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<p style="margin-top:0; margin-bottom:0">VERTCON is a "snapshot" of the differences between NGVD29 and NAVD88 as they existed in 1992. Any subsequent<i> vertical
</i>crustal movement since 1992 is not modeled in the data grids. The results for major portions of Louisiana (subsidence) and the Midwest (glacial rebound & uplift) will be in error by a number of feet. The errors will increase as time passes since the data
grids will not be updated. A similar conundrum will occur to a lesser magnitude (at first) with the new vertical datum to be published in 2022. Someday, they may publish a VTDP (Vertical Time Dependent Positioning) model that might be "reliable." So far,
it's just a dream. Clicking that mouse button will give you an answer, but it may be far from reality!</p>
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<p><font face="Copperplate Gothic Bold">Clifford J. Mugnier, c.p., c.m.s.</font></p>
<p><font face="Copperplate Gothic Bold">Chief of Geodesy</font></p>
<p><font face="Copperplate Gothic Bold">LSU Center for GeoInformatics (ERAD 266)</font></p>
<p><font face="Copperplate Gothic Bold">Dept. of Civil Engineering (P.F. Taylor 3531)</font></p>
<p><font face="Copperplate Gothic Bold">LOUISIANA STATE UNIVERSITY</font></p>
<p><font face="Copperplate Gothic Bold">Baton Rouge, LA 70803</font></p>
<p><font face="Copperplate Gothic Bold">Academic: (225) 578-8536</font></p>
<p><font face="Copperplate Gothic Bold">Research: (225) 578-4578</font></p>
<p><font face="Copperplate Gothic Bold">Cell: (225) 328-8975</font></p>
<p><font face="Copperplate Gothic Bold">honorary lifetime member, lsps</font></p>
<p><font face="Copperplate Gothic Bold">fellow emeritus, asprs</font></p>
<p><font face="Copperplate Gothic Bold">member, apsg</font></p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> PROJ <proj-bounces@lists.osgeo.org> on behalf of Greg Troxel <gdt@lexort.com><br>
<b>Sent:</b> Wednesday, February 20, 2019 10:52 AM<br>
<b>To:</b> Even Rouault<br>
<b>Cc:</b> proj@lists.osgeo.org<br>
<b>Subject:</b> Re: [PROJ] Vertical Transformations?</font>
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<div class="PlainText">Even Rouault <even.rouault@spatialys.com> writes:<br>
<br>
>> I wonder if transformations that need grid files for correct behavior<br>
>> should fail rather than skip the transformation, at least by default.<br>
><br>
> That's a complex topic. There is no "correct" behavior per-se. All <br>
> transformations are approximations, so it is a matter of how accurate your <br>
> needs are.<br>
<br>
That's strictly true, but my understanding is there is a broad consensus<br>
that VERTCON is the appropriate conversion for the general problem.<br>
<br>
> In the above example if you're fine with ~ 1 metre vertical accuracy,<br>
> you don't need the VERTCON grids.<br>
<br>
Agreed that if you are ok with ~1m, you can assume they are the same and<br>
not convert. That more or less applies to all horizontal datum<br>
conversions, with differing acceptable errors. With NAD27, the errors<br>
are so large that I suspect almost everyone would prefer an error to<br>
assuming equivalence to ITRFxx, once they understood.<br>
<br>
> The EPSG database can also reference some grids that can not necessarily <br>
> easily be found, are not free, or have no known conversion to a PROJ <br>
> recognized format.<br>
<br>
So there is no real way for most people to convert to/from those datums.<br>
That's too bad, and I agree those situations are intractable and proj<br>
declining to convert them is not necessarily helpful.<br>
<br>
> Selection of a given transformation pipeline among all possible also involve a <br>
> number of arbitrary decisions (some transformations might have intersecting <br>
> area of validity). Playing with the new projinfo utility to list available <br>
> coordinate operations can show the complexity of this...<br>
<br>
I realize this is all infinitely complicated.<br>
<br>
In this case, my concern is that there is a consensus method to go<br>
between NGVD29 and NAVD88, that I think almost everyone would agree is<br>
the standard approach. People who install proj but not optional<br>
gridfiles will silently get a much coarser approximation.<br>
<br>
So I would like to see the transform from NGVD29/NAVD88 error out when<br>
the VERTCON grid file can't be opened, perhaps with some configuration<br>
to say that it should assume 0 instead. I think it's better to fail<br>
during what I see as a misconfiguration than to fall back to a cruder<br>
approximation.<br>
<br>
Perhaps the evnironment variable "PROJ_NGVD29_COARSE" should be needed,<br>
and this hint can be returned as the error from attempting to use the<br>
gridfile.<br>
<br>
I have ensured that the pkgsrc package for proj installs all of the<br>
ok-licensed grid files, so it doesn't matter much to me personally. I<br>
see there are a lot more now, but it still seems best to include them in<br>
the package.<br>
<br>
I am assuming almost all use of proj other than by the developers is via<br>
packaging systems.<br>
<br>
It would be interesting to hear what other packagers do in terms of<br>
grids (included, separate packages, not packaged, ?).<br>
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