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<TITLE>I need some help on using the SetMercator method</TITLE>
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<P><FONT SIZE=2>I'm having some problems with the SetMercator method of OGRSpatialReference when using the GDAL Warp API. I'm writing an app the reads GeoTiff files and warps them into a memory raster. Things are ok without setting the target raster to be projected i.e. <BR>
<BR>
oSRS.SetWellKnownGeogCS( "WGS84" );<BR>
oSRS.exportToWkt( &pszSRS_WKT );<BR>
poDstDS->SetGeoTransform( adfGeoTransform );<BR>
poDstDS->SetProjection( pszSRS_WKT );<BR>
<BR>
Source data is transferred to target raster correctly by the warp operation.<BR>
<BR>
Note: I have previously calculated the GeoTransform array as follows:<BR>
<BR>
adfGeoTransform[0] = tl_lon;<BR>
<BR>
if (br_lon > tl_lon)<BR>
{<BR>
adfGeoTransform[1] = (br_lon - tl_lon) / iwidth;<BR>
}<BR>
else<BR>
{<BR>
adfGeoTransform[1] = (br_lon + 360.0 - tl_lon) / iwidth;<BR>
}<BR>
adfGeoTransform[2] = 0;<BR>
<BR>
adfGeoTransform[3] = tl_lat;<BR>
adfGeoTransform[4] = 0;<BR>
adfGeoTransform[5] = ((br_lat - tl_lat) / iheight);<BR>
<BR>
Where tl_lat, tl_lon, br_lat, br_lon are the geographic extents of my target raster and iwidth and iheight are the raster sizes.<BR>
<BR>
As soon as I set the target raster to be projected i.e.<BR>
<BR>
oSRS.SetProjCS("Mercator Projection");<BR>
oSRS.SetWellKnownGeogCS( "WGS84" );<BR>
oSRS.SetMercator (0, 0, 1.0, 0, 0);<BR>
oSRS.exportToWkt( &pszSRS_WKT );<BR>
poDstDS->SetGeoTransform( adfGeoTransform );<BR>
poDstDS->SetProjection( pszSRS_WKT );<BR>
<BR>
Now the target raster is not filled (all black). I've experimented a little with the parameters to the SetMercator method. Setting the scale factor to 0 caused one of my tiff files (large map of europe) to warp ok, however, a small map ~300km wide in Afghanistan didn't warp (all black in the target again).<BR>
<BR>
Can anyone advise on what the parameters passed to SetMercator should be? Also, what does scale factor mean in this context? Is there anything else I should be doing?<BR>
<BR>
Many thanks for any help anyone can give.<BR>
<BR>
Best regards,<BR>
<BR>
John Rawlinson</FONT>
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