The lat/lon floating point delusion • Hacker News Board

Re: The lat/lon floating point delusion

#151

Post by emergie » Sun, Aug 11, 2019, 10:35 AM UTC

parliament32 wrote: post #107

lat/lon kinda sucks because of the different notations (degrees with decimals? degrees and minutes with decimals? degrees and minutes and seconds with decimals?) and because of the way 0.1 degrees is a different amount of distance based on where you are on the earth. UTM is better in every way, and already used on most modern hiking and topo maps. https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_...

Looks like regular grid, but they did special irregular 32v cell to cover land part of Norvegia. For me this places the UTM in "not reasonable" land.

Re: The lat/lon floating point delusion

#152

Post by jiveturkey » Mon, Aug 12, 2019, 5:20 PM UTC

falcolas wrote: post #54

jjoonathan wrote: post #20

Yes, why don't people spend their time doing the extra work to figure out the exact number of meaningful digits in their measurements when the defaults work just fine for their non-scientific, non-metrological purposes? A mystery for the ages.

Because the value, as presented, is wrong. "[but it's being used for] non-scientific, non-metrological purposes" which doesn't change that it's the wrong value. Imagine, for a moment, that someone uses these values to put up a fence. Is the fence on their property? Those crap values beyond the actual significant digits of the measurement may tell you that you are, when you're not. That will cost you time and money do…

> Is the fence on their property?

Even if the values were precise, the fence can never be precisely located. Land/parcel demarcations don't work that way. Also, GPS coordinates change over time. Or rather, the land underneath moves.

Fence or other land locations are always relative (to an official surveyor's point), not absolute.

Rewinding a little:

> Imagine, for a moment, that someone uses these values to put up a fence.

There's your first problem.

Re: The lat/lon floating point delusion

#153

Post by tony_cannistra » Mon, Aug 12, 2019, 5:47 PM UTC

jandrewrogers wrote: post #126

iooi wrote: post #121

Earlier quoted context omitted.

Why do bad actors spoof transmissions? Smuggling?

Many reasons, and a significant percentage of all AIS transmissions are dodgy. Smuggling is just one reason. Other common reasons include illegal fishing in protected waters, covert meetings (e.g. a couple yachts coming together in the middle of the ocean), spy ships by state actors trying to look like ordinary commercial traffic, and misrepresenting state of shipping assets for commercial leverage in contract negoti…

Yes, these are all things we observed in our data.

Re: The lat/lon floating point delusion

#154

Post by tony_cannistra » Mon, Aug 12, 2019, 5:51 PM UTC

wpietri wrote: post #142

jandrewrogers wrote: post #126

Earlier quoted context omitted.

Many reasons, and a significant percentage of all AIS transmissions are dodgy. Smuggling is just one reason. Other common reasons include illegal fishing in protected waters, covert meetings (e.g. a couple yachts coming together in the middle of the ocean), spy ships by state actors trying to look like ordinary commercial traffic, and misrepresenting state of shipping assets for commercial leverage in contract negoti…

I would love to read more about this. Any suggestions?

Much of this kind of thing is written up in pretty terribly-written documents that come from government/law enforcement. Here's [0] a report I helped to put together which circles around some of these issues.

[0]: https://ocean.csis.org/spotlights/illuminating-the-south-chi...