Re: DTM sentence and Russian datums in US phones (yes, that's clickbait!)

[Gary E. Miller]

Yo Greg!

On Thu, 17 Jun 2021 18:35:09 -0400
Greg Troxel <gdt@lexort.com> wrote:

> "Gary E. Miller" <gem@rellim.com> writes:
> 
> >>   inexplicable buggy behavior in choice of or reprorting of datum,
> >> which has not been shown to cause a perceptible error  
> >
> > Buggy I buy, but very explixable.  
> 
> What is inexplicable is:
> 
>   Why does a cellphone with a US-designed chipset, sold in the US for
> US use, by default provide positions labeled as being in a Russian
> datum?

Yeah, that is weird.

> >> They never do but I don't think that has anything to do with
> >> assuming frame equivalnce when merging multiple constellations.  
> >
> > No assumptions needed.  All constellations use Earth Centered
> > Inertial coordinates derived from Kepler parameters.  Those are all
> > converted to ECEF so they can all be used in computing one
> > solution.  Only after an ECEF solution is computed is that
> > transformed to a datum.  
> 
> ECEF just means XYZ.

A bit more than that.  Earth Centered Earth Fixed.  Of course the
location of the "center" is still up for debate.

> One still needs a datum label for origin,
> orientation and scale.

One needs a datum to get to lat/lon/alt from EXEC XYZ i nmeter.

> >> >> What's funny is that PZ90.11 is so close to ITRF2008 that unless
> >> >> you are playing at the 10 cm level or better it can be treated
> >> >> ~= ITF2008 != WGS84(G1762).    
> >> >
> >> > And what about the other WGS84 that are > 10 cm from
> >> > WGS84(G1762).    
> >> 
> >> It's only the older ones that are that far off (TRANSIT and G730),
> >>  
> 
> > I suggest you look at the USGS error maps for the PNW.  > 10 cm.  
> 
> I have not seem the USGS deal in WGS84 at all.

Nor have I.

> What I meant is that transforms between any two WGS84(x) and
> WGS84(y), x
>  > G730, y > G730, are very small and not detectable with a  
> non-differential solution.

We keep going around on that one.  I continue to assert that in 
some location, like Mauna Kea, the differences are up to meters.
Plus the change over short distances makes the traditional 5'x5'
approximation grid insufficient to approximate.

The link I sent last time shows that.

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

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

pgpSDnwyRsNjU.pgp
Description: OpenPGP digital signature