[GRASS-user] Re: [Gdal-dev] Little experiment: comparison of vector projection OGR and ArcGIS

[Matt Wilkie]

Hi Dylan,

> A couple days ago I did another experiment, this time incorporating a
> datum shift: GCS NAD83 --> UTM NAD27. I have summarized the results
> here: http://casoilresource.lawr.ucdavis.edu/drupal/node/259
> 
> of note:
> 
> - the default tranform in ArcGIS does not produce the correct results.
> - the 'NADCON' transform in ArcGIS produces the same results as OGR.

I asked ESRI-L about this, and the near unanimous response is: there is 
no default transformation in ArcGIS, but an alphabetised list.

"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.

cheers,

-- 
matt wilkie
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