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<div class="moz-cite-prefix">On 8/2/2016 1:57 PM, Andrew Davidson
wrote:<br>
</div>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">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:
<br>
</blockquote>
<br>
WGS84 is a datum fixed; as in dated 1984 and the data does not
change. <br>
<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
1. Unless you are cleared by the US DoD you don't have access to
this level of accuracy.
<br>
</blockquote>
<br>
Or you have access to survey points/marks and their data in the
local area. <br>
<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
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".
<br>
</blockquote>
<br>
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.
<br>
Datums are definitions .. they do not rely on nor based on something
else. <br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
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.
<br>
</blockquote>
<br>
No .. WGS84 if fixed, it does not change. <br>
<a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/World_Geodetic_System">https://en.wikipedia.org/wiki/World_Geodetic_System</a><br>
And that in part is incorrect ... read
<a class="moz-txt-link-freetext" href="http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html">http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html</a>
Documents EGM 96 ... as in 1996. <br>
<u>"</u><u>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."<br>
<br>
</u>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!<br>
<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
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.
<br>
</blockquote>
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.)<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
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.
<br>
</blockquote>
<br>
Depends on what you set your GPS datum too ... mine is set to WGS84.
<br>
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 datums. <br>
<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">
<br>
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
(<a class="moz-txt-link-freetext" href="https://goo.gl/sWWfYm">https://goo.gl/sWWfYm</a>). 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 (<a class="moz-txt-link-freetext" href="https://goo.gl/B7WVfz">https://goo.gl/B7WVfz</a>).
<br>
<br>
</blockquote>
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. <br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">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.
<br>
<br>
</blockquote>
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.. <br>
<br>
It took 6 years for the NSW LPI to get over to GDA94 .. at least
publicly.
<a class="moz-txt-link-freetext" href="http://www.lpi.nsw.gov.au/__data/assets/pdf_file/0005/181526/2013_Kinlyside_APAS2013_SCIMS3.pdf">http://www.lpi.nsw.gov.au/__data/assets/pdf_file/0005/181526/2013_Kinlyside_APAS2013_SCIMS3.pdf</a>
<br>
And changing over to an earth centric datum will be more of a
challenge. <br>
<br>
<blockquote
cite="mid:de5a6a80-3f4f-0f5c-c13c-945140e1342f@gmail.com"
type="cite">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
(<a class="moz-txt-link-freetext" href="http://www.geodesy.noaa.gov/datums/newdatums/index.shtml">http://www.geodesy.noaa.gov/datums/newdatums/index.shtml</a>).
<br>
<br>
<br>
</blockquote>
<p>These are all local datums .. what will be interesting is seeing;<br>
</p>
<p>1) the translation of those dautms to WGS84 datum and visa versa.</p>
<p>2) the implementation of a global earth centric datum .. that may
take a while .. a long while. <br>
</p>
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