Hello Thomas and all
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:
* A Coordinate Reference System (CRS)
* A Reference Frame (previously known as geodetic datum)
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:
* Number of dimensions.
* Axis names, order and units of measurement.
* The type of the coordinate system (Cartesian versus Ellipsoidal, etc.)
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.
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.
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.
According above criterion, map projections also fall in the
"conversions" category. National mapping agencies decide that /by
definition/, 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:
But with enough internal state to derive transformations between two
different CRS!
Should be completed with /"if and only if there is no change of
reference frame"/. 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.
In slide 11:
Transformations are seldom unique
Yes we agree on that, but /conversions/ are unique. See above
distinction between transformations and conversions.
You’ll need geodetic context to select the right one. And that context
is not sufficiently represented in the CRS data model
Yes we agree on that, and indeed the ISO 19111 model does not try to
represent transformations in CRS data model. It represents only
/conversions/, this distinction is important (I'm excluding the BOUNDCRS
WKT element from this discussion, which is a WKT-specific thing added as
a compromise for migrating away from the legacy TOWGS84).
Last slide:
A CRS is a label
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.
Regards,
Martin
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