<div>Any coordinate system must have a "towgs84" or any other form of conversion to a global datum. In particular WGS84 itself has "+towgs84=0,0,0". You should use:</div>
<div> </div>
<div> cs2cs +proj=latlong +ellps=mod_airy +towgs84=xxxxx +to +proj=latlong +ellps=WGS84 +towgs84=0,0,0<br></div>
<div>where xxxxx is the set of 7 parameters you have. They should have signs according to the conversion from local datum to WGS84 and rotations must agree with the "position vector rotation" method (not "coordinate frame rotation"). For the conversion from WGS84 to local datum just change the two coordinate system strings, not the signs.</div>
<div> </div>
<div>Instead of using </div>
<div> +proj=latlong +ellps=WGS84 +towgs84=0,0,0</div>
<div>you can use </div>
<div> +proj=latlong +datum=WGS84 </div>
<div>which is equivalent.</div>
<div> </div>
<div>Regards </div>
<div> </div>
<div>Jose' Gonçalves</div>
<div> </div>
<div class="gmail_quote">2009/2/27 Aaron Friesen <span dir="ltr"><<a href="mailto:aaron@cartopac.com">aaron@cartopac.com</a>></span><br>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">Everyone,<br><br>I'm trying to familiarize myself with proj.<br><br>I'm trying to use cs2cs to do a datum shift where a 7-parm<br>
transformation is used. I want it to do the shift in<br>both directions.<br><br>"proj -ld" seems to provide a possible example. The listing<br>for ire65 has example towgs84 settings.<br><br>Datum: ire65<br>
Ellipse: modif_airy (sic)<br>Comment: towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15<br><br>What do I need to specify for cs2cs for both directions<br>of the transform?<br><br>+proj=latlong +ellps=mod_airy<br>
<br>+to<br><br>+proj=latlong +ellps=WGS84<br><br>seems like a resonable start. Where does the +towgs84 argument go?<br>How do things change in the reverse direction?<br><br>Thanks for your assistance,<br><br>Aaron<br><br>
_______________________________________________<br>Proj mailing list<br><a href="mailto:Proj@lists.maptools.org">Proj@lists.maptools.org</a><br><a href="http://lists.maptools.org/mailman/listinfo/proj" target="_blank">http://lists.maptools.org/mailman/listinfo/proj</a><br>
</blockquote></div><br>