Hi Paul<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im">
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It depends on what the original co-ordinate system of the image was. If it was in a different datum from ETRS89 or a different ellipsoid from GRS80/WGS84 (which is the datum/ellipsoid of your GRASS location), then there will be small errors in the reprojection because you didn't specify a datum for the target co-ordinate system.<br>
<br></blockquote><div>Gdalinfo from original file I get this:</div><div><div>river: GTiff/GeoTIFF</div><div>Files: L71203033_03320001023_B10.TIF</div><div>Size is 8141, 7161</div><div>Coordinate System is:</div><div>PROJCS["WGS 84 / UTM zone 29N",</div>
<div> GEOGCS["WGS 84",</div><div> DATUM["WGS_1984",</div><div> SPHEROID["WGS 84",6378137,298.257223563,</div><div> AUTHORITY["EPSG","7030"]],</div>
<div> AUTHORITY["EPSG","6326"]],</div><div> PRIMEM["Greenwich",0],</div><div> UNIT["degree",0.0174532925199433],</div><div> AUTHORITY["EPSG","4326"]],</div>
<div> PROJECTION["Transverse_Mercator"],</div><div> PARAMETER["latitude_of_origin",0],</div><div> PARAMETER["central_meridian",-9],</div><div> PARAMETER["scale_factor",0.9996],</div>
<div> PARAMETER["false_easting",500000],</div><div> PARAMETER["false_northing",0],</div><div> UNIT["metre",1,</div><div> AUTHORITY["EPSG","9001"]],</div><div>
AUTHORITY["EPSG","32629"]]</div><div>Origin = (524400.000000000000000,4414200.000000000000000)</div><div>Pixel Size = (30.000000000000000,-30.000000000000000)</div><div><br></div><div><br></div><div>
I just need to have this data in EPSG:3763 system so I thought that I would just need to define EPSG:3763 without anything else. Original is in WGS84 and this one is GRS80. What should I do? use in gdalwarp EPSG:3763 or the PROJ4 extension?</div>
<div>THank you for your help</div><div>Kim</div><div><br></div></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
If the two datums are the same (for our purposes it is reasonable to say this if the two ellipsoids are the same, or the difference is very small (e.g. GRS80 and WGS84)) then there is no problem at all. But in general you should specify the datum when doing a reprojection or warping as it can be important.<br>
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<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"></div></blockquote></blockquote><div><br></div><div> </div></div>