[throw0101d]

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.gov.au/scientific-topics/positioning-navigati...

* https://www.australiangeographic.com.au/topics/science-envir...

To anyone working in the GIS industry: how would one go about doing a 'mass update' of locations of an entire country in maps and firmware and such?

[taeric]

I'm also curious if anyone has a good article to read on how this is managed. My guess would be that you have reference points and sets of nearby things that are recorded relative to the closest reference point. As noted in the sibling, your plat map is likely set against a local landmark. (Though, even there, they often also include the lat/lng of where it is expected to be found.)

[gwright]

This is a good example of why capturing coordinates in a local spatial reference system may be better than capturing coordinates in a global spatial reference system.

Global coordinates of a place of interest will drift relative to a global coordinate system but remain stable relative to a local coordinate system. This article illustrates that "stable" is not the same thing as unchanging in the presence of 7.5 earthquakes.

[midasuni]

It’s similar to capturing future dates in local time including timezone. There’s no guarantee that 1300 london time on 20-Dec-2029 will be 1300UatC, as the U.K. may shift its daylight rules before then.

[sangnoir]

That depends entirely ok whether the event being reflected is local or global. If its the time of an solar eclipse, for example, then its unaffected by daylight rules - unlike business hours and such. Working with dates/timestamps is a nightmare, but appears simple at first blush.

[chrismorgan]

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 so tweaking your reference point from time to time to compensate is pragmatic (in part because it lets people skip deformation models where a metre or so of accuracy is adequate, so temper my “should”). The GDA94 → GDA2020 update is more about certain changes in ITRF1992 → ITRF2014 (~9cm changes in ellipsoidal heights), and local crust deformations. Mass updating from GDA94 to GDA2020 is an easy reprojection.

In saying GDA94 is actually fine as far as continental drift is concerned: the thing most people don’t realise is that all of these coordinate systems are actually time-dependent transformations: they’ve got forecast continental drift baked in. So later datum updates just need to record what actually happened, if it was different enough (which it will be in places).

Now real earthquakes—they make things much more interesting. I like what https://www.linz.govt.nz/guidance/geodetic-system/coordinate... says:

> Some earthquakes, such as those of the Canterbury earthquake sequence starting in 2010, have caused metres of movement. Where this has happened we have updated the coordinates rather than simply include the movement in the deformation model. This is necessary as otherwise the coordinates will not be accurate enough for many applications. The deformation model still includes the earthquake deformation, but it is applied in reverse to transform coordinates for dates before the earthquake.

The deformation model page <https://www.linz.govt.nz/guidance/geodetic-system/coordinate...> is also rather interesting, giving details of when they’ve released new deformation models, including both forward and reverse patches due to earthquakes, and it’s messy. I haven’t thought too deeply about the reverse patching technique (this isn’t a domain I work in) but I don’t think I like it (though it may be pragmatic) because it messes with epoch-era NZGD2000 points, seems to undermine the purpose of a datum unless you have recorded the coordinate times as well (rather than merely transforming to the epoch, which has now been spoiled). Did have a fun chat with a telecommunications field worker that I happened to meet when I was in New Zealand in 2021, who had a lot of interesting experience after the 2016 Kaikoura earthquake. Lots of stuff I’m not used to thinking about, coming from boring old Australia.

[bee_rider]

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.

[manarth]

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" :-)

[blacksmith_tb]

For most surveying wouldn't the the relative, nearby points be shifting together equally?

[johncalvinyoung]

not in California, Iceland, or Japan!

[midasuni]

It’s unlikely the relative position of Griffiths Observatory and the Chinese Theatre would change though, both may shift north by 74cm relative to Las Vegas sure, but I’m not aware of any faults between those two points

[fragmede]

Falsehoods programmers believe about landmasses.

[dboreham]

Locations inside a country (e.g. plat map boundaries) don't use latitude/longitude.

[eesmith]

Some places use an encoded lat/long.

Open Location Code - "Google has shown practical usage of plus codes for addressing purposes in Cape Verde,[10] parts of Kolkata[11] and Kolhapur[12] in India, and the Navajo Nation in the United States.[13]" - https://en.wikipedia.org/wiki/Geocode . https://freethoughtblogs.com/singham/2023/12/24/using-olc-co... says a friend of his sent an OLC coordinate to get to a place in Carmel-by-the-Sea, California, which (infamously?) has no street numbers.

What3words - "This population had "no consistent addressing system" until May 2016 when Mongol Post started using a geocoding system provided by what3words." - https://en.wikipedia.org/wiki/Mongol_Post

Over time (centuries?) the named coordinate will no longer be valid as the location is no longer near enough to that lat/long.

[midasuni]

I believe W3W are 1m squares, so in Japan it sounds like they may be already broken

[eesmith]

https://en.wikipedia.org/wiki/What3words says "What3words divides the world into a grid of 57 trillion 3-by-3-metre (10 ft × 10 ft) squares".

Australia moves about 7cm/year so after about 43 years a W3W coordinate no longer matches local coordinates.

It feels weird that a continent could move so much in someone's lifetime.

Looks like the 1906 San Francisco quake had displacements up to 8.5 meters, so more than two squares. https://en.wikipedia.org/wiki/1906_San_Francisco_earthquake

[taeric]

They didn't used to. And, of course you are correct that survey markers are to landmarks. But, more and more people are using data from online mapping services, that largely think they can store locations in lat/lng. People seem to trust that as "the UTC of location data."

That said, those can be just as fraught, no? Since movement will not be fully uniform for the entire continent? And landmarks aren't static things, either? I'm curious how landmarks are done to preserve this, long term?

[tmm]

> And landmarks aren't static things, either? I'm curious how landmarks are done to preserve this, long term?

At least in the US, the people in charge of this (the National Geodetic Survey) take movement into account. This[1] is the datasheet for the reference monument nearest my house. It has data for 3D velocity of the reference point.

    VX =  -0.0150 m/yr      northward =   0.0040 m/yr
    VY =  -0.0008 m/yr      eastward  =  -0.0148 m/yr
    VZ =   0.0030 m/yr      upward    =  -0.0002 m/yr

I'm not sure what the difference is between VX/VY/VZ and northward/eastward/upward, but those numbers are bigger than I thought they'd be. 15 mm/yr seems like a lot!

The official coordinates get updated from time to time, and in-between, perhaps you're supposed to adjust your measurements using that movement data (IANAS)? If there is a significant measured change in location due to an earthquake, I'm sure that data would be included in the monument datasheet or a new set of coordinates would be published.

FWIW, I went looking to see when the last earthquake was here in Maryland and it turns out there was one today[2].

[1] https://www.ngs.noaa.gov/cgi-cors/CorsSidebarSelect.prl?site... [2] https://www.abc27.com/news/top-stories/2-3-magnitude-earthqu...