Re: [gpsd-users] Create an SVG skyview image from JSON data

[Gerry Creager - NOAA Affiliate]

Apologies for not responding inline but I'm just going to comment on one of aspect of DOP. The concept was a Garmin construct back in the dark ages of consumer-grade GPS... most of which was aviation-based and non-trivial. They published a little bit about how they created the various DOPs, and if memory serves, all they started with ver VDOP and HDOP, although GDOP came along very shortly. Neither of the original estimators had much basis in reality: They were a sales tool, complete, at the time, with something like "We're the only company to calculate DOP"... 

I've been skeptical of DOP since those days, although there's a lot more agreement on how to calculate them now. Various vendors, as Gary mentions, do add something secret to make 'em look better.

Gerry

On Mon, Oct 8, 2018 at 5:05 PM Gary E. Miller <address@hidden> wrote:

Yo Niccolo!

On Sun, 7 Oct 2018 08:39:25 +0200
Niccolo Rigacci <address@hidden> wrote:

> On Sat, Oct 06, 2018 at 01:31:51PM -0700, Gary E. Miller wrote:
> >
> > Do it correctly, or don't do it.  The precision is in the
> > JSON, and it is exactly what non-experts need and expecct. 
>
> Yea! You have convinced me. Even if this means that I have to
> read two JSON sentences instead of one (SKY and TPV). But this
> has the added bonus that I can display the position!

Cool.

> Please, can you tell me if epx, epy and epv are calculated by
> GPSD or provided by the device?

Yes.

> I mean: are there some devices
> that provide HDOP/VDOP, but don't provide epx, etc.?

Yes.  And many that do not even supply DOP's.  And some that provide
ep's and not DOP's.

Also note there are six different DOP's.  HDOP and VDOP are the less
important ones.

So gpsd takes what it can get from the GPS receiver, then computes the
missing parts.  Note this is often imperfect.  Many things, like DOP's
and the ep's can be computed many ways.  Which is why gpsd defers to the
GPS for anything the GPS gives.

To give you a brief, and oversimplified, tour:  The DOPs are calculated
from the geometry of the sats.  DOP's are dimensionless ratios.  The
ep's are calculated by multiplying the DOP's by the User Range Error.

Plus many GPS companies add some 'secret sauce' to 'improve' their
results.  Different firmware often give different results for the same
imputs.

> > Good, better.  I still have an issue on some hosts:
> >
> > # display tmp.svg
> > Intrinsic has incorrect return type!
> > i8 addrspace(2)* ()* @llvm.amdgcn.dispatch.ptr
> > LLVM ERROR: Broken function found, compilation aborted! 
>
> Damn it! I suppose that your "convert" is from Imagemagick,

Of course, today it works.  Gentoo did have a number of updates over
the weekend, so maybe you fixed it, maybe updates fixed it, maybe
my error.  Dunno.

> can
> you please tell me what delegate it uses for SVG, and may be
> what version:
>
> display  -list delegate | grep -i svg

On a host that rendered your SVG well:

$ display  -list delegate | grep -i svg
        cdr =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        cgm =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        dot =>          "dot' -Tsvg '%i' -o '%o"
        dxf =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        fig =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        svg =>          "rsvg-convert' -o '%o' '%i"

$ display --version
Version: ImageMagick 7.0.8-12 Q16 x86_64 2018-10-04 https://imagemagick.org
Copyright: © 1999-2018 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC Modules OpenMP
Delegates (built-in): bzlib cairo fontconfig freetype jng jp2 jpeg lcms ltdl lzma openexr pangocairo png rsvg tiff webp wmf x xml zlib

On a host that had failed:

$  display  -list delegate | grep -i svg
        cdr =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        cgm =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        dot =>          "dot' -Tsvg '%i' -o '%o"
        dxf =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        fig =>          "uniconvertor' '%i' '%o.svg'; /bin/mv '%o.svg' '%o"
        svg =>          "rsvg-convert' -o '%o' '%i"

$ display --version
Version: ImageMagick 7.0.8-11 Q32 x86_64 2018-10-08 https://www.imagemagick.org
Copyright: © 1999-2018 ImageMagick Studio LLC
License: https://www.imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules
Delegates (built-in): bzlib cairo fftw fontconfig fpx freetype jbig jng jp2 jpeg lcms ltdl lzma openexr pangocairo png raw rsvg tiff webp wmf x xml zlib



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

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

--

Gerry Creager

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405.325.6371

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And so on to viscosity.” 

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