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

[Greg Troxel]

"Jóhannes Birgir Jensson" <joi at betra.is> writes:

> Well the current issue in Iceland is a error of 50 cm between 1993 and
> 2016 due to crust movements. So it's less than 2 meters but more than
> one cm.

That's interesting and a useful data point for later discussion about
the points that my message said this discussino isn't about!

> What is your accuracy limit if 2m is just unacceptable but a
> centimeter is?

There are several issues that must be addressed over time for improved
accuracy.  I am not claiming that any particular accuracy is required
and I am very definitely NOT suggesting that OSM adopt accuracy
requirements for data.

I am not following where "1 cm" came from in this discussion and don't
think it is relevant.  (I do think that asking the question of how to
get to 1 cm eventually is interesting, but I see that as a separate
issue.)


My point is that our current definition of "WGS84" has ~2m of fuzz
*intrinsic to the definition* because (as discussed at length on the
proj list) saying "WGS84" means "these coordinates are in one of six
datums and I am not telling you which".  There is no need for OSM to
have this definitional uncertainty, and I think that is both the biggest
issue and the easiest to address.

Regardless of the definition, it is clear that there will be data of
varying accuracy in OSM.  I am not objecting to that reality, or asking
that OSM adopt accuracy standards.  There seems to be a shared norm that
more accurate locations of  nodes are preferred to less accurate
locations.

I think there's also a shared belief that wildly inaccurate nodes are
not helpful; if I added a POI that was 10 km off (in an area where there
are many things between the mapped location and the real location), then
that is probably a bad thing to have done.  But if I add a POI that is
100m off, someone might fix it, but I don't expect they would tell me
that I should not have added it.  Certainly this is true at 10m error
(the lack of outrage; it might or might not get improved).


So about the narrow issue of the definition of OSM's coordinate
system, do you prefer

  leaving it defined as "WGS84", so that coordinates have to be treated
  as having an intrinsic uncertainty of several meters

  changing it to "WGS84, and in particular the revision currently in use
  by GPS", allowing one to treat the datum as relatively precise and
  thus only question the accuracy of the coordinates themselves?

  something else?


Note that the second option allows one to transform e.g. precise
coordinates from a geodetic control point to add to OSM.  (Yes, I
realize one could transform to a particular realization anyway.)

In all seriousness, I cannot tell if you object to this first step, or
are making comments about some later step that might happen, or at
talking about the issues of OSM perhaps adopting accuracy requirements
(not on the table!0, or something else.