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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Even, Frank or anyone in the know about gdal warp API,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I have been using the <span style='font-size:9.5pt;font-family:Consolas;color:black'>GDALCreateGenImgProjTransformer function to create my transformer arg in the warp options for the ChunkAndWarpImage function with an error threshold set to 0.0 for a while now and everything works great. I did this initially because I wanted the best possible warp results. Recently, I noticed that the performance of that operation had slowed down significantly in the newer versions and I am assuming that’s because the process has been improved to be more rigorous and produce better results (?). <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'>Anyhow, I was looking at how I could speed up the process without sacrificing too much accuracy and I noticed that the default behavior in the gdalwarp utility uses an error threshold of 0.125 and the GDALCreateApproxTransformer function to create the transformer arg. I read the documentation and I think I understand the ramifications of doing so, but I was hoping somebody could give me a more detailed explanation of how much the warp results will be affected by using the GDALCreateApproxTransformer function with a threshold of 0.125. I am assuming it must not be that bad because Frank choose that value as the default value years ago. I noticed it really improves the performance but I didn’t want to make the conversion without knowing exactly what I am sacrificing. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'>I am assuming with an error threshold of 0.0 it is a per pixel warp operation and if the threshold is greater than 0.0 and the GDALCreateApproxTransformer function is used it ends up being a grid warp. Is that correct? Does the threshold mean that the warped pixel may be off by one pixel in the results or could it be worse than that? <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'>Thanks in advance for any light you can shed on the subject!<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'>Best regards,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:9.5pt;font-family:Consolas;color:black'>Martin Chapman </span><o:p></o:p></p></div></body></html>