorne/EART118/Lecture_PDF/le...
peter303 7y ago
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.

188 points by dnetesn 7y ago

jzl 7y ago

Cool topic, but that article was a bit long and meandering. Here's a good supplement: https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/ 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 https://eos-gnss.com/elevation-for-beginners

onion2k 7y ago
If you're standing on a beach at mean tide... 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.
- globular-toast 7y ago
  Spring tides have both the highest and lowest tides. Neap tide is where the difference between high and low tide is the smallest.
  - fyfy18 7y ago
    So is the neap tide level zero elevation?
    - fhars 7y ago
      No, neap tide is the tide where the difference between low and high tide level is smallest.
- fhars 7y ago
  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.
- wffurr 7y ago
  MLLW is mean low low water. It's the average of the lowest tide each day at that location. That's the standard used on nautical charts and also used for tide charts for the depth of the tides each day.
  - znep 7y ago
    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. See https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_phase https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_ph...
- ringshall 7y ago
  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.
fhars 7y ago
Here is another nice anecdote about sea level not being sea level: https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600 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.)
Merrill 7y ago
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. http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b720a5-7339-44b0-8b58-0f5bf2843393.htm http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b72... >US datum modernization coming in 2022 https://community.esri.com/groups/coordinate-reference-systems/blog/2019/02/26/us-datum-modernization-coming-in-2022 https://community.esri.com/groups/coordinate-reference-syste...

Reventlov 7y ago

Related, and featuring openstreetmap whose processes should be more open than google maps: You thought OpenStreetMap data uses the WGS84 datum? No it doesn't! https://news.ycombinator.com/item?id=20460596 https://news.ycombinator.com/item?id=20460596

madaxe_again 7y ago

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.

capableweb 7y ago
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 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.
- madaxe_again 7y ago
  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_mboga_real 7y ago

2020. The world is ending? Here's a great google maps article to take up 10 minutes!

s_gourichon 7y ago

> 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". Here's the startup Web site : http://www.geoflex.fr/?lang=en http://www.geoflex.fr/?lang=en

wormseed 7y ago
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.

chaz6 7y ago

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.

cjbillington 7y ago

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.

Retric 7y ago
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.
- whoopdedo 7y ago
  Unless you border spans a fault line, like in Hayward, CA. And it's not like rivers and mountains can be depended on to stay in one place. Just ask Carter Lake, Iowa.
- Andrew_nenakhov 7y ago
  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.
- s0rce 7y ago
  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: https://www.google.com/maps/@33.9012327,-114.5227135,14.5z 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 https://www.google.com/maps/@32.6117771,-114.7983232,14z

aristophenes 7y ago

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. [0] http://theconversation.com/old-stone-walls-record-the-changing-location-of-magnetic-north-112827 http://theconversation.com/old-stone-walls-record-the-changi...

ajross 7y ago
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.
epc 7y ago
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.

taf2 7y ago

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?

oliveshell 7y ago
GPS satellites carry multiple synchronized atomic clocks so they can broadcast the stable, precise time signal needed for accurate position-finding. Are Starlink satellites similarly equipped?
- comboy 7y ago
  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.

OrgNet 7y ago

As the sea rise, does you altitude change?

cossatot 7y ago

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. [1]: https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-polarity/ https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-pol... [2]: https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem [3]: https://websites.pmc.ucsc.edu/~thorne/EART118/Lecture_PDF/lecture20.pdf https://websites.pmc.ucsc.edu/~thorne/EART118/Lecture_PDF/le...

peter303 7y ago

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.