[ysofunny]

the mathematics of cartographic projections possibly

but this information was really a military secret not that long ago. same as cryptography is right now!

which annoys me personally becuase I wish to learn more/better about the mathematics behind information technology and cryptography. but this is considered 'sensitive' understanding so the quantity of people that are ever taught this stuff is tightly controlled; not to say that it's difficult stuff to begin with and any efforts to make it more widespread and accessible are often derailed in education comitees and so on. but I should stop before I get any more paranoid

[AlotOfReading]

The details of specific implementations are sometimes considered secret, but broadly speaking the mathematics of both projections and cryptography are extremely public. For example, GCJ-02 (the projection being obliquely referenced by the GP) is just WGS-84 + "noise". For cryptography, you can read any of the excellent textbooks and educational resources available (e.g. [0]).

[0] http://www.cs.umd.edu/%7Ejkatz/imc.html

[ysofunny]

i'm not supposed to say this, but my above paranoia is but a coping mechanism to deal with the harsh truth that I just cannot for the life of me completely understand any of those ideas no matter how well they're explained

[mxkopy]

It would make for a good sci fi novel, but yeah. Cryptography is cutthroat in the sense that if you’re even a little imprecise people can and will break your scheme. At some level of development the material will just filter out people from learning it.

[8note]

This information is part of why the US missiles can hit their targets precisely and why Soviet ones could not. you lose accuracy dead reckoning when you don't have good corrections for the earth's gravity at your current location

[pyinstallwoes]

how much does earth's gravity fluctuate by location?

[naavis]

Enough for consumer-grade Hario coffee scales to include a geographic location setting, so that you can weigh your coffee beans accurately.

[thegrim000]

To put numbers to it, according to google there's a gravity strength max discrepancy of 0.7% between the places with the lowest and highest gravity on the planet. Assuming using ~17g of beans for a coffee, that comes out to an error of up to 0.12g. Seems kind of dumb to correct for 0.12 grams, in the absolute most extreme case. If they just manufacturer calibrated the machine for "average" gravity then there'd be a max of +-0.35% difference in gravity, for a maximum error of +-0.06g for a 17g brew.

[joe-collins]

Off by 0.35% in your morning coffee, no problem.

Off from your intended vector by 0.35% when you're moving at 6500 m/s, and you're veering off course by 22 meters every second.

[pyinstallwoes]

How is that different than air pressure?

[mschuster91]

Quite a lot, at least at the weight and speed scale a missile operates on. The biggest source of gravity force difference is the distance between the equator / pole [1], and altitude (both in terms of altitude in reference to the planet core as well as altitude in reference to ground, e.g. due to the mass of a mountain range) also adds jitter [2].

On top of that, dead-reckoning is disturbed by aerodynamic effects such as wind, as well as drift from the gyro compass itself... a fine-detailed gravity effect map can be used to compensate for both of that, and adding imagery maps and radar/laser distance measuring is even better.

To add even more context... that is the reason why Germany's Chancellor Scholz is refusing to deliver TAURUS long range missiles to Ukraine. TAURUS owes its significant increase in precision over Storm Shadow / SCALP (UK/FR equivalent) to that kind of sensor aids, but the assistance dataset aka the photogrammetry, gravity and height maps of the flight route must be individually created and loaded into the missile as part of the mission preparation. As you can imagine this kind of precision information is stuff that we really don't want to get leaked to Russia so we can't (or don't want to) deliver the systems for programming to Ukraine where it might get stolen/diverted to Russia, so Scholz is afraid that the necessary direct cooperation of German Bundeswehr soldiers with Ukrainian soldiers might be seen as an act of war of Germany by Putin. (Complete horse dung if you ask me, but I can understand where he's coming from)

[1] https://www.wired.com/story/do-you-weigh-more-at-the-equator...

[2] https://physics.stackexchange.com/a/652759

[pyinstallwoes]

Does it vary at the location or is it constant? Basically, is it more like a fluid (air) or fixed because of mass?