<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<p>Thanks for feedback so far. I'm aggregating answers into one
email, and copying your comments into the master working doc here:
<a
href="https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#">https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#</a></p>
<div class="moz-cite-prefix">On 15/7/19 9:58 pm, Greg Troxel wrote:<br>
</div>
<blockquote type="cite" cite="mid:rmi1ryrh5lu.fsf@s1.lexort.com">
<pre class="moz-quote-pre" wrap=""> I'm a little nervous that I've overwhelmed people by the number of pages
I've used to describe the web-mapping misaligned maps problem.
It might also help to have your notes in plain text and actually send
them to the list, after editing for minimum length <span class="moz-smiley-s1" title=":-)"></span></pre>
</blockquote>
<p>Greg, you are asking good questions, but it would be bad manners
for me dump my long, half baked paper onto this list. Instead I'll
copy the introduction, and hopefully you might be tempted to read
the details in doc reference above:</p>
<p><b style="font-weight:normal;"
id="docs-internal-guid-baa96872-7fff-ffe3-36b1-f010efff2c94">
<h2 dir="ltr"
style="line-height:1.38;margin-top:18pt;margin-bottom:6pt;"><span style="font-size:16pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">The challenge</span></h2>
<p dir="ltr"
style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Increasing demands for location accuracy has exposed design flaws in current “good-enough” approaches to web-mapping. Prior implementations haven’t accounted for tectonic plate shifts, resulting in systematic metre-level inaccuracies which are increasing over time.</span></p>
<p dir="ltr"
style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">The problem we’ve inherited is that a </span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">dynamic</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;"> datum is being used as if it were </span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">static</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">. (Think of a datum as a mathematical model and associated coordinate system for the earth.)</span></p>
<ul style="margin-top:0pt;margin-bottom:0pt;">
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Web-mapping applications use pre-rendered map tiles to improve performance and scalability. This freezes map tiles at a point in time.</span></p></li>
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">To facilitate interoperability, implementers have selected a defacto-standard map projection of </span><a href="https://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3857&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3857" style="text-decoration:none;"><span style="font-size:11pt;font-family:Arial;color:#1155cc;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;-webkit-text-decoration-skip:none;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">WGS84 Web Mercator</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;"> which is </span></a><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">based on the WGS84 datum.</span></p></li>
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">However the WGS84 datum is </span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">dynamic </span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">(time dependant)</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">,</span><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;"> meaning that a feature’s coordinates change over time as the tectonic plates drift across the earth.</span></p></li>
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">This results in misaligned map layers.</span></p></li>
</ul>
<br>
<p dir="ltr"
style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">To address these challenges we suggest:</span></p>
<ul style="margin-top:0pt;margin-bottom:0pt;">
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Explaining the web-mapping challenges to facilitate understanding.</span></p></li>
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Adopting a static datum for web mapping, probably using the WGS84 realisations currently in use.</span></p></li>
<li dir="ltr" style="list-style-type:disc;font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;"><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Extending spatial standards, data types, software, and workflows to address time dependence.</span></p></li>
</ul>
<p dir="ltr"
style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">This will be achieved most effectively through international collaboration.</span></p>
</b><br class="Apple-interchange-newline">
</p>
<div class="moz-cite-prefix">On 15/7/19 9:17 pm, Martin Desruisseaux
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:63f47524-ccab-489f-f949-5165fadf4dce@geomatys.com">
<pre class="moz-quote-pre" wrap="">Hello Cameron
Le 15/07/2019 à 01:59, Cameron Shorter a écrit :
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">* We are currently using the dynamic WGS84 datum as if it were a
static datum (locked to a different epoch in different regions).
* This appears to be suitable for current web mapping requirements.
* Realigning static and dynamic datums (as started by Australia, with
others to follow) is tripping a map misalignment problem.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
I'm not sure which "dynamic WGS84 datum" we are talking about. If we are
talking about EPSG::4326, in my understanding this is now an "ensemble
datum", i.e. a group of many different WGS84 datum (there is at least 6
different WGS84 datum today) to be considered as equivalent for
applications that do not need an accuracy better than a few meters. An
"ensemble datum" is not the same as a "static datum" in the sense that
it does not mean "a datum that does not move", but rather "we don't
really know which datum it is, but we don't need to care if the desired
accuracy is only a few meters".</pre>
</blockquote>
<p>Yes Martin, we discuss here: <a
href="https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#">https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#</a></p>
<b style="font-weight:normal;"
id="docs-internal-guid-64d98a70-7fff-012e-86b2-742f6fd7d106">
<p dir="ltr"
style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">Problem 1.1 Non-unique datum: </span></p>
</b>
<p><b style="font-weight:normal;"
id="docs-internal-guid-64d98a70-7fff-012e-86b2-742f6fd7d106"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">The WGS84 datum (EPSG::6326) used in web-mapping is not unique; it has been </span><a
href="https://confluence.qps.nl/qinsy/en/world-geodetic-system-1984-wgs84-29855173.html"
style="text-decoration:none;"><span style="font-size:11pt;font-family:Arial;color:#1155cc;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;-webkit-text-decoration-skip:none;text-decoration-skip-ink:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">updated six times</span></a><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;"> to date. Each update (called a realisation) refines the datum’s ellipsoid parameters to account for our improved measurements of the earth’s shape (which is different to tectonic plate movement).</span></b></p>
<div class="moz-cite-prefix">On 16/7/19 9:49 am, HIGGINS Matt wrote:<br>
</div>
<blockquote type="cite"
cite="mid:A8579F79E9007744A0741017D49253312B293B08@CHQMAIL01.lands.resnet.qg">
<pre class="moz-quote-pre" wrap="">Re: "Each update (called a realisation) refines the datum’s ellipsoid parameters to account for our improved measurements of the earth’s shape".
That is incorrect, the ellipsoidal parameters for WGS84 have never changed, i.e. the size and shape of the ellipsoid are the same it is how well the position and orientation of the ellipsoid fit the centre of the earth and rotational axis. That is why I chose my original words very carefully.</pre>
</blockquote>
Thanks Matt, I'll need to dig up your previous suggested wording.<br>
<p><br>
<b style="font-weight:normal;"
id="docs-internal-guid-64d98a70-7fff-012e-86b2-742f6fd7d106"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">On 15/7/19 9:17 pm, Martin Desruisseaux wrote:</span></b></p>
<blockquote type="cite"
cite="mid:63f47524-ccab-489f-f949-5165fadf4dce@geomatys.com">
<pre class="moz-quote-pre" wrap="">
I'm also not sure what me mean by "suitable for current web mapping
requirements". Do we we mean "an accuracy of a few meters is
sufficient"? If yes, then this requirement is addressed by above-cited
"ensemble datum", which is a new concept introduced in ISO 19111:2019.</pre>
</blockquote>
<p>To date, Australian web-mapping has had all our layers off center
by ~ 1.8 meters. This has been considered okay because layers were
topologically aligned. We are now about to see map <b>mis-alignment</b>
of 1.8 metres in web mapping. We believe this is going to be
unacceptable.</p>
<p>Described in more detail here: <a
href="https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#heading=h.bphraumy10yi">https://docs.google.com/document/d/15uBX2qICODRkiHXeksT0zEy6TuysanIxuc4anV0o5F0/edit#heading=h.bphraumy10yi</a></p>
<p><b style="font-weight:normal;"
id="docs-internal-guid-b40c9d13-7fff-95e5-a287-43450f593655"><span style="font-size:13.999999999999998pt;font-family:Arial;color:#434343;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;">4. Australia’s misaligned web-maps … is an international problem</span></b></p>
<p><b style="font-weight:normal;"
id="docs-internal-guid-b5a133d4-7fff-d096-8c8b-30bd124609fe"><span style="font-size:11pt;font-family:Arial;color:#000000;background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre;white-space:pre-wrap;"><img src="https://lh6.googleusercontent.com/pf6tok-pRzI9ipxalM6r_Vf4xTOfAGYZaJ3iEaGiTXh26w2YHNQhODek7vNLPUWAmcTj2RMedZr2f7iZ0W2nHuJqc1gjJD2LvVmzW8qjLGJWzljmY9qqdfjTucA2-ZewbvTlCobi" style="border: none; " width="369" height="249"></span></b></p>
<p>On 15/7/19 9:17 pm, Martin Desruisseaux wrote:</p>
<blockquote type="cite"
cite="mid:63f47524-ccab-489f-f949-5165fadf4dce@geomatys.com">Question
to techies:
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">* Is it technically possible to create a reference frame realisation
(datum) which is fixed to different epochs in different regions?
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">One implication of this approach would be that coordinate operations
would need to identify in which zone (e.g. using a R-Tree) are located
the coordinate values to transform, which is potentially a costly
operation. Another issue is that it would introduce discontinuities at
the frontier between different zones. It is possible that those
inconvenient would bring bigger problems than the problem we are trying
to solve.
</pre>
</blockquote>
<p>Thanks for your insights Martin, understanding the implications
of this question is likely to be very important for our decisions
moving forward.<br>
</p>
<br>
<pre class="moz-signature" cols="72">--
Cameron Shorter
Technology Demystifier
Open Technologies and Geospatial Consultant
M +61 (0) 419 142 254</pre>
</body>
</html>