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<p>Hello Jody</p>
<p>The NADCON grid files are from NAD27 to NAD83. The coordinate
transformations that Betsey wanted to do are from NAD27 to
WGS84, which is considered as a different operation in the EPSG
database. This difference was often ignored because we were used
to consider NAD83 as equivalent to WGS84. This was
approximatively true 20 years ago, but there is now a difference
of about 1.5 metres between NAD83 and latest realizations of
WGS84.</p>
The key point is that GeoTools will not necessarily provide the
same transformation for NAD27 to WGS84 than for NAD27 to NAD83. It
depends on what GeoTools finds in the EPSG database, and how the
referencing module is designed. Last time I touched to the
GeoTools referencing module, it had a bug: when it found more than
one operation for the same sourceCRS and targetCRS, GeoTools
unconditionally selected the most accurate one, regardless its
area of validity. This explain why the TOWGS84 parameters are for
Cuba: this is the operation with the smallest "positional
accuracy" found in the EPSG database, but this accuracy is true
only in a relatively small geographic area.
<p>The above bug exists (or existed) because at the time I wrote
the referencing module, I though that a sourceCRS and a
targetCRS were sufficient for uniquely identifying a coordinate
operation. I didn't realized that we can have as much as 80
different operations for the same source and target CRS
depending on the geographic area. The fix for GeoTools would be
that the CRS.findMathTransform(...) method takes another
argument, which is the geographic area where the user intend to
apply the coordinate operation. This problem has been fixed in
Apache SIS (among other issues discovered as I continue to learn
new aspects).</p>
<p>Even if GeoTools uses the grids, the grids are not the same for
USA than for Canada, so a hint about geographic area is still
desirable. For Texas, if the desired accuracy is 1 metre then
there is two possible operations depending on whether the points
are on the west side or the east side of 100°W: EPSG:8624 and
EPSG:8625. If an accuracy of 10 metres is sufficient then the
TOWGS84 parameters for USA given in my previous emails should be
okay.<br>
</p>
<p>As a side note: the above also explain why the TOWGS84 element
in WKT has been removed in the new WKT 2 specification (ISO
19162): the TOWGS84 element can not handle the real-world
complexity and is actually dangerous since it can be misleading
when given without information about its domain of validity,
like what happened with the TOWGS84 parameters for Cuba.<br>
</p>
<p> Martin</p>
<p><br>
</p>
Le 26/05/16 à 12:39, Jody Garnett a écrit :<br>
</div>
<blockquote
cite="mid:CAOhbgA=w4vS0dKOSDhboL9GkGcmiv1Bxwm7U-bLjgEpoMiw=6Q@mail.gmail.com"
type="cite">
<div dir="ltr">G'Day Martin:
<div><br>
</div>
<div>The GeoTools has been configured with GridShft files for
NAD27 - this is not something I have personally tried before
so I am not quite sure how it works.</div>
<div><br>
</div>
<div>I had a look with Betsey, the data being projected is in
Texas (so slightly different from Cuba so the <span
style="font-size:13px">TOWGS84 parameters sound legit</span>).</div>
</div>
</blockquote>
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