Re: [gpsd-dev] ✘CEP(50) 0.285 metersMime-Version: 1.0

[Gary E. Miller]

Yo Fred!

On Fri, 22 Jul 2016 16:34:14 -0700 (PDT)
Fred Wright <address@hidden> wrote:

> > IETF Standard 754-2008 "quad precision" : 128 bits  
> 
> Sure, but nothing states that "long double" == IEEE "quad precision".

Except my next  Wikipedia quote.

> >     "long double is defined as IEEE 754 extended precision (e.g.,
> > Intel 80-bit double extended precision on x86 or x86-64 platforms),
> > or some form of quad precision where available; otherwise, it is
> > double precision."  

From the c11:

    Recommended practice

    The long double type should match an IEC 60559 extended format

So, only recommended, not mandatory.  Change everything I said from 
'standard' to 'recommended by standard'.

But we are getting off in the weeds, the issue is Python, not C.

C at least 'recommends' the IETF 754-2008 standard.

Python is not standardized.  And I can't figure out how to force
it to some minimum precision past double.

> > Yeah, but then gpsprof calculates spherical distances.  Lot's of
> > squaring and square rooting.  So, double is probably good enough,
> > but somewhere I'm losing enough precision to mess up my plots.  
> 
> Spherical distances are tricky, and ISTR that GPSD switched distance
> algorithms at one point.  You almost certainly don't want the simple
> spherical law-of-cosines formula, since that has awful numerical
> sensitivities for short distances.

Yeah, tricky indeed.  Check out gps/misc.py

Check out these two function:

def MeterOffset(c1, c2):
    "Return offset in meters of second arg from first."

def EarthDistance(c1, c2):
    "Distance in meters between two points specified in degrees."

I agree, they are not optimized at all for very small angles.

If this were C I would take a shot at it.  But this is Python.

Can some Python person just improve that code?  I can provide the JSON
input to gpsprof if anyone wants to try, instead of just kvetch.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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