Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The lat/lon floating point delusion

datafix.com.au

Post reply on HN
Print view View original

51–60 of 154 posts

Re: The lat/lon floating point delusion

#51

Post by urschrei »

ISL wrote: post #47
urschrei wrote: post #44

Earlier quoted context omitted.

Survey-grade GPS (using CORS and WAAS correction) using RTK will give you accuracy to ~10 cm within a few seconds. You can get that down to several millimetres if you can tolerate post-processing several hours worth of static data.

Is accuracy at that level actually possible with GPS? Precision seems likely, but it is substantially more difficult to provide absolute accuracy at that level.

I'm by no means an expert, but yes, if you can use a modern realisation (e.g. ITRF2014) and have access to VLBI / DORIS / GNSS then sub-centimetre accuracy is possible, though not in real time.

Re: The lat/lon floating point delusion

#53

Post by scblock »

ISL wrote: post #47
urschrei wrote: post #44

Earlier quoted context omitted.

Survey-grade GPS (using CORS and WAAS correction) using RTK will give you accuracy to ~10 cm within a few seconds. You can get that down to several millimetres if you can tolerate post-processing several hours worth of static data.

Is accuracy at that level actually possible with GPS? Precision seems likely, but it is substantially more difficult to provide absolute accuracy at that level.

Yes, but not on its own, that I am aware. High accuracy GPS is is based on a combination of GPS satellite data itself with real time or post-processed positional correction using references stations which themselves are placed in a known surveyed location.

In the US, 60 cm accuracy is often possible with nothing more than WAAS corrections. 10 or even 1 cm accuracy is often possible in real time with access to an RTK network.

Re: The lat/lon floating point delusion

#54

Post by falcolas »

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 down the road.

Re: The lat/lon floating point delusion

#55

Post by Arnt »

ISL wrote: post #45
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.

Significant-digits seem like schoolroom drudgery until, one day, there is enlightenment.

That day may be today, if you enlighten us...

Re: The lat/lon floating point delusion

#58

Post by dfranke »

People have this idea that when you take a measurement, you have so-and-so number of significant figures that are probably correct and the rest are just pure noise. But that's not how measurement error works. In the real world, physical measurement errors are more-or-less normally distributed (we don't have to argue about the "more-or-less" part because my argument here holds for any distribution other than a uniform one). Let's say your measurement gives you a latitude of 45.73490534578° with a standard deviation of 0.00001° (that's about 11 meters). Those last few digits of your measurement are almost certain to be wrong. But does that make them meaningless? No! Because if your measurements are unbiased, then slightly more than half the time, 45.73490534578° is still going to be closer to the true value than 45.7349053457° is. By performing significant figure rounding, you aren't throwing very much information, you may not be throwing away any information you care about, but you are nonetheless throwing away information.

Re: The lat/lon floating point delusion

#59

Post by cafard »

sokoloff wrote: post #24
cafard wrote: post #3

Not just lat/lon. I find myself amused when The Washington Post's education columnist Jay Matthews--quite a sharp guy from all I can tell--runs his high school "challenge index" (AP tests taken / size of graduating class) out to six decimal points for small schools. Somebody give that man a slide rule.

When I see those errors, I often try to work out the exact input figures that led to the stats reported. At work, I recently saw a survey result from a small customer base reported as "31.3% of respondents" rather than the more communicative "5 out of 16..."

Quite. That way I was able to figure out how many students graduated from Washington International School that year.

For that matter, my wife taught a class one year at a local university. The class had 29 students, or at least 29 who bothered to fill out the evaluation, and the evaluation had a couple of places right of the decimal point. I found myself going reckoning, "hmm, 27 of 29 though that ...".

Re: The lat/lon floating point delusion

#60

Post by izacus »

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…

These values are as right as setting least significant digits to 0 would be. The sensor has limited precision and the other digits might as well be 0 or 584985948 and the value would be no more precise.
Post reply on HN