4 ms·

The article is kind of a tease, but doesn't really explain very well what's going on in these various coordinate systems that caused the need for the London are

The article is kind of a tease, but doesn't really explain very well what's going on in these various coordinate systems that caused the need for the London area specific survey and model, and causes different coordinate systems to put the Greenwich "zero" point all over the map. Hard to tell whether the point of the article is that precise mapping is a hard problem with various solutions optimized for global applicability giving different lattitude/longitude results in specific locales (where it kinda leaves the impression some sort of error is involved, which is not the problem; the Greenwich zero point is not 102m East in GCS84 because of error, but because it approximates the shape of the earth globally differently than standard spherical coordinates), or, as the headline says, that the curvature of the earth figures into projects that cover city-scale distances (which is hardly a surprise to anyone who thinks ten seconds about it; you can see curvature effects on a calm sea at a distance of only a few miles).

bloak 4y ago
> you can see curvature effects on a calm sea at a distance of only a few miles Open sea is rarely calm. But lakes are sometimes calm. I've sometimes thought it would be fun to set up some kind of permanent markers, something like giant clapper boards along a straight line at the same height above the water, so that people walking around a lake could easily observe the curvature of the Earth. Has this been done anywhere?
- Nzen 4y ago
  Dan Olsen published a video [0], last year, outlining his experiment on Minnewanka Curve and the ideology driving people who deny his, and similar results, demonstrating the curvature of the earth. While there is no artificial structure, he observes that trees on the opposite side of the lake are obscured by the water surface. He also published raw footage of the multiple recordings he took as a separate 28 minute video. [0] https://www.youtube.com/watch?v=JTfhYyTuT44 https://www.youtube.com/watch?v=JTfhYyTuT44 90 minutes
  - momojo 4y ago
    Great video by itself
- danielvf 4y ago
  Conveniently done already in Louisiana, where mankind has placed a line of concrete pillars twenty three miles across a lake. :) https://en.wikipedia.org/wiki/Lake_Pontchartrain_Causeway#/media/File:Lake_Pontchartrain_Causeway_From_Three_Lakeway_Center.png https://en.wikipedia.org/wiki/Lake_Pontchartrain_Causeway#/m...
  - deleted 4y ago
    [deleted]
  - burnte 4y ago
    I've driven on that one, and the bridges across the Chesapeake Bay, they're fantastic experiences, personally.
  - gowld 4y ago
    I don't understand how that image (And this simulation[1]) is caused by curvature. 23 miles is ~0.001 of the Earth's circumference. How can that cause a visible difference in tower height? Even more so, the visible difference is in the last few miles, even smaller fraction. cos(0.001*2pi) = 0.999980 [1] http://walter.bislins.ch/bloge/index.asp?page=Finding+the+Curvature+of+the+Earth%3A+Stand%2DAlone+App&demo=TrnsmLine#App http://walter.bislins.ch/bloge/index.asp?page=Finding+the+Cu...
    - modo_mario 4y ago
      >23 miles is ~0.001 of the Earth's circumference. How can that cause a visible difference in tower height? It seems fairly intuitive that a tiny amount of an insane amount can still be a lot no? I don't want to get too deep in the math right now since it's morning where i live nor am I good at it.... but if we'd just assume the earth was a nice sphere and a quick google was correct about a 12742 diameter then every degree down from the northpole to the equater would be on average a 70.79km difference in "height". Take a thousand of that and you're still dealing with an easily perceptible >7 meter difference. Now all of this is shoddy quick thinking since the earth is not a nice sphere, this heigh difference is better not measured perpendicular to the equator but to the ground or center of the earth, etc but you get the idea i hope.
simonh 4y ago
If GCS84 has the zero point 102m east, what are they using as a reference from which to determine that? How do they choose the position of their zero line? I’d have thought the zero line would be the reference from with other points would be determined. Unless they used the international date line as the reference perhaps.
- jandrewrogers 4y ago
  As I recall, the WGS84 meridian is registered to the center of the Earth from space, whereas the classic Greenwich meridian is essentially registered to the local direction of gravity at a point on the surface. If the local direction of gravity passed through the center of the Earth then the meridians would be the same. However, the Earth's gravity is non-uniform which causes the local direction to be slightly deflected away from a path that would pass through the center of the Earth as is the case in Greenwich. The WGS84 prime meridian is parallel to the Greenwich meridian but slightly shifted so that it actually passes through the center of the Earth. Because WGS84 meridian is not registered to a point on the surface (for good reason), it was always going to drift away from the Greenwich meridian over time regardless.
- walnutclosefarm 4y ago
  The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" vertical that was used to establish the Greenwich meridian as the origin for universal time does not pass through the center of mass of the earth. The coordinate system used by GPS (WGS84) has the center of mass of the earth as it's origin. The plane that defines the reference meridian (the 0th meridian that is) in that system has to be parallel to the one used to establish universal time, in order to maintain UT consistency. So, when you extend the plane of the WGS84 coordinate system from the center of mass of the earth, parallel to the vertical used at Greenwich, which, again, does not point exactly at the center of the earth, the plane cuts the earth's surface 102 meters East of the original 0th meridian.
  - simonh 4y ago
    This is why I love HN, thanks (and jandrewrogers).

bloak 4y ago

> you can see curvature effects on a calm sea at a distance of only a few miles Open sea is rarely calm. But lakes are sometimes calm. I've sometimes thought it would be fun to set up some kind of permanent markers, something like giant clapper boards along a straight line at the same height above the water, so that people walking around a lake could easily observe the curvature of the Earth. Has this been done anywhere?

Nzen 4y ago
Dan Olsen published a video [0], last year, outlining his experiment on Minnewanka Curve and the ideology driving people who deny his, and similar results, demonstrating the curvature of the earth. While there is no artificial structure, he observes that trees on the opposite side of the lake are obscured by the water surface. He also published raw footage of the multiple recordings he took as a separate 28 minute video. [0] https://www.youtube.com/watch?v=JTfhYyTuT44 https://www.youtube.com/watch?v=JTfhYyTuT44 90 minutes
- momojo 4y ago
  Great video by itself
danielvf 4y ago
Conveniently done already in Louisiana, where mankind has placed a line of concrete pillars twenty three miles across a lake. :) https://en.wikipedia.org/wiki/Lake_Pontchartrain_Causeway#/media/File:Lake_Pontchartrain_Causeway_From_Three_Lakeway_Center.png https://en.wikipedia.org/wiki/Lake_Pontchartrain_Causeway#/m...
- deleted 4y ago
  [deleted]
- burnte 4y ago
  I've driven on that one, and the bridges across the Chesapeake Bay, they're fantastic experiences, personally.
- gowld 4y ago
  I don't understand how that image (And this simulation[1]) is caused by curvature. 23 miles is ~0.001 of the Earth's circumference. How can that cause a visible difference in tower height? Even more so, the visible difference is in the last few miles, even smaller fraction. cos(0.001*2pi) = 0.999980 [1] http://walter.bislins.ch/bloge/index.asp?page=Finding+the+Curvature+of+the+Earth%3A+Stand%2DAlone+App&demo=TrnsmLine#App http://walter.bislins.ch/bloge/index.asp?page=Finding+the+Cu...
  - modo_mario 4y ago
    >23 miles is ~0.001 of the Earth's circumference. How can that cause a visible difference in tower height? It seems fairly intuitive that a tiny amount of an insane amount can still be a lot no? I don't want to get too deep in the math right now since it's morning where i live nor am I good at it.... but if we'd just assume the earth was a nice sphere and a quick google was correct about a 12742 diameter then every degree down from the northpole to the equater would be on average a 70.79km difference in "height". Take a thousand of that and you're still dealing with an easily perceptible >7 meter difference. Now all of this is shoddy quick thinking since the earth is not a nice sphere, this heigh difference is better not measured perpendicular to the equator but to the ground or center of the earth, etc but you get the idea i hope.

simonh 4y ago

If GCS84 has the zero point 102m east, what are they using as a reference from which to determine that? How do they choose the position of their zero line? I’d have thought the zero line would be the reference from with other points would be determined. Unless they used the international date line as the reference perhaps.

jandrewrogers 4y ago
As I recall, the WGS84 meridian is registered to the center of the Earth from space, whereas the classic Greenwich meridian is essentially registered to the local direction of gravity at a point on the surface. If the local direction of gravity passed through the center of the Earth then the meridians would be the same. However, the Earth's gravity is non-uniform which causes the local direction to be slightly deflected away from a path that would pass through the center of the Earth as is the case in Greenwich. The WGS84 prime meridian is parallel to the Greenwich meridian but slightly shifted so that it actually passes through the center of the Earth. Because WGS84 meridian is not registered to a point on the surface (for good reason), it was always going to drift away from the Greenwich meridian over time regardless.
walnutclosefarm 4y ago
The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" vertical that was used to establish the Greenwich meridian as the origin for universal time does not pass through the center of mass of the earth. The coordinate system used by GPS (WGS84) has the center of mass of the earth as it's origin. The plane that defines the reference meridian (the 0th meridian that is) in that system has to be parallel to the one used to establish universal time, in order to maintain UT consistency. So, when you extend the plane of the WGS84 coordinate system from the center of mass of the earth, parallel to the vertical used at Greenwich, which, again, does not point exactly at the center of the earth, the plane cuts the earth's surface 102 meters East of the original 0th meridian.
- simonh 4y ago
  This is why I love HN, thanks (and jandrewrogers).