<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Dec 26, 2016 at 11:47 PM, <span dir="ltr"><<a href="mailto:support@mnspoint.com" target="_blank">support@mnspoint.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="font-size:10pt">
<p>Hello,</p>
<p>It looks like the reasons why grid shift files are not much used or referenced is that usually there are other ways available to fix small map errors ... and using grid shift files would require too much knowledge in the subject ... and also give too much unrequired accuracy since the source material is already so much off the line. So most users and applications are happy with the towgs84 mechanism.</p></div></blockquote><div>I don't think most users have any idea what datum transformation is being used. But they do expect results that match other geospatial software.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="font-size:10pt">
<p><span style="font-size:10pt">Other questions with grid shift files are:</span><br></p>
<p>How much their usage slows down the end product that uses Proj.4 library?</p></div></blockquote><div> I'd venture to guess that grid transformations are computationally less expensive than 7 parameter transformations. </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="font-size:10pt">
<p>How difficult their usage is for end users to be realistic that they ever used them?</p></div></blockquote><div><br></div><div>I'm proposing changes only to the "epsg" file that is a part of the Proj.4 distribution. However this would require that the hpgn grids here <a href="https://trac.osgeo.org/proj/wiki/HarnGrids">https://trac.osgeo.org/proj/wiki/HarnGrids</a> be included in the Proj.4 distribution. They are 354kB zipped. There are at least two grid files in the nad/ directory that are already part of the distribution. But as time goes by we see more, and larger, grid shift files. So I can see some resistance to adding more.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="font-size:10pt">
<p>How could the above questions (or problems) be fixed the most easy way (for the end user)? Meaning that adding more good code to Proj.4 and/or choosing better data formats instead of writing more complex manuals would be preferred.</p></div></blockquote><div>Even Rouault suggested adding code that would use the +nadgrids definition if the required grid shift file was present, otherwise the +towgs84 definition would be used.</div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="font-size:10pt">
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<p>Janne.</p>
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<p>Richard Greenwood kirjoitti 2016-12-22 22:40:</p>
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<div dir="ltr">I've been following a few of the recent threads on this list regarding enhancements to Proj's datum transformations. Pretty exciting stuff!
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<div>One group of transformations that seems to have been ignored for a number of years is applying the HARN hpgn grids in the Proj epsg file. For example the Proj epsg entry for epsg:3758 uses +towgs84=0,0,0,0,0,0,0 but I believe that is should use +nadgrids=wyhpgn.gsb, or at least that's what I change it to, which produces results that match NADCON.
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<div>Is anyone aware of a reason that nadgrids are not referenced in the epsg file? Is it maybe just an oversight, something that nobody ever got around to? Or because distributing nadgrids would become burdensome?</div>
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<div>It seems like pretty low hanging fruit. HARN has been around for about 20 years. The nadgrid files are already there. Differences between +towgs84=0,0,0,0,0,0,0 and +nadgrids=XXhpgn.gsb are on the order of a meter in the few places I have looked, so it's a fairly significant correction (or error, depending on your point of view). "grep -c HARN epsg" returned 264 and "grep -c 'hpgn' epsg" returned 0, so effects a lot of definitions.</div>
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<div>Maybe I'm missing something. I'd be interested in any comments.</div>
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<div>Rich</div>
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<a href="http://lists.maptools.org/mailman/listinfo/proj" rel="noreferrer" target="_blank">http://lists.maptools.org/<wbr>mailman/listinfo/proj</a><br></blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature"><div dir="ltr">Richard W. Greenwood, PLS<br><a href="http://www.greenwoodmap.com" target="_blank">www.greenwoodmap.com</a></div></div>
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