I agree with your proposal. <br><br><div><span class="gmail_quote">On 8/26/06, <b class="gmail_sendername">Paul Kelly</b> <<a href="mailto:paul-grass@stjohnspoint.co.uk">paul-grass@stjohnspoint.co.uk</a>> wrote:</span>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">I suspected v.proj was doing this for years (since the introduction of the<br>new vector format) but never got round to verifying the behaviour until
<br>yesterday. When re-projecting a 3-D vector file, if datum transformation<br>parameters are specified using towgs84= notation (rather than nadgrids=),<br>v.proj will assume the z co-ordinates are ellipsoidal heights and
<br>transform them accordingly.<br><br>I think there are probably very few circumstances in which this behaviour<br>would be desirable. It is much more common for height data to be specified<br>relative to a vertical datum,
e.g. mean sea level, than relative to the<br>ellipsoid (which is normally only used for horizontal distances).<br><br>GPS data is more likely to include ellipsoidal height, but again it is<br>(IMHO) of questionable usefulness to transform it to another ellipsoid:
<br>the geoid models that exist to transform GPS data to the local vertical<br>datum in regular use (OSGM02 for Great Britain and Northern Ireland is the<br>one I'm familiar with) convert directly from WGS84 ellipsoidal height data
<br>to height above mean sea level.<br><br>I propose the attached patch to add a specific flag to v.proj to enable<br>the current behaviour, otherwise the z co-ordinates are always left<br>untouched. I thought it was worth explaining and discussing properly
<br>because it changes the current behaviour and will give different results<br>in certain circumstances, but IMHO the current behaviour is wrong.<br><br>Any comments?<br><br>Paul<br><br>_______________________________________________
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</div><br><br clear="all"><br>-- <br>David Finlayson