[HN Gopher] How Crossrail was affected by the curvature of the E...
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How Crossrail was affected by the curvature of the Earth (2018)
Author : zeristor
Score : 131 points
Date : 2022-05-05 11:40 UTC (11 hours ago)
(HTM) web link (www.ianvisits.co.uk)
(TXT) w3m dump (www.ianvisits.co.uk)
| londons_explore wrote:
| A good chunk of this problem is because they don't just use the
| global WGS-84 standard, and instead decide to try to use locally-
| flat custom coordinate systems. It's like trying to design your
| own car from scratch to travel somewhere rather than just using
| one from Ford. Lots of extra effort with little gain.
|
| If WGS-84 had been used throughout (and all old measurements
| converted), everything would have been far simpler and cheaper,
| both now and for future modifications.
| pacaro wrote:
| I worked for a small British GIS company in the mid-nineties.
| Having an entire country on a single rectilinear coordinate
| system really makes life easier. The OS grid makes sense for
| mapping a country the shape, size and location of the UK. And
| on 16-bit windows machines, not having to deal with WGS-84 or
| UTM, was a reasonable call
| londons_explore wrote:
| But now all the complex math is hidden away behind
| abstractions, and the extra computation to do everything in
| WGS-84 space is insignificant. Yet the benefits are huge in
| that every engineer who works on the design won't need all
| this extra knowledge of many custom coordinate systems and
| the way conversions and interactions between them work. Not
| to mention the huge possible costs when someone messes up and
| uses the wrong coordinate system for something and a beam
| ends up only half the thickness it should have been because
| the top was constrained by something in one coordinate system
| and the bottom by something in another...
|
| I think a close parallel is the complexity of UTF-8 is now
| worth it over encoding things using codepages.
| lmc wrote:
| > the extra computation to do everything in WGS-84 space is
| insignificant
|
| There is still significant overhead involved in conversion
| - arbitrary trig functions, even optimised, are still
| really expensive compared to planar geometry. Remember
| these folks will be working at super high resolutions too.
| teruakohatu wrote:
| > But now all the complex math is hidden away behind
| abstractions, and the extra computation to do everything in
| WGS-84 space is insignificant... UTF-8 is now worth it over
| encoding things using codepages.
|
| There is a reason for having GIS professionals. WGS-84 is
| not nearly accurate enough for many purposes. Good enough
| to find your home, not good enough for precision surveys.
| In my country to get sub-metre (cm) precision using
| NZGT2000 is necessary.
|
| WGS-84 also has no deformation component. The tectonic
| plates moves, in parts of my country movement is
| 5cm/year,and so a WGS-84 coordinate taken at one point in
| time won't precisely point to the save bit of dirt at a
| later time. Other geographic coordinate systems can take
| this into account.
|
| Nobody would use UTF-8 if every 1000th character was lost.
| and_viceversa wrote:
| With a dual band survey grade receiver, WGS84 coordinates
| can be derived to 2cm accuracy in the vertical [0] and
| even better in the horizontal.
|
| WGS 84 is a time dependent datum - each addition to the
| ensemble is known as a realization. The software used to
| convert across time and reference frames is called HTDP
| [1]. This can also be done for the vertical using VDatum,
| which wraps HTDP.
|
| The confusion around this issue usually has to do with
| how analysts actually handle coordinate information in
| software. For example ESRI, arguably the dominant GIS,
| does not have time dependent conversion capability. In
| the US there is also a false equivalence that NAD83 ==
| WGS84. Looks like NZ has a similar issue [2].
|
| Developers also have to deal with this issue, since web
| mercator and the tiling scheme were designed for
| convenience and not sub meter accuracy.
|
| [0]
| https://www.ngs.noaa.gov/PUBS_LIB/NGS592008069FINAL2.pdf
| [1] https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml [2]
| https://www.linz.govt.nz/data/geodetic-system/datums-
| project...
| and_viceversa wrote:
| For a U.S. analog, we use the State Plane system. The states
| have (multiple) custom projections to suit local accuracy
| needs. Like another commenter said, WGS 84 uses ellipsoidal
| coordinates that are not suitable for the Crossrail's
| construction. If it were really so easy, American states would
| "just use WGS 84" as well.
| lmc wrote:
| WGS-84 has angular coordinates, you can't measure distances
| like they need for construction without projecting to Cartesian
| coords, hence the local CRS they developed.
| JackFr wrote:
| One of my favorite curvature-of-the-earth/engineering details is
| that the tops of the towers of the Verrazon Narrows Bridge are 1
| 5/8 inches farther apart at their tops than at their bases due to
| the curvature of the earth.
|
| https://en.wikipedia.org/wiki/Verrazzano-Narrows_Bridge
| adolph wrote:
| I was thinking about this too from the perspective of
| potentially being easier to observe than ships at sea.
|
| On the other hand, the earth's curvature is imperfectly
| correlated to gravitational vector. Given a stable
| gravitational anomaly, would two towers such as that bridge be
| oriented to gravity or curvature? My guess the former, meaning
| that the change in distance between the base and top of two
| tall towers wouldn't add to understanding curvature.
| ninju wrote:
| > ...in three different coordinate systems (OSGB36, WGS84 and
| ED50)
|
| > Crossrail made use of a *customised projection* with a meridian
| that runs through central London
|
| https://xkcd.com/927/
| burnte wrote:
| "... so I decided to use regex. Now I have two problems."
| avs733 wrote:
| a number of years ago when I traveled frequently for work I was
| very focused on optimizing my airline miles and points.
|
| At some point as part of a computer system cutover, United
| Airlines suddenly started shorting people miles on airport to
| airport differences. Not a lot, but a few here and there. A group
| of folks on flyertalk slowly realized that the change was
| perfectly explained by the difference between treating the globe
| as a perfect sphere and treating it as an oblate spheroid.
|
| The previous model had been technically correct and precise.
| Whoever built the new model had simplified the code for some
| reason. After much kvetching United fixed it.
| aidenn0 wrote:
| I propose that to simplify geographic calculations, we should
| all pack up and move to venus.
| simonh wrote:
| Or the Earth could be shrinking.
| burnte wrote:
| This is the truth no one wants to admit. In ten years we'll
| be orbiting the moon!
| ArchitectAnon wrote:
| Also Kansai airport [0] had to cope with this issue, complicated
| by the fact that it's built on reclaimed land and the whole thing
| is (or was) on software controlled hydraulic jacks which control
| differential settlement.
|
| [0] http://www.rpbw.com/project/kansai-international-airport-
| ter...
|
| [1] http://engineering-
| timelines.com/scripts/engineeringItem.asp...
| boredumb wrote:
| Extremely interesting and an amazing feat of engineering.
|
| "The terminal was completed in less than 36 months." Blows me
| away the most I think.
| walnutclosefarm wrote:
| 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).
| simonh wrote:
| 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 wrote:
| 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.
| bloak wrote:
| > 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?
| danielvf wrote:
| 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#/m.
| ..
| burnte wrote:
| I've driven on that one, and the bridges across the
| Chesapeake Bay, they're fantastic experiences, personally.
| [deleted]
| gowld wrote:
| 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+t
| he+Cu...
| Nzen wrote:
| 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 90 minutes
| chx wrote:
| What the article mentions in passing was called "threading the
| needle" where an earth pressure balance tunnel-boring machine
| mined the section of tunnel directly above the operational
| Northern line platform tunnels at Tottenham Court Road, directly
| below the London Underground station structures, with less than
| 800mm clearance to each. Read more at
| https://www.theengineer.co.uk/content/in-depth/your-question...
| heavenlyblue wrote:
| It's not as impressive if you know that we can use ultrasound
| to know how far away we are from the tunnels in the first place
| lifeisstillgood wrote:
| Wait what? There are three tunnels under London separated by a
| _two feet_ of earth?!
|
| I doubt we even need a Bond villain to break through that !
|
| I am not sure I trust engineering _that_ much?
| timthorn wrote:
| They posted an engineer (and a film crew[1]) to stand on the
| relevant platform at Tottenham Court Road while the TBM was
| boring through, to watch for anything going wrong. That's
| trust :)
|
| 1. For this documentary. The title hasn't aged well... :
| https://www.bbc.co.uk/programmes/b08ry6fy
|
| I wonder if the Bond villain is responsible for the late
| opening of the eponymous Station?
| lifeisstillgood wrote:
| It's more, is that going to hold for a hundred years? I
| mean it's just ... clay under London.
|
| Then again what do I know. I look up at Jumbo Jets flying
| past and think, "no way can that stay up there"
|
| If we turn out to live in the Matrix and heavier than air
| flight is done with a continuous loop in a Perl script, I
| will not be surprised.
| avs733 wrote:
| make the project engineers who signed the plans stand there.
| TYPE_FASTER wrote:
| The first time I drove across the US, I wondered why the two lane
| roads we were driving on in the midwest (scenic route) did a
| quick dogleg every so often.
|
| https://kottke.org/18/01/us-road-grid-corrections-because-of...
| rob74 wrote:
| Ok, the title "US road grid corrections because of the Earth's
| curvature" is slightly misleading. The doglegs are not there
| because of the curvature of the earth, but because of the
| "Jefferson Grid", which divided a large portion of the US into
| plots which were each a square mile. But, due to the curvature
| of the earth, the north-south boundaries between these plots
| couldn't be exactly straight, and that led to these
| "misalignments".
| helsinkiandrew wrote:
| > Ok, the title "US road grid corrections because of the
| Earth's curvature" is slightly misleading
|
| OK, but "US road grid corrections because of the north-south
| boundaries between Jefferson Grid plots that aren't straight
| due to the Earth's curvature" doesn't roll off the tongue
| quite as well.
| InCityDreams wrote:
| I always figured it was to give drivers something to do to keep
| them awake.
| drewzero1 wrote:
| I had heard of that idea in relation to the design of the US
| interstate highway system. I looked into it[0] and found that
| it may be a consideration, but not a requirement as I thought
| I remembered.
|
| https://www.fhwa.dot.gov/interstate/faq.cfm#question31a
| Mountain_Skies wrote:
| Though not the intended purpose, it probably also helps serve
| as a ground level windbreak.
| triggercut wrote:
| Funny, I was just reminiscing about this very problem this
| afternoon.
|
| When you are working to specific locations at each end it is a
| really tricky problem, but local grids like the ones described
| here help. I've worked on rail projects for mines in remote areas
| that need a few hundred kms of new track and span across "zones".
| The problem is exacerbated (at least then, early 00s) by the way
| you represent these in various CAD products, especially when
| sharing data between them. There are two main 3D modeling kernels
| that most popular professional CAD products rely on, both from
| the 70's 80's with minor updates neither 64bit native. Why is
| this an issue? Well, you see, the standard for civil design is to
| model in "real world" coordinates. That means you could be
| dealing with a file at the resolutions of mm for engineering
| purposes, but thousands of kms away from the origin. Now we enter
| the world off floating point calculation errors and subsequent
| kernel issues.
|
| There are ways around it. Essentially offsets that move the
| origin of the file temporarily with a note to itself that
| everything must also account for the offset before displaying in
| the UI. But it can get confusing, fast, every product does it its
| own way and may not recognise this trickery. Working digitally
| with sparse real world reference data of varying qualities can be
| a big risk. I remember being in a large workshop with two
| surveying companies, our client and some civil engineers. I was
| horrified that I knew more about the various grids and their
| issues than any one else. I did one unit of surveying at uni, not
| long before and had to do a lot of research to get my head around
| the issues. These people were meant to be the experts.
|
| The good thing about linear projects is that on those scales you
| can usually get somewhere where you can "work it out" on site
| (i.e. fudge it to make it fit). But It potentially affects
| everything, like, how many meters of track are we ordering? What
| contingency do we need? What are our expected mass haul volumes
| and estimated fuel costs? How big should we make our margin of
| error? It'll all work itself out, it'll just cost.
|
| What happens when we start building beyond the planet and need to
| accomodate for curvatures in space time? Even ones that are
| brought about by the very thing you are building?
| gonzo41 wrote:
| --What happens when we start building beyond the planet and
| need to accommodate for curvatures in space time?
|
| By the looks of things, Star Trek fixes this issue by
| eliminating money.
|
| In Avatar, there's a whole other interesting world of economic
| star travel where the mineral unobtanium is valued at 20
| million per kg. Interstellar travel with back and forth trips
| to another world actually works out to be profitable for a
| company and that's set in 2154. With inflation at current rates
| 20 million ain't going to be much, so it seems like this
| problem get's solved. Or the company in that film has worse
| margins than air travel.
| ArchitectAnon wrote:
| Back in the mid 2000's I tried to point out to the people
| writing the UK BIM standards why it was a bad idea to have all
| buildings drawn relative to the OS grid datum which is
| somewhere southwest of the Scilly Isles for exactly this
| reason. I even tried to explain that computers can't do
| accurate floating point calculations and they didn't believe
| me! And then CAD consultants are surprised when automatic 2D
| drawing generation from 3D models randomly fails to work
| properly (usually at 10pm the night before a big deadline) and
| then everyone wonders why architects are backwards and don't
| want to adopt 3D processes...
| idleproc wrote:
| I used CAD software back in the late 90's called Spirit. The
| floating point calcs were infuriating. You could draw a bunch
| of lines with exact length and offset them by exact distances
| and trim them and they'd all end up x.01mm etc. when you
| measured or dimmed them.
|
| Hah, people used to say. We've always used drawing boards,
| that kind of accuarcy isn't important.
|
| But I'd argue that it was. For my own sanity. Sadly, a
| standard UK brick is 215 x 102.5 x 65mm. Are bricks
| manufactured to a tolerance of 0.5mm? Can a builder measure
| to 0.5mm? No.
|
| But when you're digital, and you have a large building, small
| errors start to accumulate. Next thing is you have the
| builder on the phone saying the overall length of your
| building on opposite sides don't match, which one is correct?
| Tabular-Iceberg wrote:
| This is why I can't fathom why fractional inches didn't
| become the dominant system once engineers switched to CAD.
| Or not necessarily inches, but at least a fractional
| representation.
| aidenn0 wrote:
| This is one of many reasons why floating-point should never be
| used for coordinates. It's also a bit frustrating because
| 64-bit integer support is easier to implement in hardware than
| double-precision (or even single-precision) floating point, but
| the latter was widely available long before the former.
|
| There are coordinate systems that can represent any point on
| earth to the precision of a micron that use 64 bit integers.
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