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<p>Hello Thomas</p>
<p>Thanks for your email (and no worry if my email was "dry" - I
just tried to focus on some points without making the mail too
long). Just a few notes about my understanding of ISO 19111
subtleties:<br>
</p>
<p>Le 02/06/2022 à 16:02, Thomas Knudsen a écrit :</p>
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<p>Regarding "internal state" and ISO19111, there is still a
number of places that the notion that a that a CRS should be
"defined" pops up, cf. e.g. ISO19111 clause C.5.1: "The
associated map projection effectively **defines** the
projected CRS from the geodetic CRS.</p>
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<p align="justify">ISO 19111 makes a distinction between
"conversion" and "transformation". In ISO 19111 terminology, a
conversion is always between two CRS using the same datum. So
conversions are not impacted by changes in the figure of the Earth
(plate motions, etc), is defined by pure mathematical formulas and
has theoretically an infinite precision (ignoring floating point
rounding errors and approximations by series expansions). By
contrast, a transformation is between two CRS using different
datum. Transformation are determined empirically, have unavoidable
stochastic errors (not caused by software limitation), change with
time and location, etc.</p>
<p align="justify">Map projections fall in the "conversion" category
while datum shifts fall in the "transformation" category. In ISO
19111 model, a projected CRS is a special case of derived CRS,
i.e. a CRS related to another CRS by a conversion. The model
restricts this relationship to conversions; it is not possible in
ISO 19111 model to define a derived CRS using a transformation. So
the ISO model forces the use of a database such as EPSG for all
empirically-defined relationship between CRS. Only the
mathematically-defined relationship can be associated to the CRS.
I presume this is because the latter are considered stable.<br>
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<p><br>
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<p>But the real deluge of internal state first becomes clear
when turning to ISO19162/OGC18-010, "WKT representation of
CRS", which, I believe, you are one of the primary authors of.</p>
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<p>The merit goes to Roger Lott; I only provided some comments. Even
also contributed well.</p>
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<div dir="ltr">(…snip…) Which in my book could be reduced to (…)
just<br>
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PROJCRS["SWEREF99 TM"]<br>
<br>
i.e: "There exists a projected coordinate system called
SWEREF99 TM". We also know that it's known as EPSG:3006, since
that is the key we used to look it up. To have any use of it,
we need it supplemented by an entry in the transformation
table telling the "CONVERSION" part of the story.<br>
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<p align="justify">Yes we agree :-) With a nuance: the labels are
not applied on the CRS, but on the datum. And indeed any
transformation between different datum requires the use of a
database (ignoring "bound CRS"). So the model is already the way
you describe, if we keep in mind that as soon as there is a change
of datum, the relationship between two CRS is no longer a
conversion (it become something with at least one transformation)
and consequently can not be used anymore in the definition of a
derived or projected CRS. The "CONVERSION" element in "PROJCRS"
definition is restricted by the model to a narrower case bounded
by mathematics. By contrast, the problem about the "TOWGS84"
element was that is was associated to the datum, not the CRS. So
it was defining a transformation (as opposed to conversion), which
we agree should not appear in a WKT.<br>
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<p> Martin</p>
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