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Earthquake in Japan yesterday may have shifted land 1.3 meters

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Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#81

Post by arghwhat »

Retric wrote: post #48
arghwhat wrote: post #35

Earlier quoted context omitted.

Not necessarily. Place a grid of receivers in fixed locations and observe their reported position. That grid is then the transform you need between the map when they were placed and the current state of the globe. A question would be how many you need, but these movements are on a large scale and receivers are dirt cheap.

A low resolution grid is cheap, but having enough censors for local accuracy is expensive because fault lines are so complicated: https://strangesounds.org/wp-content/uploads/2014/09/earthqu... What you really want is censers placed based on the underlying geology rather than a grid and then to combine that with a model of the geology.

Whether it is a grid or a selectively placed point cloud was not the point. Either way you end up with a mesh you can use for a relatively simple warp.

Granted, using it as input for a model might be more accurate, but also more complex. The simple warp might be enough to get within reasonable mapping tolerances though, while simultaneously revealing which areas are in most need of update.

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#82

Post by manarth »

bee_rider wrote: post #40
chrismorgan wrote: post #30

Earlier quoted context omitted.

Australia’s geocentric datum is pretty boring, because Australia’s tectonics are pretty boring. We just don’t get large earthquakes. So the GDA94 → GDA2020 update is actually pretty minor. If my understanding is correct, the 1.8m difference sometimes talked of between ITRF92 and GDA94 by 2020 is actually irrelevant so long as you do proper projection of your coordinates, which you should , but some things don’t—and s…

Hi from New England, middle-of-the-plate buddy. In fact, it had never occurred to me that the mapping of parts of the ground to, like, latitude-longitude could be time dependent, haha. I guess this could make a big difference in surveying.

Latitude-Longitude are references within a CRS (Coordinate Reference System).

US mapping can be based on USNG (United States National Grid) which is a cartesian geometry reference system, and if the entire USA moved a meter to the east, you may find that your USNG coordinates do not change at all.

The most common CRS is called WGS84, as this is the reference CRS for GPS. If the USA moved a meter to the east, the WGS84 coordinates would change.

There's a field of study devoted to translations between different coordinate reference systems, projections, geometries and more – and that's before we even get to mapping Mars or the Moon!

Geospatial definitions have a similar delight to date definitions, in that any and every reference could be annotated with "It depends" :-)

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#83

Post by manarth »

cm2187 wrote: post #6
sandworm101 wrote: post #2

The amount of energy released in such events boggles the mind. Shifting cubic-kilometers of rock, in this case several hundred or even thousands of cubic-kilometers, even a few inches requires more energy than all the worlds nukes many times over. We live atop an immense heat engine, every little vibration of which could power our entire civilization for years.

It's even harder than harvesting lightnings.

For starters, the scythe keeps melting

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#84

Post by manarth »

throw0101d wrote: post #19
badcppdev wrote: post #15

Earlier quoted context omitted.

I assume that should be 6.56 feet or some similar miscalculation converting from metric to imperial measurements

> 6.56 feet Statute feet or survey feet? * https://www.nist.gov/pml/us-surveyfoot * https://oceanservice.noaa.gov/geodesy/international-foot.htm... * https://en.wikipedia.org/wiki/Foot_(unit)#U.S._survey_foot

Or Swedish feet (29.69 cm), perhaps an Amsterdam foot (28.3133 cm)?

Perhaps make it half and half (and undocumented) so that neighbours can embark on generational land feuds.

https://en.wikipedia.org/wiki/Swedish_units_of_measurement#O... https://en.wikipedia.org/wiki/Dutch_units_of_measurement#Voe...

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#85

Post by Retric »

arghwhat wrote: post #81
Retric wrote: post #48

Earlier quoted context omitted.

A low resolution grid is cheap, but having enough censors for local accuracy is expensive because fault lines are so complicated: https://strangesounds.org/wp-content/uploads/2014/09/earthqu... What you really want is censers placed based on the underlying geology rather than a grid and then to combine that with a model of the geology.

Whether it is a grid or a selectively placed point cloud was not the point. Either way you end up with a mesh you can use for a relatively simple warp. Granted, using it as input for a model might be more accurate, but also more complex. The simple warp might be enough to get within reasonable mapping tolerances though, while simultaneously revealing which areas are in most need of update.

The wider point was you assumed it was easy because you didn’t understand the underlying complexity. GIS is a surprisingly deep rabbit hole once you start digging into the specifics.

Apply corrections on the device and Google Maps may want to use a more accurate model which then requires undoing the first correction before applying the second etc.

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#86

Post by fragmede »

throw0101d wrote: post #3

Australia updated its latitude and longitude somewhat recently because of continental drift: > The Geocentric Datum of Australia, the country’s local coordinate system, was last updated in 1994, and Australia is now about 1.5m further north-north-east (or, to give the metric used in a BBC infographic: about the height of a kangaroo). * https://www.theguardian.com/science/2016/aug/03/mind-the-gap... * https://www.ga.g…

Falsehoods programmers believe about landmasses.

Re: Earthquake in Japan yesterday may have shifted land 1.3 meters

#87

Post by oasisbob »

dnedic wrote: post #76
helen___keller wrote: post #72

Earlier quoted context omitted.

While you’re correct that GNSS is not 100% accurate (like everything in the physical world, it comes with error bars), I think the point stands that the error is unrelated to the movement of continents. Ultimately GNSS is just measuring your position relative to some celestial objects using the time it takes for signals to propagate space (and some other info: their ephemerides, and a shared-ish clock). The fact that…

GNSS satellites talk to base stations on earth to get correction data using the measurements they obtain (including measurements obtained by tracking satellites from the ground). I believe this is what the poster above is reffering to. RTK is a whole another beast and the meaning of an RTK base station is something else.

RTK is exactly what I was referring to.
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