Re: [boost] GGL Review

Subject: Re: [boost] GGL Review
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2009-11-17 14:27:22

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]] On
Behalf Of
> Phil Endecott
> Sent: Tuesday, November 17, 2009 5:39 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] GGL Review
> 
> Paul A. Bristow wrote:
> > Phil Endecott wrote:
> >> Personally, I find it better to use fixed point (e.g. for
> >> latitude/longitude) and relax knowing that I don't have to worry.
> >
> > But isn't this assuming that your latitude/longitude (or whatever) is exact?
> 
> Both fixed and floating-point formats represent latitude and longitude
> values snapped to a grid.  In the case of fixed point it's a grid of
> squares (on a flat earth, anyway); in the case of floating point it's a
> grid of rectangles whose sizes change depending on how far you are from
> the equator and prime meridian.  Floating point is only "less exact" in
> the sense that we often don't worry about what the grid size is, but it
> is still there.

But the *actual position* is never known exactly, it always has some
uncertainty, say + or - 1 grid unit.  So, for example, if it appears to be in
the wrong (next door or so) grid, you could decide that it doesn't matter to
you.

Is this why users seem to be happy with a potentially uncertain FP
implementation?

Is an interval based implementation another way to go - except that it will
probably be too slow to be useful?

(But I note that one author Sylvian Pion
http://www.mpi-inf.mpg.de/~kettner/pub/nonrobust_cgta_06.pdf an interval
enthusiast and Boost Interval author doesn't mention it, so perhaps not?).

Paul

---
Paul A. Bristow
Prizet Farmhouse
Kendal, UK   LA8 8AB
[phone], mobile [phone]
pbristow_at_[hidden]