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Comment by jp57 | Hacker News Reader

FYI: nm = nautical miles, not nanometers.

It's quite amusing that they used the incorrect, lowercase abbreviation for "nautical mile" which means something else ("nanometer") in an article about a major issue caused by two things sharing the same abbreviation.

I learned navigation around 2005, and we used "nm" then. I honestly don't remember the moment when this abbreviation was granted to nanometer.

I was wondering; it seemed like if the to airports were 36000 angstroms apart (3600 nanometers), it'd be reasonable to give them the same airport code since they'd be pretty much on top of each other.

I've also seen "DANGER!! 12000000 μVolts!!!" on tiny little model railroad signs.

That's so adorable (for model railroads)

Even though I knew this was about aviation, I still read nm as nanometres. Now I'm wondering what this says about how my brain works.

It says ‘metric’. Good.

Though one could argue that the (original) definition of a meter and the definition of a nautical mile are equally arbitrary and yet similarly based.

Originally 1 meter was one ten-millionth of the distance over the surface of the earth from the equator to the pole.

One nautical mile is the length one arc-minute of latitude along a meridian. (About 1.85km).

Yes, the nautical mile is actually les arbitrary than the 'normal' mile.

> Originally 1 meter was one ten-millionth of the distance over the surface of the earth from the equator to the pole.

Even more originally, they wanted to use the length of a pendulum that takes one second to swing. But they discovered that this varies from place to place. So they came up with the newer definition based on the size of the earth. And just like with all the subsequent redefinitions (like the one based on the speed of light etc), the new length of the metre matches the old length of the metre:

> [The] length of the string will be approximately 993.6 millimetres, i.e. less than a centimetre short of one metre everywhere on Earth. This is because the value of g, expressed in m/s^2, is very close to π^2.

The definitions matching is by design, not an accident.

See https://en.wikipedia.org/wiki/History_of_the_metre and https://en.wikipedia.org/wiki/Seconds_pendulum

If you want something less arbitrary, you can pick 'Natural Units': https://en.wikipedia.org/wiki/Natural_units

They were. The difference is one is part of a standard system of units with sensible relations between different units in the system, and the other isn't; the specifics aren't what's important, the relations are.

1 foot is allllllllmost the distance light travels in a nanosecond. If we’d just adjusted the length a tiny bit it would be an excellent unit of measurement

Indeed. There are plenty of things in aviation where they care so much about compatibility that something survives decades after it should reasonably be obsolete and replaced.

Inches of mercury, magnetic bearings (the magnetic poles move! but they put up with that) and gallons of fuel, all just accepted.

Got a safety-of-life emergency on an ocean liner, oil tanker or whatever? Everywhere in the entire world mandates GMDSS which includes Digital Selective Calling, the boring but complicated problems with radio communication are solved by a machine, you just need to know who you want to talk to (for Mayday calls it's everyone) and what you want to tell them (where you are, that you need urgent assistance and maybe the nature of the emergency)

On an big plane? Well good luck, they only have analogue radio and it's your problem to cope with the extensive troubles as a result.

I'm actually impressed that COSPAS/SARSAT wasn't obliged to keep the analogue plane transmitters working, despite obsoleting (and no longer providing rescue for) analogue boat or personal transmitters. But on that, at least, they were able to say no, if you don't want to spend a few grand on the upgrade for your million dollar plane we don't plan to spend billions of dollars to maintain the satellites just so you can keep your worse system limping along.

> Inches of mercury, magnetic bearings (the magnetic poles move! but they put up with that) and gallons of fuel, all just accepted.

Here in Europe we use hectopascals for pressure, as does pretty much everywhere else. It’s important to have a magnetic bearing in case your glass dies and you’re reliant on a paper map and compass, if you didn’t plan with magnetic bearings you’d be screwed if this happened in an area of high magnetic variation.

Air pressure is reported in hectopascals and fuel quantity in kilograms (or tons). It's only in America where this isn't the case. We're still using feet for altitude in most places though (it's mainly Russia that uses metres).

Feet are "optionally accepted" due to USA being China of aviation and flooding the market with feet-marked hardware

"Hacker News failure caused by two units 12 orders of magnitude apart sharing 2-letter code"

We better get this sorted out before open source manned Mars missions.

That all programming languages, down to statically typed assembly, don’t support something as simple to validate as unit consistency says something strange about how the science of replacing unreliable manual processes with automated systems is really bad at the practice of replacing its own risky manual processes with automated systems.

If numeric types just required a given unit, without even supporting automated conversions, it would make incorrectly unit-ed/scaled literals vastly less likely.

> That all programming languages, down to statically typed assembly, don’t support something as simple to validate as unit consistency [...]

Many programming languages are flexible and strong enough support this. We just don't do it by default, and you'd need libraries.

Btw, units by themselves are useful, but not enough. Eg angular momentum and energy have the same units of Newton * metre, but adding them up is not recommended.

> Eg angular momentum and energy have the same units of Newton * metre, but adding them up is not recommended.

The unit of angular momentum is kg.m^2.s^-1, you're thinking of torque. Although even then we distinguish the Newton meter (Nm) from the Joule (J) even if they have the same dimensionality.

Thanks, yes, I meant torque.

Well, 1 J = 1 Nm, the differentiation you mention only helps humans a bit, but would be really hard to make work for a computer.

I don't think it would be too difficult for a computer to handle. We already deal with situations like float(1) = int(1), it doesn't seem any harder to handle torque(1) = energy(1).

The problem is that you want torque not being equal to energy.

Specifically you have:

torque = force * distance

energy = force * distance

The only difference being that in the former the farce is perpendicular to the distance, and in the latter it's in line with the distance.

A vector based system could distinguish the two, but you don't always want to deal with vectors in your computations. (And I'm fairly sure there are problems where even using vectors ain't enough to avoid this problem.)

Some languages do; F# and Ada have units.

I agree no sexy languages have it, and almost all languages have terrible support or anti-support for correctness in numerical programming. It's very strange.

(By anti-support I mean things that waste your time and make it harder. For instance, a lot of languages think "static typing" means they need to prevent you from doing `int a,b; short c = a * b;` even if this is totally well-defined.)

`short c = a * b` can be both well-defined and a serious bug if (a * b) is greater than `sizeof(short)`. Whether it _is_ a bug depends on what you're doing.

Swift has units as part of the standard library. In the sense that matters here, Rust and C++ could also have units. It requires a level of expressiveness in the type system that most modern languages do have, if you put it to use.

It can be a serious bug if it overflows and you didn't intend it to happen or you expected overflow behavior to do something different. But that's also true of `int c = a*b`, and yet that's not a compiler error.

int/short should be thought of as storage size optimizations for memory. They're very bad ways to specify the correct range of values for a variable.

(Ada has explicitly ranged integers though!)

Whenever it's vague, I include the units as part of the parameter name (eg. delay_ms). Not perfect, but for practical purposes it helps. It's simple and I'm not sure why there aren't more people/libraries doing this.

I wouldn't have guessed until I read the comments. My assumption was somebody just mistyped km and somehow nobody cared to fix it.

Yeah, I went into the article thinking this because I expected someone had created waypoints right on top of each other and in the process also somehow generating the same code for them.

Ah! I thought this was a case where the locations were just BARELY different from each other, not that they're very far apart.

Given the context, I'd say NM actually https://en.wikipedia.org/wiki/Nautical_mile

I was clarifying the post title, which uses "nm".

Yes, it looks like they should have written "NM" instead of "nm".

No one is using nanometers in aviation navigation. Quite a few aviation systems are case insensitive or all caps only so you can't always make a distinction.

In fact, if you say "miles", you mean nautical miles. You have to use "sm" to mean statute miles if you're using that unit, which is often used for measuring visibility.

Sure but I could imagine some kind of software failure caused by trying to divide by a distance that rounded two zero because the same location was listed in two databases that were almost but not exactly the same location. In fact I did when I first read the headline, then realized that it was probably nautical miles.

That would be roughly consistent with the title and not a totally absurd thing to happen in the world.

This is exactly what I thought q when I first read the title.

Indeed, having locations internally represented in software with a resolution of nanometers is as ridiculous as having your calendar's internal times represented as milliseconds since some arbitrary moment more than fifty years ago!

Indeed, but you can easily imagine a software glitch over what looks like a single location but which the computer sees as two separate ones.

Yes. And, to quote the Wikipedia article: "Symbol: M, NM, or nmi". Not nm (as used in the title, but the article also uses it).

Thanks, from the title I was confused on why there was such a high resolution on positions.

Wow, I read this article because I could not understand how two labeled points on an air path could be 3600 nanometers apart. Never occurred to me that someone would use 'nm' to mean nautical miles.

Nanometers would be a very short flight.

I could imagine conflict arising when switching between single and double precision causing inequality like this.

Exactly the first thing that came to my mind when I saw that abbreviation.

Ah, yes, like when people put in extraordinary amounts of effort to avoid sending a millibit (mb) of data over the wire.

And for those like myself wondering how much 3600nm is, it is of course 0.0036mm

We all need to stop using abbreviations, in my opinion.

EDIT: I mean the point of abbreviations is to facilitate communication. However with the world wide web connecting multiple countries, languages and fields of endeavour there are simply too many (for example) three letter acronyms in use. There are too many sources of ambiguity and confusion. Better to embrace long-form writing.

It took me a while, tbh..

man, this ruins everything.

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