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

Greg Troxel gdt at lexort.com
Thu Dec 19 14:33:07 UTC 2019


(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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