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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

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?

ArchitectAnon 4y ago
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 4y ago
  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 4y ago
    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.
  - iggldiggl 4y ago
    Interesting. For infrastructure projects like roads and railways the standard I'm familiar with is still to just draw them directly based on the national coordinate system, but then again we're also still mostly outputting 2D drawings. Thankfully it seems that at least with 2D drawings and AutoCAD the worst side effect these days is that under certain circumstances hardware acceleration causes curves/curve segments to be displayed slightly offset. Strangely (but also luckily) enough it never happens when using just AutoCAD, but only when our road/railway design add-in is active (which displays all its output inside the AutoCAD drawing itself) [1], and it doesn't affect points picked via object snapping, i.e. it's really only the display that's affected. Our friends in structural engineering or architecture on the other hand do indeed use local coordinate systems for their 3D models, though I can't say whether that practice is absolutely universal [1] Edit: And also in the layout view, but thankfully definitively not in regular model space, where you'd be actually editing things.
gonzo41 4y ago
--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.
aidenn0 4y ago
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.

ArchitectAnon 4y ago

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 4y ago
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 4y ago
  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.
- iggldiggl 4y ago
  Interesting. For infrastructure projects like roads and railways the standard I'm familiar with is still to just draw them directly based on the national coordinate system, but then again we're also still mostly outputting 2D drawings. Thankfully it seems that at least with 2D drawings and AutoCAD the worst side effect these days is that under certain circumstances hardware acceleration causes curves/curve segments to be displayed slightly offset. Strangely (but also luckily) enough it never happens when using just AutoCAD, but only when our road/railway design add-in is active (which displays all its output inside the AutoCAD drawing itself) [1], and it doesn't affect points picked via object snapping, i.e. it's really only the display that's affected. Our friends in structural engineering or architecture on the other hand do indeed use local coordinate systems for their 3D models, though I can't say whether that practice is absolutely universal [1] Edit: And also in the layout view, but thankfully definitively not in regular model space, where you'd be actually editing things.

gonzo41 4y ago

--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.

aidenn0 4y ago

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.