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<p>Hello Thomas and all</p>
<p>On the ISO 19111 model, I have a different view. One possible
source of confusion is that a label such as "WGS84" or "ETRS89" is
used for two different things:</p>
<ul>
<li>A Coordinate Reference System (CRS)</li>
<li>A Reference Frame (previously known as geodetic datum)</li>
</ul>
<p align="justify">In the ISO 19111 model, this is two different
objects. There is an "ETRS89" reference frame. But there is also a
"ETRS89" CRS. A reference frame is basically only a label
("ETRS89"). But a CRS is a reference frame augmented with the
following information:</p>
<ul>
<li>Number of dimensions.</li>
<li>Axis names, order and units of measurement.</li>
<li>The type of the coordinate system (Cartesian versus
Ellipsoidal, etc.)</li>
</ul>
<p align="justify">So a CRS is not just a label. We also need to
know if axis order is (longitude, latitude) or (latitude,
longitude), if units of measurement are degrees or grad (or
metres, kilometres, foot, miles…), etc. Those information are
independent of coordinate transformations and are necessary for
correct understanding of the numerical values in a coordinate
tuple.<br>
</p>
<p align="justify">When a transformation needs to be applied between
two CRS, then indeed we use the CRS as labels and search for a
transformation in a database. The ISO 19111 specification is
already designed that way. But there is also some operations that
do not need database. For example if we just switch axis order
from (latitude, longitude) to (longitude, latitude), there is no
need for a database for this operation. The result is two distinct
CRS (because axis order is part of CRS definition), but both CRS
are associated to the same reference frame.</p>
<p align="justify">When talking about coordinate operations, ISO
19111 makes a distinction between transformations and conversions.
It can be understood as a distinction between "operations that
need a database" and "operations that do not need a database"
respectively. Changes of axis order and units conversions fall in
the latter category. The key criterion for determining if an
operation is a conversion or a transformation is whether the
operation involves a change of reference frame. If the reference
frame of both CRS is the same, then this is a conversion.
Otherwise this is a transformation.<br>
</p>
<p align="justify">According above criterion, map projections also
fall in the "conversions" category. National mapping agencies
decide that <i>by definition</i>, the national CRS is the result
of applying a specific set of formula (the map projection) on a
geodetic CRS. Because the geodetic reference frame is not changed,
the operation has theoretically an infinite precision (ignoring
rounding errors). This is different than transformations, where
change of geodetic reference frame brings stochastic errors. So in
the slides shown at NKG General Assembly 2022, the following
sentence in slice 6:</p>
<p align="justify">
<blockquote type="cite">But with enough internal state to derive
transformations between two different CRS!</blockquote>
Should be completed with <i>"if and only if there is no change of
reference frame"</i>. The ISO 19111 model does not have internal
state for deriving transformation between two different reference
frames. It only has enough internal state for conversions between
two CRS having the same reference frame. It means change of axis
order, units conversion or map projection, but not datum shifts.</p>
<p align="justify">In slide 11:</p>
<p align="justify">
<blockquote type="cite">Transformations are seldom unique</blockquote>
Yes we agree on that, but <i>conversions</i> are unique. See
above distinction between transformations and conversions.</p>
<p align="justify">
<blockquote type="cite">You’ll need geodetic context to select the
right one. And that context is not sufficiently represented in
the CRS data model</blockquote>
Yes we agree on that, and indeed the ISO 19111 model does not try
to represent transformations in CRS data model. It represents only
<i>conversions</i>, this distinction is important (I'm excluding
the <font face="monospace">BOUNDCRS</font> WKT element from this
discussion, which is a WKT-specific thing added as a compromise
for migrating away from the legacy <font face="monospace">TOWGS84</font>).</p>
<p align="justify">Last slide:</p>
<p align="justify">
<blockquote type="cite">A CRS is a label</blockquote>
In ISO 19111 model, a Reference Frame is a label, and all the
discussion in the NKG presentation about transformations is true,
but applied to Reference Frame, not to CRS (again using ISO 19111
terminology). As said above, a possible source of confusion is
that the same label (e.g. "WGS84") is often used for both kinds of
objects.<br>
</p>
<p align="justify"> Regards,</p>
<p align="justify"> Martin</p>
<p align="justify"><br>
</p>
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