Submitted by matt wilkie (not verified) on Wed, 2006-08-02 16:58.
I asked ESRI about this, their response: The 'default' transformation that Dylan refers to, NAD_1927_To_NAD_1983_6, is just the first in the alphabetical list. As it turns it out, it uses NTv2 and is designed for Quebec. ArcGIS generally doesn't set any default transformations but leaves it to you to decide which one is most appropriate. Dylan's data is outside the area covered by the NTv2 grid file, so essentially no tranformation occurs when he uses NAD_1927_To_NAD_1983_6. A better reference for him, which does list the methods, parameters and areas of use for the transformations is:
http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=21327
-- Melita Kennedy Product Specialist ESRI, Inc.
Even if mistaken about the "default", thank you for conducting the experiment and telling us about it. Otherwise I would sailed along quite happily usiing the wrong selections myself. Also thanks to you we also know that provided the appropriate transformation method is chosen equivalent results can be achieved with both ArcGIS and GDAL, so we are all free to choose the tool most readily at hand.
NAD_1927_To_NAD_1983_6 is not the default for ArcGIS
I asked ESRI about this, their response: The 'default' transformation that Dylan refers to, NAD_1927_To_NAD_1983_6, is just the first in the alphabetical list. As it turns it out, it uses NTv2 and is designed for Quebec. ArcGIS generally doesn't set any default transformations but leaves it to you to decide which one is most appropriate. Dylan's data is outside the area covered by the NTv2 grid file, so essentially no tranformation occurs when he uses NAD_1927_To_NAD_1983_6. A better reference for him, which does list the methods, parameters and areas of use for the transformations is:
http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=21327
-- Melita Kennedy Product Specialist ESRI, Inc.
Even if mistaken about the "default", thank you for conducting the experiment and telling us about it. Otherwise I would sailed along quite happily usiing the wrong selections myself. Also thanks to you we also know that provided the appropriate transformation method is chosen equivalent results can be achieved with both ArcGIS and GDAL, so we are all free to choose the tool most readily at hand.