<div dir="ltr">I meant to reply to list.<div class="gmail_extra"><br><div class="gmail_quote">On Thu, Dec 22, 2016 at 2:21 PM, Richard Greenwood <span dir="ltr"><<a href="mailto:richard.greenwood@gmail.com" target="_blank">richard.greenwood@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><span class="">On Thu, Dec 22, 2016 at 2:00 PM, Even Rouault <span dir="ltr"><<a href="mailto:even.rouault@spatialys.com" target="_blank">even.rouault@spatialys.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"><u></u>
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<p style="margin:0px;text-indent:0px">On jeudi 22 décembre 2016 13:40:23 CET Richard Greenwood wrote:</p>
<p style="margin:0px;text-indent:0px">> I've been following a few of the recent threads on this list regarding</p>
<p style="margin:0px;text-indent:0px">> enhancements to Proj's datum transformations. Pretty exciting stuff!</p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> One group of transformations that seems to have been ignored for a number</p>
<p style="margin:0px;text-indent:0px">> of years is applying the HARN hpgn grids in the Proj epsg file. For example</p>
<p style="margin:0px;text-indent:0px">> the Proj epsg entry for epsg:3758 uses +towgs84=0,0,0,0,0,0,0 but I believe</p>
<p style="margin:0px;text-indent:0px">> that is should use +nadgrids=wyhpgn.gsb, or at least that's what I change</p>
<p style="margin:0px;text-indent:0px">> it to, which produces results that match NADCON.</p>
<p style="margin:0px;text-indent:0px">> </p>
<p style="margin:0px;text-indent:0px">> Is anyone aware of a reason that nadgrids are not referenced in the epsg</p>
<p style="margin:0px;text-indent:0px">> file? Is it maybe just an oversight, something that nobody ever got around</p>
<p style="margin:0px;text-indent:0px">> to? Or because distributing nadgrids would become burdensome?</p>
<p style="margin:0px;text-indent:0px"> </p>
</span><p style="margin:0px;text-indent:0px">Richard,</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">I don't think we would want to force people using grids to be able to use EPSG codes (because of the required space or licensing terms),</p></div></blockquote><div><br></div></span><div>Space might be an argument, but I don't think licensing is. The grids are produced by the USGS and USGS data is freely available (one thing we can still take pride in post-election).</div><span class=""><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-family:monospace;font-size:9pt;font-weight:400;font-style:normal"><p style="margin:0px;text-indent:0px"> but we could generate +nadgrids=@bla syntax to make them optional. What could be useful is to have both +nadgrids= and +towgs84 with the priority given to the grids when they are present </p></div></blockquote><div><br></div></span><div>That would be a very nice solution.</div><span class=""><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-family:monospace;font-size:9pt;font-weight:400;font-style:normal"><p style="margin:0px;text-indent:0px">but this isn't implemented right now in proj.4 (if you have both +nadgrids and +towgs84, I believe +nadgrids is always used, even if the grids are missing on the system)</p>
<p style="margin:0px;text-indent:0px"> </p>
<p style="margin:0px;text-indent:0px">As far as adding the grid info, in the current workflow, this would be a matter of doing that in the <a href="https://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/build_pcs.py" target="_blank">https://svn.osgeo.org/metacrs/<wbr>geotiff/trunk/libgeotiff/csv/b<wbr>uild_pcs.py</a> script, presumably adding a new column in the gcs.csv file, and some logic/dictionary to match the datum name with a grid name, and then updating GDAL to use that new information when reading the .csv files.</p></div></blockquote><div><br></div></span><div>I took a look at that workflow a few years ago before the modernization and backed off from it, but I'm interested in looking at it again. Not being very familiar with build_pcs.py or the epsg database, this is a pretty naive question, but why aren't the nadgrids being reference already? Is the issue with the build_pcs.py script or with the epsg database? </div><div><br></div></div><span class="">-- <br><div class="m_-2221379181228118250gmail_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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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature" data-smartmail="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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