<HTML dir=ltr><HEAD><TITLE>Re: [Proj] Portuguese Modified Grid - Datum 73</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Bravo, Professor Gonçalves!<BR></FONT></DIV></DIV>
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<DIV><FONT face="Times New Roman" color=#000000 size=2><SPAN style="FONT-SIZE: 10pt">Clifford J. Mugnier,</SPAN></FONT><FONT face="Times New Roman" color=#000000 size=2><SPAN style="FONT-SIZE: 10pt"><BR><STRONG>LOUISIANA STATE UNIVERSITY</STRONG> <BR></SPAN></FONT><BR>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Jose Gonçalves<BR><B>Sent:</B> Tue 16-Feb-10 11:34<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] Portuguese Modified Grid - Datum 73<BR></FONT><BR></DIV></DIV>
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<P><FONT size=2>Hello<BR><BR>The proj string you refer is not correct. What is called in the EPSG<BR>database "Portuguese Modified Grid - Datum 73" (epsg code 27492) is<BR>the projection used in Portugal for the local datum called Datum 73.<BR>Datum 73 was established in 1973 and is based on the Hayford<BR>ellipsoid.<BR>The correct definition of the projection can be found in the epsg file<BR>distributed with proj:<BR><BR># Datum 73 / Modified Portuguese Grid (deprecated)<BR><27492> +proj=tmerc +lat_0=39d00' +lon_0=-8.131906111111112 +k=1<BR>+x_0=180.598 +y_0=-86.98999999999999 +ellps=intl +units=m +no_defs <><BR><BR>It is a Transverse Mercator projection with central point:<BR> lat_0=39d00'00.000"N<BR> lon_0= 8d07'54.862"W<BR>No scale factor in central meridian is applied (+k_0=1)<BR>It has a small false easting/northing<BR> x_0=180.598<BR> y_0=-86.990<BR><BR>This coordinate system was replaced by a new projection based on datum<BR>ETRS89. The new projection, that the Portuguese Geographic Institute<BR>(IGP) called PT-TM06, has a different central point. It's correct<BR>definition is:<BR><BR># ETRS89 / Portugal TM06<BR><3763> +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334<BR>+k=1 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs <><BR><BR>In your proj string you had this central point mixed with the Datum 73<BR>projection.<BR><BR>For a test point you can use the following (a geodetic point called Cabreira):<BR><BR>D73 Geographic - epsg:4274 (8d02'39.1243"W 41d38'17.2443"N)<BR>D73 projected - epsg:27492 (7488.375 218847.838)<BR>ETRS89 Geographic - epsg:4258 (8d02'35.8302"W 41d38'20.2812"N)<BR>ETRS89 Projected - epsg:3763 (7483.752 218845.648)<BR><BR>To convert from Datum 73 to ETRS89 a Bursa-Wolf transformation can be<BR>used. Using program cs2cs you can have, for example:<BR><BR>cs2cs +init=epsg:27492<BR>+towgs84=-230.994,102.591,25.199,0.633,-0.239,0.900,1.950 +to<BR>+init=epsg:4258 +towgs84=0,0,0<BR><BR>These parameters are given by IGP<BR>(<A href="http://www.igeo.pt/produtos/Geodesia/Inf_tecnica/parametros_transformacao/parametros_Portugal.htm">http://www.igeo.pt/produtos/Geodesia/Inf_tecnica/parametros_transformacao/parametros_Portugal.htm</A>).<BR><BR>I have created NTv2 grid files to convert from datum 73 (and other<BR>older national systems) to ETRS89 with better accuracy. They are<BR>available in my page: <A href="http://www.fc.up.pt/pessoas/jagoncal/coordenadas">http://www.fc.up.pt/pessoas/jagoncal/coordenadas</A><BR>(sorry, only in Portuguese).<BR><BR>Regards<BR><BR>José A. Gonçalves<BR>University of Porto, Portugal<BR><BR><BR><BR><BR><BR><BR>2010/2/16 Roger Oberholtzer <roger@opq.se>:<BR>> On Tue, 2010-02-16 at 14:53 +0100, OvV_HN wrote:<BR>>> Dunno, but doesn't the utm projection uses a couple of preset values for<BR>>> lat_0, k_0, x_0, y_0, and a calculated value of lon_0?<BR>>> The tmerc projection seems more appropriate.<BR>><BR>> You are surely correct. I will give this on a shot:<BR>><BR>> +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334 +k=1 +x_0=180.598 +y_0=86.990 +ellps=intl +units=m +no_defs<BR>><BR>> Too bad it is always so hard to get appropriate test data for<BR>> verification.<BR>><BR>><BR>> --<BR>> Roger Oberholtzer<BR>><BR>> OPQ Systems / Ramböll RST<BR>><BR>> Ramböll Sverige AB<BR>> Krukmakargatan 21<BR>> P.O. Box 17009<BR>> SE-104 62 Stockholm, Sweden<BR>><BR>> Office: Int +46 10-615 60 20<BR>> Mobile: Int +46 70-815 1696<BR>><BR>> _______________________________________________<BR>> Proj mailing list<BR>> Proj@lists.maptools.org<BR>> <A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR><A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR></FONT></P></DIV></BODY></HTML>