The lat/lon floating point delusion

Post by iooi » Fri, Aug 09, 2019, 6:27 PM UTC

tony_cannistra wrote: post #119

wpietri wrote: post #79

In contrast to his 4-digits-are-fine notions, let me offer a counterexample: http://scissor.com/transient/destination_heatmap/ I am working on a Twitter bot called @sfships, which monitors the comings and goings of large ships in the San Francisco Bay: https://twitter.com/sfships As part of that, I generated the above map from AIS data. [1] It's basically where ships stop. If you zoom in on the San Francisco waterfro…

I used to have a job where I analyzed global ship tracking data (AIS and other less-open (i.e. more classified) data streams) for law enforcement and environmental conservation purposes, and learned one thing: man is AIS data a bitch. This is especially true when you're dealing with potential bad actors who can spoof their transmissions, combined with incredibly poor vessel metadata databases and strange satellite AI…

Post by 7952 » Fri, Aug 09, 2019, 6:36 PM UTC

falcolas wrote: post #92

7952 wrote: post #82

Earlier quoted context omitted.

No, I meant that it is an approximation or the thing in the real world that it represents. For example, consider a restraunt. Should the point be at the entrance to the building, in the middle of the building, or maybe at the door round the back where mail is delivered, or at the centroid of the parcel. In order to actually use this excessive precision you would need to know that. In the examle cases the data is not…

Then the coordinates presented should match that approximation. Anyone who has worked with plans knows that the number of significant digits presented on the plans has a meaning; it represents the margin of error. Online maps aren't respecting this, they're giving decimeter precision values on maps that are lucky if they're accurate to within meters, if not tens of meters. That's the problem.

Post by wpietri » Fri, Aug 09, 2019, 6:56 PM UTC

tony_cannistra wrote: post #119

wpietri wrote: post #79

In contrast to his 4-digits-are-fine notions, let me offer a counterexample: http://scissor.com/transient/destination_heatmap/ I am working on a Twitter bot called @sfships, which monitors the comings and goings of large ships in the San Francisco Bay: https://twitter.com/sfships As part of that, I generated the above map from AIS data. [1] It's basically where ships stop. If you zoom in on the San Francisco waterfro…

I used to have a job where I analyzed global ship tracking data (AIS and other less-open (i.e. more classified) data streams) for law enforcement and environmental conservation purposes, and learned one thing: man is AIS data a bitch. This is especially true when you're dealing with potential bad actors who can spoof their transmissions, combined with incredibly poor vessel metadata databases and strange satellite AI…

Post by itroot » Fri, Aug 09, 2019, 7:03 PM UTC

Post by seandougall » Fri, Aug 09, 2019, 7:14 PM UTC

ramshorns wrote: post #113

rumanator wrote: post #111

Earlier quoted context omitted.

> Because the value, as presented, is wrong. That's not true nor is it the problem at all. The thesis was that the values, albeit correct, have too many significant digits, which in turn reflect differences which lie somewhere between having no practical use or being absurd. > Imagine, for a moment, that someone uses these values to put up a fence. That example is very poor. Any engineer can tell you that a measureme…

> a measurement is meaningless without tolerances/margin of error Reporting a value with too many significant digits is the same as reporting an incorrect margin of error, which (arguably) makes the value wrong.