<div class="gmail_quote">On Wed, Jul 29, 2009 at 5:25 AM, Matt Wilkie <span dir="ltr"><<a href="mailto:maphew@gmail.com">maphew@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br>
Let's see if I have this straight then: if I specify -t_srs epsg:4617 and not<br>
+nadgrid, no datum transformation occurs, even though the output .prj file<br>
says it has?</blockquote><div><br></div><div>If your projection definition in Proj4/OGR has a datum defined it will go through one of the following (in order of preference) if the datum's are different:</div><div><br>
</div><div> - a gridshift (the first matching one if there are >1)</div><div> - or a 7 parameter transform (+towgs84=#,#,#,#,#,#,# if specified)</div><div> - or a 3 parameter transform (+towgs84=#,#,# if specified)</div>
<div> - or do nothing</div><div><br></div><div>It does that from source-datum to WGS84, then from WGS84 to the dest-datum. A datum in Proj is defined as an ellipse and a towgs84 or gridshift combination, +datum= just pre-populates this info. For Proj's purposes, GRS80 and WGS84 are the same ellipse.</div>
<div><br></div><div>cs2cs -v is quite clear about what it's doing, OGR not so much. </div><div><br></div><div>Using the "straight" Proj EPSG defs on my system:</div><div><br></div><div><div>$ cs2cs -v +init=epsg:3579 +to +init=epsg:4617</div>
<div># ---- From Coordinate System ----</div><div>#Albers Equal Area</div><div># Conic Sph&Ell</div><div># lat_1= lat_2=</div><div># +init=epsg:3579 +proj=aea +lat_1=61.66666666666666 +lat_2=68 +lat_0=59</div>
<div># +lon_0=-132.5 +x_0=500000 +y_0=500000 +ellps=GRS80 +units=m +no_defs</div><div># ---- To Coordinate System ----</div><div>#Lat/long (Geodetic alias)</div><div>#</div><div># +init=epsg:4617 +proj=longlat +ellps=GRS80 +no_defs</div>
</div><div><br></div><div>No datum transform will happen (ellipses match, and +towgs84 is ommitted (ie ==0)).</div><div><br></div><div>HTH,</div><div><br></div><div>Rob :)</div></div>