Re: [boost] GGL Review

Subject: Re: [boost] GGL Review
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-11-17 12:39:08

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 others (like you and Luke) assert your input is exact and demand an exact
> result?

Really I'd just like to know what the limitations of GGL's FP 
algorithms are, and I'd like to know that the possibility of gross 
errors, invalid output, etc., have been understood and documented.  The 
"Classroom Examples" paper shows one way in which one can construct 
test cases that are likely to trigger any problems.  I don't think that 
testing with random inputs or real-world data is very likely to find anything.

> Paul A. Bristow
> Prizet Farmhouse
> Kendal, UK   LA8 8AB

I'm going to be in Kendal at the weekend for the Mountain Film 
Festival.  Fancy a beer?  We could have a mini-boostcon-a-deux :-)

Phil.