[OSM-talk] What does WGS84 mean for openstreetmap these days?

[Jóhannes Birgir Jensson]

Hello Greg

I don't think we can or will be providing accuracy up to cm when most of the stuff we map from our chairs is off by a meter or two anyways - the beauty is that it doesn't matter for 99,99% of users. If a centimeter matters then we are probably dealing with legal matters and there OSM makes it quite clear it is not suitable for such.

Also regarding the accuracy, as another fast moving country Iceland is actually splitting in the middle and so it edges west and east and south as well, depending on where you are in the country. We've had 3 official national datums now, ISN93, ISN2004 and ISN2016 (helpfully naming them after years). The fact is that pretty much everything is still running in ISN93, ISN2004 saw very little uptake and ISN2016 has started very slowly.

So for Iceland we do know that we are never going to achieve a centimeter accuracy, pretty much ever, and don't expect a free people sourced geographical database to reach it.

--Jóhannes / Stalfur


19. desember 2019 kl. 14:37, skrifaði "Greg Troxel" <gdt at lexort.com>:

> (This is a long and complicated subject and I am intentionally asking
> only part of the question.)
> 
> It's been said from the beginning that coordinates in the openstreetmap
> datbase are in "WGS84". That more or less meant "what a GPS receiver
> showed", back in the days when GPS was the GNSS system of choice and
> accuracies were low compared to talking about versions of WGS84.
> 
> In discussion on the proj list, it seems the consensus view is that
> WGS84 is now a term that refers to any one of the 6 realizations of
> WGS84 over time. This makes sense when you have data that is merely
> labeled WGS84, without a more specific label such as WGS84(G1762). This
> means that WGS84 is considered low accuracy (because the original was),
> and thus any transforms involving it are assigned high error values.
> 
> This page has a good overview of the various WGS84 realizations and
> their relationship to ITRF realizations:
> 
> https://www.e-education.psu.edu/geog862/node/1804
> 
> As normal people (or at least normal nerds) get access to more accurate
> positions, this question begins to matter, as in North America positions
> in original WGS84 and modern WGS84 differ by more than a meter.
> 
> I should note that now that WGS84 has converged to ITRF, and new ITRF
> realizations seem to be at most cm-level changes from previous ones, I
> do not expect future WGS84 revisions to be signficantly different from
> either the current one.
> 
> So, I wonder if we want to change the definition for OSM coordinates
> from "WGS84" to "the realization of WGS84 currently in use by GPS".
> That doesn't change older coordindates (and I am not suggesting any
> automated changes!!!). But it does give a notion of what coordinates
> should be, both in using them and in producing new ones for editing. I
> expect that this will have zero practical effect for most people, but
> will allow higher accuracy for those who are into extreme accuracy.
> 
> postscript:
> 
> I am intentionally leaving out of this discussion two more issues (which
> could result in further changes, with much more complexity). I list
> them so that those with some background in geodesy can begin to ponder,
> and to explain that my stopping at the proposal above was intentional.
> 
> 1) WGS84 is a US datum. BEIDOU, GALILEO, GLONASS use different datums.
> SBAS systems also use different datums -- WAAS seems to give
> coordinates in "ITRF2000 (current epoch)". It seems most are
> equivalent to some modern ITRF, with possibly differing epochs.
> 
> (I will assume for point 2 that there OSM redefines coordinates to be a
> particular ITRF at a particular epoch, probably matching the current
> WGS84.)
> 
> 2) ITRF is global, but objects we map are generally crust-fixed on some
> plate. The US has a (mostly, if you're not in CA) crust-fixed datum,
> NAD83, and other countries do too. This is particularly acute in
> Australia which is a notably fast-moving country :-) The modern trend is
> for stations to have velocities and not just coordinates. Over 20
> years, this starts to matter. Several countries are introducing new
> national datums that are intended to address some of these issues. I
> don't think it makes sense for OSM to deal with this issue for a few
> years.
> 
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