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Re: [gpsd-dev] ✘CEP(50) 0.009 meters

From: Gary E. Miller
Subject: Re: [gpsd-dev] ✘CEP(50) 0.009 meters
Date: Mon, 25 Jul 2016 12:48:07 -0700

Yo Hal!

On Mon, 25 Jul 2016 12:30:07 -0700
Hal Murray <address@hidden> wrote:

> address@hidden said:
> > Looks like I have a bit more work to do, but close.  Changes so far
> > are in git head.  Next I'll add WGS84 ellipsoid correction for
> > latitude.   
> Why?  Is that going to be significant?

It could be.  Skytraq claims, not yet verified, that they can get
cEnti-meter accuracy off a 1km baseline.  At that distance the
slightly less than 1% error would be noticeable.  At least to someone
very picky.

Even if not important, if the last guy that worked on gpsprof had
worked to rediculous overkill in the math then I would not have had to
suffer to fix it.  So I'm paying it forward to the next guy pushing the

> Assuming you start with doubles, after you do the subtract, how many
> bits of precision do you have left?

Right now I am subtracting these two:

- 121.31405411


I figure the latitude is 35 binary digits of precision.  Plus a sign bit.

An IEEE 754 double float is 53 bits of binary precision.  Plus a sign bit.

So that leaves 18 bits binary precision to play with.  About 5.5 decimal
digits of precision.

If I only know the meters inn a degree (lat or lon) to about 1% that
seems gross compared to the rest.

The next step, that I'll not take, would be altitude correction.  I've been
reading PhD papers that do indeed go that far.

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

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