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Subject: Re: [boost] GGL Review
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-11-17 16:31:21

Paul A. Bristow wrote:
> Phil Endecott wrote:
>> 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.

Yes, for both fixed and FP.

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

I hope that's not why they seem happy with it, because it overlooks the
more serious consequences i.e. gross errors as illustrated in the
"Classroom Examples" paper.


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