[talk-au] Australia "changing coordinates"
61sundowner at gmail.com
Tue Aug 2 05:19:23 UTC 2016
On 8/2/2016 1:57 PM, Andrew Davidson wrote:
> It's interesting because it highlights one of the foundation myths of
> OSM; which is that it uses the "WGS84" co-ordinate system. This is a
> convenient myth and if you're talking about only mapping to the
> nearest 5m then it is in effect true. However, once you start talking
> about sub-metre accuracy it stops being true for a number of reasons:
WGS84 is a datum fixed; as in dated 1984 and the data does not change.
> 1. Unless you are cleared by the US DoD you don't have access to this
> level of accuracy.
Or you have access to survey points/marks and their data in the local
> 2. If the accuracy of your GPS is supplemented by external data then
> this is inevitably based on one of the realisations of the ITRS; which
> is not "WGS84".
No. The survey points can be in one datum and coordinate system and
those can be translated to any other datum and/or coordinate system.
Datums are definitions .. they do not rely on nor based on something else.
> 3. WGS84 is a semi-dynamic datum. Every 1st of January the USAF
> uploads new co-ordinates for each of their ground control/monitoring
> stations (where they will be at 1st July). Which means that every
> location in WGS84 co-ordinates needs an epoch to indicate when the
> measurement was taken but the database schema for OpenStreetMap
> doesn't have an entry for this.
No .. WGS84 if fixed, it does not change.
And that in part is incorrect ... read
Documents EGM 96 ... as in 1996.
_"__Third Edition, 4 July 1997 has been amended to correct errata found
in the original printing of this edition. There are no changes to the
definition of WGS 84 and this does not constitute a "new" WGS 84."
_If WGS84 were to be 'updated' then I would expect a different number
e.g. WGS94 for a 1994 model. Otherwise it is not possible to
distinguish between the two datums!
> So what happens in OSM is that each country gets mapped to their local
> approximation of WGS84. In North America this is NAD83, Europe has the
> ETRS89 and the various national variations. The UK must be the most
> confusing as the OS seems to be maintaining two system OSGB36 and OS
> Net but a least they are far enough apart to be obvious.
WGS84 is not approximated to any local datum. Rather a local datum is
modelled for the best fit of the local surface. A global model must make
more compromises/approximations so will always be worse than a local
model (assuming the same data, effort and level of complexity).
Translation of data from one datum to another has approximations, but
the datums themselves have no approximation, they may be derived
(contrived) from approximations but they define the thing. If 'the' inch
is defined as the length of the Quenn Of Britians thumb .. then that is
what it is .. no approximation. (I would buy before the thumb nail is
trimmed, and sell after it is trimmed.)
> In Australia it means that we've been happily mapping to GDA94 and
> ignoring the fact that we're racing north at something like 70-90 mm a
> year. People mapping with consumer grade GPS are only within 5m anyway
> so don't care. People with access to survey grade GPS understand the
> difference and convert back to GDA94.
Depends on what you set your GPS datum too ... mine is set to WGS84.
The GPS knows what datum the map is, and adjusts for the required datum.
With more or less accuracy depending on the model and the complexity
implemented inside the GPS. As my GPS is obsolete by at least 2
generations I don't see it having any capacity for the new earth centric
> What difference will it make to OSM? It depends. If you're mapping
> some where that has good quality aerial imagery and people have
> carefully traced things then it's going to be obvious
> (https://goo.gl/sWWfYm). On the other hand most of OSM in Australia
> can be described as roughly sketched and if all you have is Bing
> imagery and some crudely traced streets then I doubt you'll be able to
> tell (https://goo.gl/B7WVfz).
As the imagery is set for WGS84 it should not be a problem'. Problems
may arise if (when) open sourced data is in some 'other' datum and
imported without adjustment to whatever datum OSM uses.
> What to do about it? Unfortunately the changeover to GDA2020 is not
> going to happen overnight, the plan is for a three year transition
> starting in 2017. If it did happen overnight then we could just shift
> all of the nodes in Australia to their new positions can keep on
> mapping. What's actually going to happen is that the various different
> organizations are going to switch at different timr84 o WGes and the
> situation for aerial imagery mosaics will be interesting if they
> aren't retrospectively re-projected (you'll have some places on
> GDA2020 and others still on GD94). So I'm guessing the approach of
> wait and see might be the one to take. Obviously if organizations
> start releasing data in GDA2020 and people start importing it into OSM
> we're going to have to make the call: move everything to GDA2020 or
> convert imports back to GDA94? It's only going to be a real issue once
> we get access to aerial imagery that's in GDA2020 or people get
> general access to GPS with decimetre accuracy.
As OSM is global .. OSM will need a global datum ... unless the OSM
inuse datum changes for different areas .. and that could be a problem
at the junctions..
It took 6 years for the NSW LPI to get over to GDA94 .. at least
And changing over to an earth centric datum will be more of a challenge.
> Whatever we do it'll be a good test to see what should happen in OSM
> when NAD83 gets replaced by 2022 and everything in the USA moves
> 1~1.5m (http://www.geodesy.noaa.gov/datums/newdatums/index.shtml).
These are all local datums .. what will be interesting is seeing;
1) the translation of those dautms to WGS84 datum and visa versa.
2) the implementation of a global earth centric datum .. that may take a
while .. a long while.
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