<div dir="ltr"><br><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Jun 11, 2013 at 8:40 PM, Ben Caradoc-Davies <span dir="ltr"><<a href="mailto:Ben.Caradoc-Davies@csiro.au" target="_blank">Ben.Caradoc-Davies@csiro.au</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">On 12/06/13 07:33, Frank Warmerdam wrote:<br>
</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">
In a somewhat recent version of PROJ.4 the behavior was changed so that<br>
if no proper datum information is given, no attempt is made to convert<br>
between datums based on bare ellipsoid information. So in the cases<br>
given, there is no attempt made to translate between the ellipses.<br>
The behavior used to be different but it seemed to lead to many<br>
unexpected results when people did not provide datum shift info and it<br>
was decided those results were doing more harm than good.<br>
Of course it is still a bit invisible to the user whether datum shifting<br>
is being applied or not which is unfortunate.<br>
Contrast what you see with:<br>
echo "6378137 0 0" | cs2cs +proj=geocent +datum=WGS84 +no_defs +to<br>
+proj=longlat +ellps=WGS66 +towgs84=0,0,0 +no_defs<br></div>
0dE0dN -8.000<div class="im"><br>
In this case the eight meter change in height is entirely done based on<br>
the ellipsoid difference but it is only applied because we have provided<br>
information declaring datum relationship of the two datums (the first as<br>
WGS84, and the second as "equivelent to WGS84").<br>
</div></blockquote>
<br>
Thanks, Frank. That is a huge help and explains the behaviour I see.<br>
<br>
One unfortunate side-effect of this new behaviour is that published Proj.4 settings lacking +towgs84=0,0,0 such as this are incomplete:<br>
<a href="http://spatialreference.org/ref/epsg/4891/proj4/" target="_blank">http://spatialreference.org/<u></u>ref/epsg/4891/proj4/</a></blockquote><div><br></div><div style>Ben,</div><div style><br></div><div style>Yes, that is true, though I'd claim there is no clear right transformation to use in this case.</div>
<div style> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
Furthermore, it appears that OGR does not set enough datum information when importing OGC WKT, so conversions between WGS84 and WGS66 do not change height. Or do you consider this OGC WKT incomplete because it lacks TOWGS84?<br>
<a href="http://spatialreference.org/ref/epsg/4891/prettywkt/" target="_blank">http://spatialreference.org/<u></u>ref/epsg/4891/prettywkt/</a></blockquote><div><br></div><div style>It might be nice if GDAL/OGR would lookup a preferred datum shift when there is an explicit reference to an OGC datum like this. It certainly does not provide explicit information on how to transform to other datums in the WKT itself. </div>
<div style><br></div><div style> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
The OGR workaround is to import Proj.4 with +towgs84=0,0,0 rather than OGC WKT.</blockquote><div><br></div><div style>Assuming towgs84=0,0,0 is an acceptable approximation. I would claim it often is not a very good choice.</div>
<div style><br></div><div style>Best regards,</div><div style>Frank</div><div style> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">
<br>
<br>
Kind regards,<br>
<br>
-- <br>
Ben Caradoc-Davies <Ben.Caradoc-Davies@csiro.au><br>
Software Engineer<br>
CSIRO Earth Science and Resource Engineering<br>
Australian Resources Research Centre<br>
</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br>---------------------------------------+--------------------------------------<br>I set the clouds in motion - turn up | Frank Warmerdam, <a href="mailto:warmerdam@pobox.com" target="_blank">warmerdam@pobox.com</a><br>
light and sound - activate the windows | <a href="http://pobox.com/~warmerdam" target="_blank">http://pobox.com/~warmerdam</a><br>and watch the world go round - Rush | Geospatial Software Developer<br>
</div></div>