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<p>Hello Jochem<br>
</p>
<p>Le 24/03/2017 à 11:29, Jochem a écrit :</p>
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<blockquote cite="mid:1490351360448-5314039.post@n6.nabble.com"
type="cite">
<p wrap="">I think we should make a distinction between reference
systems and reference frames here (I assume you are familiar
with that). As each frame of a system will have different
parameters. For a frame only one set of parameters is needed. If
these are multiple sets, one could combine the information of
all these stochastic transformations in to one best
least-squares estimated set of parameters.</p>
</blockquote>
<p>In the case of NAD27 to WGS84 transformations in the EPSG
database, all the 80 transformations are between the same geodetic
datum (I think "geodetic datum" was the old ISO 19111 terminology
for "reference frame", but they are renaming that in the new ISO
19111 revision). There is +towgs84 parameters for the whole USA,
then different +towgs84 for various smaller geographic areas (e.g.
East of Texas, West of Texas, etc.). The transformations for local
areas have smaller errors than the transformations for the whole
USA. I do not know how those parameters were determined, but I
could imagine starting from the same set of stations and using
e.g. only the stations in Texas for determining the +towgs84
parameters in Texas, etc.</p>
<p>For map projection libraries, this imply that:</p>
<ul>
<li>For coordinate conversions, we only need the source and target
CRS.</li>
<li>For coordinate transformations, in addition of the source and
target CRS we also need the area of interest if we want to
choose the most accurate transformation among the various
possibilities.<br>
</li>
</ul>
<p><br>
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<blockquote cite="mid:1490351360448-5314039.post@n6.nabble.com"
type="cite">
<pre wrap="">However, this is the case for the national grid (RD) of the Netherlands. RD
is defined as the transformation (in ISO19111 terminology this would be a
conversion?) from ETRS89, including a 7 parameter transformation, but also
an correction grid on projected coordinates is used:
ETRF2000(R05) <--7par--> pseudo_NL_Bessel <--proj--> pseudo_RD <--corr-->
true_RD
</pre>
</blockquote>
<p>This look like a datum shift to me, so it would still be a
transformation in ISO 19111 terminology. Even if the new CRS is
officially defined by Bursa-Wolf parameters and all software
produce the same numerical results, the relationship with objects
in the real world would still have stochastic errors: if we have a
ETRF2000 coordinate that describes the location of a physical
object, and if we transform that coordinate to RD, then the
location described by the new coordinate may be slightly aside the
physical object (otherwise we would not have to do least-square
fitting). Or did I missed something?</p>
<p>Note: this is not an objection to your change proposal. This is
rather an emphasis that coordinate operations should be associated
to metadata that allow users to evaluate implications of what they
are doing.<br>
</p>
<p> Martin</p>
<p><br>
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